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Uninterruptible Power Supplies (UPS) provide short-term emergency backup power in the event of any disturbances or disruptions to the mains electricity supply.  

A UPS system protects computers, IT equipment, telecommunications networks, and other vital electrical equipment, infrastructure, and machinery against unexpected problems with the input power source.  

Clean, continuous power is an essential requirement of 21st-century day-to-day life, with sectors as diverse as banking, utilities, manufacturing, transportation, retail, healthcare, and entertainment all dependent on complex computer and communications technologies.  

Any unexpected power disruption to these sophisticated mission-critical networks can lead to damaging downtime and data loss, costing businesses thousands of pounds a minute in sales and hours of lost productivity. Even worse, it could even lead to death and serious injury.  

Uninterruptible power supply systems reduce these risks and form a fundamental part of any organisation’s continuity planning. 

How Does A UPS System Work?  

A UPS differs to other standby power systems such as a generator by using energy stored in batteries, supercapacitors, or flywheels to maintain power to the load when there’s a disruption to the mains supply.  

The battery runtime (or autonomy) of most UPS systems is relatively short, often only a few minutes. This battery backup provides enough time for either the connected equipment to safely shut down with minimal risk of damage or data loss, or for an alternative power source such as a backup generator to kick-in. 

An uninterruptible power supply’s primary role is to provide short-term emergency power when there’s a complete mains failure or blackout.  

In addition, most modern online UPS can correct a wide range of common power problems, including: 

  • Sags: short periods when the voltage is below the usual mains supply level 
  • Brownouts: an undervoltage similar to a sag, but over a longer period of time (several hours or even days) 
  • Power surges: a sudden increase in voltage caused by an event such as a lightning strike. 
  • Harmonics: a distortion from the ideal sinusoidal waveform 
  • Unstable frequency: where the power oscillates at something other than 50 Hertz (sometimes referred to as electrical noise)
  • Spikes: very short millisecond bursts of energy on the power line 

Design life is how long a UPS battery will last for assuming perfect operating conditions, while service life is the length of time before the battery needs replacing.

All UPS batteries have a stipulated design life. Standard design life tends to be 5 years, although 10, 15, and even 20-year design life batteries are available too.

Under internationally-recognised EUROBAT (Association of European Automotive and Industrial Battery Manufacturers) guidelines, a UPS battery is considered at the end of its service life when capacity falls 80% below the original value.

So in effect, a 10-year design life battery will last for 10 years, assuming perfect operating conditions. However, it’s performance will reduce over time to a degree where it isn’t safe to carry on using it.

No installation can ever be perfect though. It would be technically impractical and cost too much.

Several factors can influence battery service life, such as operational and storage temperatures, the frequency and depth of discharge, and the battery maintenance regime.

Before such influences are even taken into account, the operational capacity of a 10-year design life battery will drop below 100% capacity by year 6 and will continue to fall to 80% over the next 4 years, as outlined in the image below.

When Should UPS Batteries Be Replaced?

Taking into account the EUROBAT recommendation that batteries should be swapped out before capacity falls below 80%, it’s clear why it is accepted industry best practice to replace 10-year design life batteries in year 7 or 8.

Doing so makes allowance for all the external factors that impact on service life whilst providing a safe enough margin for potential failure without compromising the load.

Similar performance drop-offs occur in 5-year design life batteries too. This is why it’s advisable to replace them in either year 3 or 4 of service life.

Central Power Supply Systems (CPSS) are a specific type of standby power solution used with emergency and safety-related applications such as lighting, alarms and security systems. Also known as Central Supply Systems (CSS), they share many of the design and operational features of a typical online UPS. 

To be classified as a CPSS, the product must meet the strict criteria outlined in EN 50171 for use with essential safety systems. This standard covers emergency systems permanently connected to AC supply voltages not exceeding 1,000V that use batteries as an alternative power source.  

CPS systems are used to power a broad range of emergency-related applications and devices in the event of a mains failure: 

  • Emergency lighting 
  • Fire safety and prevention i.e. fire extinguishers, smoke extractors, lifts 
  • Alarms and other detection systems 
  • Warning systems such as carbon monoxide detectors 
  • Safety signs and other signalling installations 

A standard UPS is unlikely to meet the requirements of EN 50171 and should not be used for such critical emergency applications. However, some specially-designed emergency UPS models do meet the strict criteria.  

Central Power Supply Systems require a combination of resilience, regulatory compliance, and ease of use. To meet these objectives, Riello UPS’s emergency UPS typically incorporate the following features: 

  • Battery autonomy up to three hours (longer if necessary) 
  • Short battery recharge times (typically less than 12 hours) 
  • Sophisticated battery monitoring and battery care systems 
  • Optional Galvanic isolation of the input and output 
  • High short circuit (in-rush) current capacity 
  • Advanced diagnostics accessible through user-friendly display panel 

Temperature compensated charging helps to prolong battery life by dynamically adjusting the voltage depending on the ambient temperature.

It ensures maximum UPS battery charge in colder conditions. While it reduces the risk of overcharging in warmer operating conditions.

Ambient temperatures alter the chemical reactions that occur within batteries. It is generally accepted that battery life halves for every 10°C rise in temperature above the recommended 20-25°C.

In fixed charge voltage systems, there is a danger that batteries undercharge when kept in cold conditions. On the flip side, they can overcharge if the temperature is too warm.

How Does Temperature Compensated Battery Charging Work?

Temperature compensated battery charging offers a solution to these issues. Special circuitry fitted to the UPS dynamically adjusts the recharge voltage depending on changes to the ambient temperature. In essence, the higher the temperature, the lower the recharge voltage.

Reducing the charge voltage at higher temperatures optimises the chemical reaction. Without this compensation, the batteries may overcharge. This produces excess hydrogen that can build up pressure inside the cell.

In a worst-case scenario, this pressure may be enough to vent a sealed battery. This unbalances its chemical equilibrium and causes permanent damage.

On the opposite end of the spectrum, batteries require a higher charge voltage in colder conditions. Failure to increase the voltage will undercharge the battery and limit its capacity.

All batteries have a finite service life and will deteriorate over time. But compensating the voltage based on ambient temperature helps keep the chemical balance stable and prolong its life.

What Are The Benefits Of Temperature Compensated Charging?

  • It reduces the risk of venting and permanent damage to cells in warm ambient temperatures
  • It ensures batteries are 100% charged in cooler conditions, which maximises UPS runtime
  • Prolongs battery life by limiting deterioration in float charging applications
  • Limits electrolyte maintenance on flooded battery cells

Galvanic isolation separates the input and output supplies to a device so that energy flows through a field rather than via electrical connections. It enables power transfer between two circuits that must not be connected. 

In a UPS system, isolation transformers transfer AC current to connected equipment while isolating them from the power source. This protects against electric shock and suppresses harmful electrical noise.  

Typically, transformer-based uninterruptible power supplies are the choice for industrial environments and medical applications that tend to demand galvanic isolation.

However, it is possible to add two isolation transformers to the input supplies of a transformerless UPS, which helps ensure complete neutral separation. (Learn more about the differences between transformer-based and transformer-free UPS).

The image below outlines the simplest installation for a UPS with a single power supply and input: 

  • Figure 1 shows how an isolation transformer can be installed at point A, point B, or both (NB if point A, the transformer must be oversized in accordance with the UPS input supply rating)
  • Figure 2 highlights a more complex installation where the UPS has a dual input supply. In this case, the isolation transformer can be installed at point C, point D, or both.  

Also known as ‘Active Standby’ or ‘Economy’, ECO mode is the most energy-efficient UPS operating mode. Capable of exceptional efficiency up to 99%, ECO mode sees the bypass line (raw mains supply) power the load, with the inverter powered but remaining off as long as the mains is in tolerance. 

It is similar to the basic operating mode of an offline UPS, where the inverter is on standby and ready to switch on if there’s a power problem. 

When there’s an issue with the mains supply, the load experiences a fractional break in supply (perhaps up to 15ms), while the UPS system’s automatic bypass switches back to the inverter. 

This short break in continuous power is the main drawback to using ECO mode, particularly for facilities with sensitive equipment, such as data centres.

Another disadvantage of pure ECO mode is the lack of power conditioning provided by true double-conversion online UPS mode.

How Much More Efficient Is ECO Mode Compared To Online UPS?

The main benefit of ECO mode is the increased efficiency of the bypass line, which typically runs at 98-99% compared to standard online UPS efficiency of 93-97%.

That difference of anywhere between 2-6% has the potential to deliver significant savings. Consider a large-scale facility, where even a 1% improvement in efficiency could equate to tens or even hundreds of thousands of pounds a year in lower energy costs.

ECO mode is only recommended on sites where the utility supply is relatively stable and the load generates low harmonics so isn’t sensitive to any mains interference.

What Is Active ECO Mode? 

In recent years, some UPS systems (such as Riello UPS’s NextEnergy range) now offer an operating mode called Active ECO Mode, sometimes referred to as Advanced ECO.  

Just like in standard ECO mode, the bypass line (mains supply) powers the load. But with Active ECO, the inverter remains on at all times and runs in parallel with the input without actually carrying the load.  

This means that power transfer is far quicker in the event of a mains failure, ensuring higher availability than true Economy mode. And even though the inverter isn’t processing the load, it does absorb harmonic currents and provide power filtering in a similar way to online UPS.

Obviously, powering the inverter all the time in Active ECO does require more energy than standard ECO mode. The trade-off for this is that efficiency in Active ECO is between 0.5-1% lower than normal ECO mode. But efficiency is still considerably higher than online UPS mode, so it offers something of a happy medium.

In the event of a mains failure, the inverter takes over far quicker in Active ECO mode, meaning it offers higher availability than standard ECO mode.  

Obviously, the efficiency using Active ECO is slightly less (around 1%) than it is with pure ECO, but this is still considerably better than online, so offers users something of a happy medium. 

When To Use ECO Or Active ECO Modes

Mission-critical sites such as data centres are reluctant to use these energy-saving operating modes, with the trade-off in resilience and power quality putting operators off.

While it may not be practical to have a UPS running in energy-saving modes all of the time, it might be practical to use when a site’s most critical loads are inactive, such as overnight or out of hours.

Parallel UPS systems offer an opportunity too. In such an example, one UPS would run in online mode as the ‘master’, with the remaining units operating in ECO mode until the condition of the mains supply changes and they’re required to actively support the load. 

A static inverter is designed for use with emergency lighting systems and other safety-related applications, as well as in remote settings such as offshore.

It performs a similar role to a standard uninterruptible power supply, but provides backup for a longer time, usually either 1 or 3 hours.

Inverters provide either a continuous or standby source of AC power from a DC supply, typically a sealed lead-acid battery or photovoltaic cell used as part of a solar panel array.

In effect, a static inverter performs in a similar way to a UPS system running in Standby operating mode with the bypass supplying the load and no filtering of the mains supply. When a mains failure occurs, the static inverter switches to running off batteries. 

Static inverters must comply with the EN 50171 safety standard for Central Power Supply Systems. One of the criteria this involves is the capability to clear a final circuit fault and continue running without breaking its own circuit protection device. 

Standard UPS systems are not designed to meet EN 50171 specifications and therefore should not be used to back up emergency lighting and similar applications. However, there are specially-designed UPS for emergency applications to meet these requirements.

BS or British Standard is the generic term given to high integrity batteries that fully comply with BS6290 part 4 (and IEC60896-2) in terms of construction, performance and design life. Usually costing slightly more than standard sealed lead acid products, they offer 10-12 year design life, threaded copper insert terminals, flame retardent case material (UL94-VO) and are generally selected for premium installations such as Hospitals and Telecommunications.

Automatic Transfer Switches (ATS) are typically used to provide resilience for smaller uninterruptible power supplies (below 10 kVA) that can’t operate in parallel.  

The ATS has two AC input power sources (‘A’ and ‘B’) and if/when one fails the loads are automatically and instantaneously transferred to the other. 

Automatic Transfer Switches like Riello UPS’s Multi Switch ATS can also be used instead of Power Distribution Units (PDUs) for plug and play loads. In addition, ATSs protect against short-circuits and offer the ability to switch output power connections on and off remotely over a network. 

The ‘A’ and ‘B’ power sources feeding an Automatic Transfer Switch can be configured in a variety of combinations: 

  • ‘A’ supplied from the output of the UPS and ‘B’ from the mains supply 
  • Both ‘A’ and ‘B’ supplied from two separate UPS outputs 
  • Both ‘A’ and ‘B’ supplied from two separate mains supplies. 

The above example highlights an ATS with two input supplies and up to eight outputs. 

Automatic Transfer Switches (ATS) are typically used to provide resilience for smaller uninterruptible power supplies (below 10 kVA) that can’t operate in parallel.  

The ATS has two AC input power sources (‘A’ and ‘B’) and if/when one fails the loads are automatically and instantaneously transferred to the other. 

Automatic Transfer Switches like Riello UPS’s Multi Switch ATS can also be used instead of Power Distribution Units (PDUs) for plug and play loads. In addition, ATSs protect against short-circuits and offer the ability to switch output power connections on and off remotely over a network. 

The ‘A’ and ‘B’ power sources feeding an Automatic Transfer Switch can be configured in a variety of combinations: 

  • ‘A’ supplied from the output of the UPS and ‘B’ from the mains supply 
  • Both ‘A’ and ‘B’ supplied from two separate UPS outputs 
  • Both ‘A’ and ‘B’ supplied from two separate mains supplies. 

The above example highlights an ATS with two input supplies and up to eight outputs. 

A Parallel Systems Joiner (PSJ) is an output power coupling switch that enables two independent groups of UPS operating in parallel to join together. 

A PSJ performs a similar role to a UPG Group Synchronisers. It connects the outputs from two distinct groups of parallel UPS into a dynamic dual bus for system expansion, fault tolerance and ease of maintenance.  

If one of the UPS within the two parallel systems fails or needs to be switched off for maintenance and emergency service work, the PSJ merges the two outputs to allow power sharing.  

A UPS Group Synchroniser (UGS) is a device that harmonises the outputs from two separate groups of UPS systems operating in parallel, even if they are supplied from different AC power sources.  

It enables the outputs from two groups to be configured into a dual bus format, maintaining synchronisation regardless of input supply variations. As an example, one group of UPS could be powered by the mains, with the other running on batteries. As a rule, a UGS can be used with up to eight UPS across two separate groups. 

Adding a Static Transfer Switch (STS) to the output allows the protected load to be supplied from either of the two parallel groups. In such a configuration, if one of the groups fails, the STS transfers the load to the other (as long as there is spare capacity).  

A power conditioner is a device used to protect sensitive loads by smoothing out voltage fluctuations such as spikes, transients and electrical noise. It can be electronic or transformer-based. 

Also known as a line conditioner, it protects equipment from power surges, helps to correct voltage and waveform distortions, and removes external electrical noise (i.e. frequency and electromagnetic interference) caused by devices such as radios and motors.  

A static bypass switch automatically and instantaneously transfers the load to the mains electricity supply when there’s an internal fault or failure with the UPS system.  

In effect, running on bypass the circumvents the UPS (i.e. rectifier, batteries, and inverter) and ensures power continuity while the UPS is fixed or swapped out.  

However, operating on bypass does not filter or condition the input or output supply as is the case with typical double conversion online UPS systems, so should be used sparingly.  

Internal static bypass switches should not be confused with an external maintenance bypass switch, which is an option that enables the UPS to be powered down manually for maintenance while the critical load is powered directly from the mains. 

External maintenance bypass switches tend to be mechanical in the form of a wraparound or rotary switch or set of circuit breakers.  

Examples Of Static And Maintenance Bypass In A UPS System

With current recommendations demanding lower levels of input harmonics (THDi) it is more common for larger systems to require some form of input filtering to achieve this. Twelve pulse rectifiers can improve upon more standard 6 pulse systems and offer the ability to correct the problem across the whole load range. Passive input filters added to 6 pulse designs are usually cost effective and improve the input power factor, but often only effective at higher load levels of the load, typically above 50%. For very large installations it is quite common to have a combination of a 12 pulse rectifier with extra input filtering, to dramatically reduce harmonics as a result of a restricted input supply or standby generator limitations. Recent developments in rectifier design have resulted in IPFC (input power factor corrected) IGBT rectifiers offering very low THDi and a high Input PF as standard.

There are several factors that influence sizing a UPS system, including the combined load of all the equipment the UPS will protect, scope for further system expansion, battery runtime and redundancy.  

As well as choosing the right UPS topology, correctly sizing an uninterruptible power supply is crucial – undersizing inevitably causes immediate problems, while initial oversizing will waste energy, money and valuable floor space.  

The easiest way to ensure a correctly sized UPS system is to get prospective suppliers to undertake a full site survey where they can accurately assess your requirements. However, it is possible to broadly size a UPS yourself by following a step-by-step process. 

Critical or Non-Critical Loads 

This starts with listing and reviewing all the equipment that will need to be protected by the UPS. Establish whether an item of equipment is critical – and therefore will need the emergency backup provided by the UPS – or non-critical, which can be allowed to fail when the mains power supply does so. (Learn more about the difference between critical and non-critical loads).

Power Range 

The next step is to calculate the total power range for the combined critical load that needs protecting. It’s important to base this on use during peak working hours, rather than on quieter times such as an office network during the night. 

Equipment labels and supporting technical data will provide information such as the supply voltage, frequency, number of phases, load current, power factor and power consumption.  

The power consumption of electrical equipment is stated in either Watts (W) or Volt-Amperes (VA). Because UPS systems are rated by VA or kVA ratings, this may require a conversion from W to VA, which can be calculated by dividing the power consumption (W) by the power factor.  

Add up all the VA, then multiply this by a figure such as 1.2 or 1.25, which factors in future growth and system expansion. That figure is the maximum size in VA or kVA that your UPS should be.  

Note that a UPS should never be sized to run at 100% load capacity, as this isn’t recommended for safe, stable and reliable performance. 

Potentially Problematic Loads 

Certain equipment (i.e. laser printers, blade servers, air conditioners, certain lighting systems, motors and compressors) have an inrush of current during start-up or draw higher currents in normal operation, which can cause the UPS to overload. This can lead to intermittent alarms or potentially send the UPS into bypass mode.  

For these types of load, good practice suggests two options: either remove them from the power protection system (if the equipment can safely power down on mains failure) or oversize the UPS by a factor of at least three.  

Battery Runtime 

This is the amount of time you want the UPS to keep equipment operating in the event of a power failure. How to size a UPS battery depends on the nature of the equipment. In some circumstances, runtime only needs to be for a few minutes as a bridge to let the standby generators kick-in and take over. 


An external maintenance bypass switch enables a UPS system to be electrically isolated – taken out of the critical power circuit – for safe UPS maintenance, service work, or unit replacement without any disruption to the load.  

Without an external bypass switch, such as Riello UPS’s Multi Pass, to allow for preventive maintenance opportunities, the only safe alternative is to power down the entire critical load, not just the UPS system.  

The external maintenance bypass will use a manual switching method to transfer the load between UPS output and the mains power supply. This can comprise of a single transfer switch (three or four pole) or an array of isolators.

An external bypass can often be wall or rack-mounted – known as a Wrap-Around Bypass, these are generally installed next to the UPS system. It can also incorporate mechanical interlocks, known as Castell, to make sure the bypass operates in the correct sequence and doesn’t damage the UPS or the load.

External maintenance bypass shouldn’t be confused with the internal maintenance bypass built into some UPS, which only allows for safe maintenance work in isolated parts of the UPS – there may still be other “live” parts such as the busbar connections.  

In addition, external maintenance bypass shouldn’t be mistaken for the static bypass switch in online UPS, which is a safety function in case of fault or failure with the uninterruptible power supply itself. Note that some external maintenance bypasses do incorporate an automatic transfer function which acts in the same way as a static bypass within a UPS.  

External maintenance bypasses tend to be circuit breaker or rotary switch-based.  

Benefits Of Using An External Maintenance Bypass 

  • Capability for total isolation that allows for UPS maintenance with no disturbance to the load 
  • Safe and user-friendly with a simple switching sequence that doesn’t risk back-feeding the UPS 
  • Electrical interlocking ensures no-break power transfer without the need for complex and expensive key interlock arrangements 
  • Eliminates the need for additional maintenance switches or key exchange boxes, which can reduce costs 
  • Is a standard system using readily available switchgear 
  • Versatile as can be built to include full MCB (Miniature Circuit Breaker) or MCCB (Moulded Case Circuit Breaker) protection or just isolation, depending on site requirements 
  • Easily customisable to incorporate larger terminals for oversized cables 
  • Option to incorporate shut trip facilities for Emergency Power Off (EPO) on input and bypass switches 
  • Can be installed outside normal hours, which enables the UPS to be installed during normal hours without the need for further shutdown. 

Isolation transformers transfer electricity from a source of alternating current (AC) to equipment or devices while isolating these devices from the power source, typically for safety reasons.  

This galvanic isolation protects against electric shock, suppresses electrical noise to sensitive devices, and enables power transfer between two circuits that must not be connected. 

There are several reasons why isolation transformers are used: 

  • Keep the load from generating harmful harmonics 
  • Keep harmonics on the distribution bus from continuing downstream to sensitive loads 
  • The output of the UPS may be at a different voltage to the load, so the transformer acts as a step-up/down voltage converter 

If the isolation transformer is located between the distribution bus and the UPS system, anything generated by the UPS or load is isolated from the bus, while anything from the bus is isolated from the load, even in bypass mode. 

For example, when a generator is installed, it is common to use four pole changeover switchgear or contactors when transferring from mains to the generator. This results in the traditional neutral-earth reference being lost during transition, which can cause phase voltages to rise and damage sensitive single-phase loads. 

Adding a bypass isolation transformer allows an electrical contractor to earth the UPS output neutral, eliminating this problem.  

Single-phase bypass transformers should be installed on smaller systems where the UPS output neutral needs to be earthed.  

For a transformerless UPS, two isolation transformers are required on the input supplies to provide complete neutral separation, including the bypass supply.

Site surveys are essential to help customers get the best UPS system to meet their specific circumstances and budget. They cover key areas such as power needs, logistics, and electrical installation.  

Onsite visits are necessary for any uninterruptible power supply that will be hard-wired to the electrical connections. Depending on the size of the installation, a site survey usually takes up to a couple of hours, with full recommendations and product quotations provided upon completion.  

The UPS site survey itself consists of fairly standard questions to identify site-specific needs and highlight any areas that may require further investigation. It will typically cover: 

  • Power requirements i.e. load, voltage, frequency, battery runtime, recharge times,  
  • Location and physical environment i.e. dimensions, room temperature, risk of leak or ingress, which floor will the UPS be installed 
  • Logistics i.e. whether there are any delivery restrictions such as height, weight, time, or vehicle access; what loading equipment will be needed; whether lifts are available  
  • Optional extras i.e. whether the UPS needs add-ons such as external bypass, isolation transformersremote monitoring software, battery extension packs 
  • Electrical installation i.e. whether the customer or manufacturer is responsible for installation works, commissioning, battery builds, removal and disposal of old UPS; health and safety RAMS; need for PPE 
  • Project timescales i.e. preferred installation timeframe, need for ongoing maintenance, monitoring and technical support 

Surveys should be carried out for both new installations and projects where an existing UPS is being replaced. With the latter, a site survey for UPS will evaluate the viability and efficiency of the current uninterruptible power supply compared to the latest, most technologically-advanced solutions.  

Riello UPS offers FREE, no-obligation site surveys where one of our highly-qualified experts will visit during standard working hours to assess your needs. 

Contact our team today to arrange a visit

Emergency Power Off (EPO) functionality enables an uninterruptible power supply and other related equipment such as generators to be remotely shut down in the event of a fire or building evacuation. 

The process is known by several other terms, including Emergency Shutdown (EDS), Remote Shutdown (RSD) and Remote Emergency Power Off (REPO).  

It can be controlled from a central EPO push-button, fire alarm panel or Building Management System (BMS) software and it aims to ensure there’s no risk of electrical shock when fire extinguishers or sprinklers are set off inside the room. 

The EPO circuit is typically closed and will open to initiate the shutdown, which will stop the inverter and rectifier, as well as disconnecting the battery (certain cheaper EPO circuits may only stop just the inverter). Caution is needed, however, as older systems may use an open circuit, so it is important to check beforehand.  

When installing an EPO circuit, it is essential to ensure the connection method used matches the type of UPS. For a single UPS unit only one switch contact is necessary. However, with a parallel UPS installation with multiple units, an individual switch contact must be provided for each of the UPSs (as outlined in the image below).  

The positioning of the EPO, particularly if it takes the form of a push-button, requires careful positioning to reduce the risk of accidental or even malicious use. It is common for most EPO functionality to be monitored by CCTV. 

Following an EPO, it is usual for a UPS and other affected equipment to require a manual restart. 

Generators are used by organisations whose machinery and IT equipment must remain online at all times during an extended power outage.

While uninterruptible power supplies are also designed to protect against such power outages, their purpose isn’t to deliver backup power indefinitely.

The primary role of a UPS system is to bridge the gap before the generator has time to start up and synchronise, or to keep equipment running long enough to enable a safe shutdown. It also cleans the power, whether from the mains or generator, to smooth out problems such as sags or surges.

Backup generators are typically found in mission-critical environments such as data centres, hospitals, call centres, and financial institutions. Most of these organisations will have large, stationary generators that are designed to handle multiple devices.

Diesel generators have built-in fuel tanks that typically last 8-12 hours, although some models are fitted with external tanks that extend running time for up to several days. An alternative would be a gas-powered generator.

Critical loads directly affect an organisation’s ability to maintain key operations and must be kept running during a mains power supply failure, for example, a data centre’s servers or life support equipment in a hospital. 

As the name suggests, non-critical (or non-essential) loads can be dropped during a power cut as they aren’t fundamental to the organisation’s operations. Printers, office lighting, and desk fans are good examples of non-essential loads. 

In modern life, many organisations are now dependent on data and voice processing systems. They’ve become such a fundamental part of the everyday infrastructure that a total or even partial failure can have catastrophic consequences.  

That’s why uninterruptible power supplies and other standby power solutions such as generators have a vital role in ensuring business continuity, providing instantaneous emergency backup to the most important electronic systems, devices and equipment when there’s a mains failure.  

These types of load are defined as ‘Critical Loads’ – loads that directly affect the ability of an organisation to operate and must either be kept running (without any break in power) when their mains supply fails or be powered down in an orderly manner to prevent system crashes, data corruption and life-shortening hardware damage. 

For most organisations, there are two further classes of load. ‘Essential Loads’ provide secondary support services that even though they aren’t operationally critical, may still be required for health and safety reasons, such as emergency lighting. These loads must still have some form of backup but do not require uninterrupted power, so can be allowed to fail or ride through the time it takes for a generator to start. 

Finally, there are ‘Non-Essential Loads’ that an organisation can afford to lose when the mains power supply fails, for example printers, general lighting and desk fans. 

Factors To Consider When Classifying Loads As Critical Or Non-Critical

Whether a load is classified as critical or non-critical depends on its importance to the organisation in terms of: 

  • Financial penalties, lost business and impact on customer service 
  • Service provision 
  • Lost production and productivity 
  • Quality, health and safety, and environmental systems 
  • Security breaches and loss of control 
  • Organisational reputation and stakeholder confidence 

Once critical loads have been identified, they should be prioritised by their importance and how long they need to be kept running during a mains failure.  

For some critical loads like a local file server, it may only need to be backed up to enable a safe system shutdown. 

Others such as medical life support systems or telecoms networks may need to be kept running for as long as possible. This prioritisation is known as load shedding. 

UPS systems should be used in combination with a standby generator in situations where equipment and machinery must remain operational at all times, even during prolonged periods without mains electricity.

Where possible, it is beneficial to deal with a single supplier for both items as this eliminates any potential conflicts if the equipment doesn’t work well together.

It is important to correctly match a standby generator to the uninterruptible power supply it will be working with.

The generator must have the capacity to handle the load from the UPS, to recharge the batteries, and cover any conversion losses from the UPS. Factor in ancillary equipment such as air conditioning, emergency lighting, alarms, and communications and monitoring systems too. For most generators, the maximum step load is approximately 65% of the total rated capacity.

Another point to consider is frequency regulation. The worst-case scenario is that the UPS and generator aren’t able to synchronise. The two main causes of this are if the rate of change is too fast for the UPS to follow without endangering the load, or the frequency is too wide for the UPS to accept.

Other issues that can affect poorly matched UPS systems and generators include input harmonics, load acceptance, voltage intolerance, and poor response times.

Battery autonomy is the period of time (in minutes or hours) a UPS battery will last for at a specified load level in the event of a power outage. In essence, it is how long the inverter will run off battery power.

Autonomy can also be referred to as backup, discharge time, or runtime. It is a function of battery charge state, load size, and capacity. 

UPS battery autonomy can support loads for just a few minutes through to several hours, although for longer runtime applications, it is more cost-effective to combine a UPS with a backup generator. 

Adding extra battery strings connected in parallel will increase autonomy, although it is important to consider the charging capabilities of the UPS. 

How Is UPS Battery Autonomy Calculated?

To size a UPS battery correctly, battery runtime is a factor of the Ampere-hour (Ah) rating of the battery set and the applied load – the lower the load, the longer the runtime for a specific Ah rating.

For example, a 10 kVA/8 kW three-phase UPS with a 14Ah battery set will offer approximately 15 minutes runtime at full load. But this might be extended to 30 minutes or more when powering a reduced load, such as 5 kVA/ 4 kW i.e. 50% capacity.

Load shedding is deliberately reducing the total load placed on a device or network. With uninterruptible power supplies, prioritising which loads power down in which particular sequence when the mains supply fails can help to maximise the amount of available battery runtime.  

This process is also known as a priority-based shutdown. Divide all the loads protected by the UPS into groups, for example, A, B and C – category ‘A’ loads are those that must be kept running on battery for the longest possible time, while group ‘C’ will need the shortest (as outlined in the image below).  

For example, a local file server only requires sufficient backup to safely shut down, while a Private Branch Exchange (PBX) supporting critical servers will need to be kept running as long as possible.

During an outage, as the hardware is powered down according to the pre-defined sequence, the load reduces on the UPS, which in turn increases the available battery runtime for the remaining equipment. 

Load shedding is common practice to manage potential downtime on networks where there are multiple servers or equipment clusters that need protection.  

There are three ways to achieve such a priority-based shutdown: 

Load shedding is deliberately reducing the total load placed on a device or network. With uninterruptible power supplies, prioritising which loads power down in which particular sequence when the mains supply fails can help to maximise the amount of available battery runtime.  

This process is also known as a priority-based shutdown. Divide all the loads protected by the UPS into groups, for example, A, B and C – category ‘A’ loads are those that must be kept running on battery for the longest possible time, while group ‘C’ will need the shortest (as outlined in the image below).  

For example, a local file server only requires sufficient backup to safely shut down, while a Private Branch Exchange (PBX) supporting critical servers will need to be kept running as long as possible.

During an outage, as the hardware is powered down according to the pre-defined sequence, the load reduces on the UPS, which in turn increases the available battery runtime for the remaining equipment. 

Load shedding is common practice to manage potential downtime on networks where there are multiple servers or equipment clusters that need protection.  

There are three ways to achieve such a priority-based shutdown: 

Witness testing for UPS systems takes place prior to full site installation and commissioning. It simulates real-life load conditions and aims to ensure the entire UPS and its ancillary components such as switchgear work properly and meet the agreed customer and contractual specifications. 

Also known as a Factory Acceptance Test (FAT), witness testing for uninterruptible power supplies incorporates a variety of inspections and simulations based on the customer’s specific requirements. These can include: 

  • Visual inspections 
  • Static-state tests i.e. input/output stability, harmonics, operational efficiency 
  • Dynamic tests i.e. different operating modes, overloads 
  • Failure simulations i.e. battery failure, AC mains failure 

In addition to these operational tests, a UPS witness test will also incorporate conformity checks and an audit of the original contract agreement to ensure all obligations are met.  

What Are The Benefits Of A UPS Factory Acceptance Test? 

Witness tests are most common for larger-scale UPS, especially the multi-megawatt systems typically found in modern data centres. Customer representatives, specifiers, project managers and other key personnel visit the manufacturer’s site, where their technicians and engineers oversee all tests. 

From a customer’s perspective, a FAT is the ideal opportunity to get a hands-on look at their new UPS in full operational mode prior to installation. It identifies and rectifies any fundamental issues and allows for last-minute modifications. 

Manufacturers can offer some practical training and insight, which will help boost customers’ confidence when they’re operating the UPS for real. 

It also gives both sides the chance to review maintenance procedures and discuss the spare parts required for both system start-up and its ongoing operation. 

Final FAT reports also act as a template for a Site Acceptance Test (SAT) that takes place during installation and commissioning. Replicating the FAT tests during a SAT is the most effective way to confirm the UPS hasn’t been damaged during transportation.  

Factory Acceptance Tests are also undertaken at Riello UPS’s state-of-the-art Italian facilities in Legnago and Cormano. These temperature-controlled environments enable our uninterruptible power supplies to be put through their paces using real-life loads.  

Where possible, air conditioning units should be backed up by a generator rather than an uninterruptible power supply. 

Air conditioning systems are considered something of a “dirty load” because of the continual switching, which causes spikes and surges to both the voltage and current. While UPSs can handle certain levels of overload for a small duration, the current surges typically found with air conditioning units tends to be greater than the UPS’s capacity. 

If there’s no option other to use a UPS to support A/C, it is recommended to oversize the inverter by as much as four to five times to ensure the UPS will be able to support the load and eliminate the risk of failure. 

Another option that reduces the risk is to operate a separate, dedicated UPS system just for the air conditioning system, with another UPS backing up the critical load.  

However, this obviously does significantly increase costs and require a bigger footprint.  

At first glance, specifying an uninterruptible power supply seems a complex task. But in essence, whether you’re buying a new UPS system or replacing an existing unit, the choice boils down to two main factors: required load and battery autonomy. 

To establish the required power consumption, separate out the critical loads (that will need protecting by the UPS) from the non-critical. In a call centre, for example, telephones and computers would be classified as critical, but desk fans not. For a factory, production lines and machinery are critical, but non-essential lighting or telephone lines not so much.  

Establishing the load requirement will determine which UPS topology is best-suited (standby/offline, line-interactive, or online). 

The second main criteria is battery autonomy. How much runtime do you need to carry out a safe shutdown? When you’re thinking about runtime, you should also consider how much space you have to install batteries. 

Other Factors Influencing Your Choice Of UPS

  • Installation environment: do you need a freestanding tower (typically for industrial sites) or rack-mounted (usually with IT loads) UPS? And where will the UPS be fitted? UPS systems work best in dry, dust-free, well-ventilated rooms with a consistent and controlled temperature. If the UPS is rear rather than front-venting, you’ll need to leave a 300mm gap. 
  • Installation configuration: do you need to build in redundancy, such as an N+1 parallel system, to mitigate against the risk of serious system failure? 
  • UPS maintenance: does the UPS have front panel access for maintenance or does it require side access? If it’s the latter, it has implications for configuring the system.  
  • Warranties: the industry norm for UPS systems is a two or perhaps three year warranty. Riello UPS offers a five-year extended warranty as standard on all UPS up to and including 3 kVA. 
  • Accessories: It’s recommended to always incorporate a bypass switch. Depending on your networking and communications requirements, SNMP cards or other BMS connections might be advisable.  
  • Features: does the UPS include an LCD or even touchscreen display panel that gives you important performance information? What operating modes does it have (i.e. ECO mode, Smart Active, Frequency Converter)? 

Specifying the correct UPS system often requires a site survey.

There are two key concepts to measuring the resilience of an uninterruptible power supply: Reliability covers the ability of the UPS to perform its necessary functions under stated operating conditions for a specified period of time. Availability focuses on whether a UPS is operational when required for use and takes into account both system running time and downtime. 

Several metrics help to measure UPS resilience, although a certain element of caution is advised. Firstly, UPS configuration can have a direct impact on resilience and availability i.e. a parallel-redundant installation can achieve higher availability than a single UPS.  

There’s also no universally accepted standard with most of the widely-used metrics, leading to competing UPS manufacturers having differing favoured approaches to measurement. This makes accurate comparisons tricky.  

Finally, there are factors outside the manufacturer’s control. For example, UPS systems installed in temperature-controlled, highly regulated environments are less likely to experience faults and failures than units faced with extremes in heat, cold, vibration and dust.  

Mean Time To Repair (MTTR) 

Mean Time To Repair (MTTR) measures the average time it takes to repair a UPS and restore it to full operating functionality following a failure. Also known as Mean Time To Restore or Mean Time To Recover, it is calculated by dividing the total maintenance time by the total number of maintenance actions over a set period of time.  

It’s extremely unlikely a UPS service engineer will happen to be onsite at the moment a UPS system fails. That’s why MTTR should factor in the time it takes for an engineer to arrive on site to produce an accurate figure.  

Some UPS manufacturers will only base their MTTR figure based on the actual repair time. A truly comprehensive MTTR should measure the entire time from which the failure is first discovered through to when the UPS returns to full working operation.  

Mean Time Between Failures (MTBF) 

Mean Time Between Failures (MTBF) measures the average length of operational time between powering up a UPS and system shutdown caused by a failure. It predicts the time (usually in hours) that passes between one UPS system breakdown and the next and is calculated by dividing the total operational time by the number of failures. 

MTBF is incumbent on the presumption that the equipment will fail at some point. It doesn’t take into account when a UPS is voluntarily shut down for preventive maintenance or routine parts replacement. Instead, MTBF only focuses on failures that require a UPS to be taken out of operation in order for it to be repaired. 

Naturally, failure rates aren’t constant. UPS systems mirror other sensitive electronic devices by following what’s known as the ‘Bathtub Curve’, which outlines three distinct periods ‘A’, ‘B’ and ‘C’: 

  • Period A – ‘Infant Mortality’ failures: corresponds to early failures caused by a component or manufacturing defect or transportation problem.  
  • Period B – ‘Random’ failures: during the normal working life of a UPS the rate of these failures is normally low and fairly constant. 
  • Period C – ‘Wear Out’ failures: towards the end of working life, system failure rates increase significantly.  

Be aware that while MTBF is an indication of reliability and availability, it does not represent the expected service life of a UPS.  

An online UPS has a typical MTBF of roughly 250,000 hours, although this will vary depending on the manufacturer. Note that when the UPS is in bypass mode and the load is connected to the mains, the MTBF of the system drops to that of the mains electricity supply.  

Mean Time To Failure (MTTF) 

Another, lesser-used, metric is Mean Time To Failure (MTTF). It measures the reliability of products and systems that cannot be repaired, and is comparable to MTBF, which is used in cases where repairing and returning to use is possible.  

MTTF offers a reasonable indication of how long a product is expected to last until it fails. In other words, it provides an estimate of service life.  

MTTF is calculated by dividing the total hours of operation by the number of products being tracked – most MTTF data is collected by running multiple products (often thousands) over an extended period of time to provide the most accurate possible figure.  


Availability measures both system running time and downtime. It combines the MTBF and MTTR metrics to produce a result rated in ‘nines of availability’ using the formula: Availability = (1 – (MTTR/MTBF)) x 100%. 

The greater the number of ‘nines’, the higher system availability. In mission-critical environments such as data centres, ‘5 nines’ and above is fast becoming the desired standard.  

AvailabilityLevelDowntime Per Year
99.9999%6 nines32 seconds
99.999%5 nines5 minutes 35 seconds
99.99%4 nines52 minutes 33 seconds
99.9%3 nines8 hours 46 minutes
99%2 nines87 hours 36 minutes
90%1 nine36 days 12 hours

By selecting a UPS from our MHT range you can rely on the advanced rectifier design to dramatically reduce input current harmonics to <3%, thereby eliminating the need for a 12 pulse rectifier or Input harmonic filters.

Yes, the MST, MPS and MHT ranges employ a plug and play parallel system that allows additional units to be added to an existing system at a later date. Additional UPS units can be added to either provide redundancy or extra capacity if the load increases. Sometimes it’s worth investing in additional switchgear early on so that more units can be accommodated much more easily.

Power Quality

A brownout happens when the voltage drops below the usual mains supply level. It is similar to a sag – a short-term drop in voltage – but can last for anywhere between a few minutes or even several hours and days.

Brownouts tend to be caused by increased demand for power or extreme weather conditions putting additional strain on the electricity network.

Sometimes, brownouts can be initiated intentionally. This occurs when National Grid deliberately reduces voltage to ration electricity supplies and prevent a possible blackout.

Tell-tale signs of a brownout include flickering lights, electrical appliances quickly switching off and turning back on again, and intermittent internet connections.

What’s The Difference Between A Brownout And A Blackout?

Brownouts are damaging for IT loads – in many ways they can be more disruptive than a total blackout when the power simply goes off. During a brownout, devices continue to receive power but at a reduced level. This can cause some devices to malfunction.

Uninterruptible power supplies can handle the reduction in voltage due to its input voltage window. When the voltage drops outside this window, the UPS’s batteries kick-in and take over without any disturbance to the load.

Learn more about the most common power problems such as brownouts, sags, and surges with our ‘UPS Basics’ video:


A three-phase (3-phase) UPS can deliver more electrical power than a single-phase (1-phase) supply because it uses the full three phases generated from the grid. Three-phase UPS tend to be used in industrial and business settings, whereas single-phase UPS systems are used for domestic appliances or equipment with lower power requirements.

Single-Phase (1-Phase) UPS Overview

Single-phase UPS have a single input and output source to the electrical equipment. With just one sinewave voltage, it only requires two wires to complete the circuit, one conductor and one neutral. 

Single-phase uninterruptible power supplies typically cover requirements up to 20 kVA and are used for smaller installations such as rack-mounted servers, telecoms or computer systems, and network switches, along with any device that runs directly from a standard three-pin plug.  

Three-Phase (3-Phase) UPS Overview

Three-phase UPS use three separate conductors providing three sinewaves, each out of phase and spaced 120° apart from each other, to provide continuous power to the load. This means a three-phase system needs a minimum of four wires (three conductors plus one neutral), which enables it to support a single-phase or three-phase output.  

Three-phase UPS are the standard choice for larger installations with critical loads such as data centres, industrial applications, and medical environments, as well as protecting equipment with motors such as lifts, pumps, and fans.  

Single-phase may also be referred to as 1 phase or 1-phase, while three-phase is known as 3 phase or 3-phase. 

For UPS systems, it is common to refer to a single-phase UPS by just its kVA/kW rating i.e. 10 kVA. With three-phase systems, the kVW/kW rating is accompanied by the number of output phases i.e. 20 kVA (3:1) or 200 kVA (3:3) 

Key Differences Between Single-Phase And Three-Phase UPS: 

  • Number of conductors (one versus three) 
  • Number of sinewaves (one versus three) 
  • Single-phase voltage is 230V, three-phase voltage is 415V 
  • Single-phase connection is less complicated than a three-phase UPS 
  • Three-phase offers higher efficiency 

Online UPS are the preferred choice for mission-critical applications as they ensure ‘no-break switchover’ in the event of any mains power supply failure and an automatic internal bypass for safe failure to mains in the event of a fault.

EN/IEC 62040-3 defines three generic Static UPS topologies. Each topology is then further classified based on its output waveform and dynamic performance:

  • VFI – Voltage and Frequency Independent: more generally known as ‘Online UPS‘ or ‘Double Conversion’, where the UPS output is completely independent of any voltage and/or frequency abnormalities on the incoming mains supply. The highest classification for an Online UPS is VFI-SS-111. There are two main approaches to Online UPS – transformer-based or transformerless. It is known as ‘Double Conversion’ because of its two voltage conversion stages: rectifier (AC to DC) and inverter (DC to AC).
  • VI – Voltage Independent: typically referred to as ‘Line Interactive UPS‘, where voltage fluctuations are stabilised by built-in regulation devices. Output frequency tracks the input frequency when the mains supply is present. 
  • VFD – Voltage and Frequency Dependent: more commonly referred to as Offline UPS‘ or ‘Standby UPS, where the output of the UPS output tracks the voltage and frequency of the input mains supply. When the mains supply fails, there’s a brief (4-8msec) break in supply while the inverter switches on and powers the load from the batteries.

What Power Problems Do The Different UPS Topologies Protect Against?

Power ProblemOnline (VFI)Line Interactive (VI)Offline (VFD)
Electrical NoiseYesYesYes
Frequency VariationYes
Mains FailureYesYesYes

Parallel UPS systems see several uninterruptible power supplies installed together to either increase reliability (redundant systems) or to increase load handling capability (capacity systems).

In a typical parallel redundant UPS system, the total load demand is met by operating two or more UPS in an N+X configuration.

Each unit is sized to support the full load in the event of a system failure. The UPSs share the load between them equally and if one UPS fails, there’s a seamless transfer and the remaining UPSs continue to share the load.

An alternative solution is known as cascade redundancy. This sees the output of one UPS fed into the bypass line of the second unit. The first unit feeds the load, with the second constantly ready in a “hot-standby” position.

Under mains failure, the first unit supports the load until the batteries are exhausted. Then the load is transferred to the second unit via the static bypass.

Likewise, if there’s a system failure in the first UPS, the second seamlessly takes over the load. In the unlikely and rare event that both UPSs experience a malfunction, the load is transferred to bypass to maintain power.

Cascade redundancy provides very high reliability and maximum redundancy, as in a two UPS system there are a total of four power sources available to support the critical load: the AC mains supply; the batteries in the first UPS; the batteries in the second UPS; and the bypass source.

An important point to bear in mind with cascade redundant systems is that each UPS must be correctly sized to handle the entire load.

Offline UPS – also referred to as VFD (Voltage and Frequency Dependent) or Standby UPS – offer the most basic level of power protection.

When mains supply is present, the UPS output is supplied via a built-in EMI/RFI filter which provides the load with protection from spikes and transients by clamping peak voltage to pre-defined levels.

When the mains supply fails or fluctuates outside of the UPS’s operating window, a relay connects the load to the inverter output (resulting in a 4-8ms transfer time). In normal operation, with mains supply present, both output voltage and frequency will track the input voltage and frequency respectively.

As the inverter is switched off when the UPS is operating normally, the term ‘Offline’ is given to any UPS of this design. The inverter output on Offline UPS is typically a square-wave.

Offline UPS Line Diagram

Offline UPS are the most basic models and designed for use in small, non-critical applications that require protection against momentary loss of power. They are used to protect workstations, terminals, or equipment below 1 kVA.

Typical internal battery autonomy with Offline UPS lasts for just a few minutes and there doesn’t tend to be the option to add external battery packs for additional autonomy.

It is not advised to use a VFD UPS to protect critical loads or sensitive electrical equipment.

What Power Problems Do The Different UPS Topologies Protect Against?

The table below offers an easy comparison of the protection provided by Offline UPS, Line Interactive UPS, and double-conversion Online UPS

Power ProblemOnline (VFI)Line Interactive (VI)Offline (VFD)
Electrical NoiseYesYesYes
Frequency VariationYes
Mains FailureYesYesYes

A supercapacitor is a high power density energy storage device that can be used in smaller UPS systems (up to 30 kVA) instead of the usual batteries to protect against momentary mains power supply failures.  

Supercapacitors differ from an ordinary electrolytic capacitor in two main ways: their plates are much bigger, while the distance between them is smaller because the separator works differently to a conventional dielectric. This means supercapacitors can store 10-10,000 times more energy per unit volume than a standard capacitor. 

Compared to traditional sealed lead-acid UPS batteries, which have a higher energy density, a supercapacitor’s power density can be up to 100 times greater, so it can release energy far quicker.  

This means supercapacitors are perfect for critical power applications where loads might be sensitive to small breaks in mains electricity (i.e. from 1 second to 1 minute). For example, they can provide sufficient backup for a standby generator to start-up. 

Supercapacitors have a typical service life of up to 10 years and can operate at a wide temperature range of -30°C to +45°C.  

They currently remain an emerging technology so are more expensive than a standard UPS battery set. However, their longer lifespans and high number of cycles offer long-term savings if you take into account reduced battery monitoring, maintenance, replacement and removal costs.

Line Interactive UPS systems – also referred to as VI – Voltage Independent – operate similarly to an Offline UPS, with the addition of a built-in Automatic Voltage Stabiliser (AVS).

The AVS ensures the output voltage remains within a pre-defined voltage window regardless of any voltage variations on the mains input supply. This enables Line Interactive UPS to provide protection against power sags, surges, and brownouts

An EMI/RFI filter may also be used to protect the AVS and load from spikes and transients by clamping their peak voltages to more acceptable levels.

When the mains power supply fails or fluctuates outside of the pre-set window, the load is transferred via a relay (introducing a 4-8ms transfer time) to the inverter output. 

Inverters within Line Interactive UPS can vary in design and performance, and ultimately output waveform. Most Line Interactive UPS will provide a modified sinewave (more commonly known as a ‘step-wave’) whereas more premium designs will provide a sinewave output.

Line Interative UPS Line Diagram

Line Interactive UPS are typically used in smaller, less critical applications, such as PCs, telephone systems, non-critical networking equipment and small motor loads.

Typically a line-interactive UPS is only available up to 3kVA – however, larger Online UPS will have a specific ‘Line Interactive’ operating mode, which allows an Online UPS to mirror the operation of a Line Interactive model.

What Power Problems Do The Different UPS Topologies Protect Against?

Power ProblemOnline (VFI)Line Interactive (VI)Offline (VFD)
Electrical NoiseYesYesYes
Frequency VariationYes
Mains FailureYesYesYes

Also known as an Automatic Voltage Regulator (AVR) or Voltage Regulator (VR), an Automatic Voltage Stabiliser (AVS) stabilises the mains power supply voltage to a load. 

It is a feature of Line Interactive uninterruptible power supplies and provides protection from power problems such as sags, brownouts and surges.  

Automatic Voltage Stabilisers have a wide input voltage window (+20/-40%). If the input supply voltage is too low, the AVS uses a transformer to boost (step-up) the output voltage. On the other hand, if the input supply voltage is too high, the AVS reduces the voltage to a safe operating range (this process is known as buck or step-down). 

Output voltage tends to track the input voltage window as there’s no need for any voltage regulation.  

Some AVS incorporate a filter that provide both the load and the stabiliser itself with limited protection from spikes, transients and electrical noise.  

There are four main components in any online double conversion uninterruptible power supply (UPS) system: Rectifier; UPS Batteries; Inverter; and Static Bypass Switch.  


The rectifier carries out several key functions. The first is to convert the input power from AC (Alternating Current) to DC (Direct Current). Its second main role is to recharge the batteries, while the DC power routes to the inverter too. 

Depending on the size of the UPS, the rectifier module may incorporate the battery charger. With smaller uninterruptible power supply systems (i.e. below 3 kVA) it is not uncommon for the rectifier and battery charger to be separate components.  

UPS rectifiers can accept wide input voltage fluctuations, meaning the system can handle overloads or surges without having to engage the batteries.  

UPS Batteries 

The batteries in a UPS system provide emergency power when the mains supply fails. Either the rectifier or a separate charger ensures that the batteries are always charged.  

UPS battery systems have at least one string of batteries, with the number of batteries required depending on the DC voltage of the UPS. Batteries within a string are connected in series, so if a single battery fails, so too does the entire string.  

For smaller UPS systems, the batteries are often internal to the unit. Whereas in larger solutions, UPS batteries are often housed in their own standalone cabinets.


This component fulfils the second half of the double conversion by switching the DC voltage from the rectifier or battery back to an AC output that powers the critical load. 

This conversion process (AC to DC to AC) and filtering smooths out events such as spikes, sags, surges, and electrical noise, ensuring the final output is a pure sine waveform.  

Static Bypass Switch 

This component is a safeguard in case there’s a failure within the UPS system. In the event of a UPS fault, the static bypass switch automatically connects the load to the mains supply, bypassing the rectifier, batteries, and inverter.  

Having to transfer to mains supply isn’t ideal as the power won’t be filtered or conditioned as usual in an online double-conversion UPS, but it does enable equipment to continue functioning while the UPS is repaired or replaced.  

Other UPS Components

Depending on the size and type of UPS, there are several other common components that may be included, for example, fans or capacitors

In addition, there are also components such as an External Maintenance Bypass, which enables the UPS to be removed and/or replaced without interrupting the load, Transient Volt Surge Suppressors (TVSS), and Simple Network Management Protocol (SNMP)-compliant monitoring and communications applications. 

Transformerless UPS systems were first developed in the 1990s and offered a number of benefits over traditional transformer-based systems in terms of higher efficiency, reduced size and weight, and cost savings. 

Transformerless uninterruptible power supplies are now common in data centre environments and with smaller installations. They are the typical technology for the smallest power ratings (below 10 kVA) and are available up to around 300 kVA at the higher end of the spectrum. Riello UPS’s range of transformer-free solutions includes the Sentryum, Multi Sentry, and NextEnergy series. 

Available from 10 kVA and above, transformer-based UPS are still a popular choice in industrial process environments or installations requiring galvanic isolation

A transformer is a wound component consisting of windings around a core, with iron sheet laminates that can be used to change voltage levels and provide galvanic isolation. 

How Do Transformer-Based And Transformer-Free UPS Work?

In a traditional transformer-based UPS, the power flows via the rectifier, inverter and transformer to the output, with the transformer used to step up the AC voltage levels, protect the UPS from load disruptions and provide galvanic isolation. 

Transformerless UPS operate in the same way, apart from one key difference. It uses insulated-gate bipolar transistors (IGBTs) that are capable of dealing with high voltages, eliminating the need for a step-up transformer after the inverter. This improves the energy efficiency of transformer-free uninterruptible power supplies.  

Thanks to R&D and technological improvements, the latest transformer-based UPS can achieve similar levels of efficiency as transformerless systems (95-96%), although the latter still has the efficiency edge when carrying lower loads. 

What Are The Advantages Of Transformer-Based UPS? 

There are two main benefits of a transformer-based UPS. Firstly, it is generally accepted that they are more robust – there are less points of failure. Secondly, the transformer provides galvanic isolation, a separation of the input and output supplies, which protects the load from any spikes, surges, or electrical noise. 

Transformer-based UPS are the typical technology for 100 kVA and above and the choice to achieve large kW sizes or provide redundancy.  

Main benefits of Transformer-based UPS: 

  • Galvanic isolation 
  • Independent mains power supplies 
  • Dual load protection from DC voltage 
  • Providing a higher phase-neutral inverter short circuit current than a phase-phase short circuit current 
  • Superior power protection when presented with power quality problems 
  • Greater robustness with respect to back feed protection 

What Are The Advantages Of Transformerless UPS? 

The obvious benefit of a transformerless UPS is the lack of a big, bulky, and heat-generating transformer. Transformers are expensive too, so eliminating them reduces initial capital costs. 

Main benefits of Transformeless UPS: 

  • Physical: reduced size and weight (a factor for space-restricted data centres) 
  • Operational: higher energy efficiency (particularly at lower loads), lower noise levels, and less heat 
  • Cost: lower purchase, installation, and running costs (i.e. needs less air conditioning) 

One of the main drawbacks with transformer-free UPS systems is it can’t clear and isolate internal faults as well as a transformer-based unit.  

A solution to this is installing isolation transformers to mirror the strength of a transformer-based system, but this would significantly increase cost and footprint, while also introducing additional points of failure. 

Another issue with transformerless UPS power supplies is its power strength limitations. To achieve larger kW size or redundancy, several transformer-free UPS modules need to be paralleled together – the more modules (and components), the greater the likelihood of failure. 

There are three main types of batteries used in uninterruptible power supplies: Nickel-Cadmium, Lead-Acid, and Lithium-Ion. There isn’t a single “best” UPS battery technology – the choice should be made on a case-by-case basis. 

Lead-Acid UPS Batteries 

Lead-Acid batteries have a proven track record for reliability when used in an uninterruptible power supply system. In large power applications, where weight isn’t the overriding concern, they provide the most economical choice.

This cost-effectiveness is combined with other performance qualities such as low internal impedance and high tolerance. 

Lead-Acid batteries come in two different types: 

  • Valve Regulated (VRLA) 

Also known as Sealed Lead-Acid (SLA), this is the most common type found in modern UPS systems. They typically come with a 5 or 10-year design life and are best stored in a dry, climate-controlled room at a temperature of 20-25°C.  

VRLA batteries are sealed inside a case which has a valve that vents to release gas if internal pressure gets too much, hence the term “valve regulated”.  

Because they are sealed, they can be mounted either vertically or horizontally, so are suitable for use within battery compartments, rackmount trays or external cabinets. In addition, they don’t need any direct maintenance such as being regularly topped up with water.  

There are two main types of electrolyte composition used in a VRLA UPS battery: Absorbed Glass Mat (AGM), where the electrolyte is held in a porous microfiber glass separator; and Gel, which is made from a mix of sulphuric acid and silica. 

AGM technology is the norm for UPS batteries because of its lower cost, lower internal resistance and higher charge/discharge rates.  

By comparison, Gel-filled VRLA has a higher internal resistance, which makes it less suitable for high-rate discharging common with UPS applications. It does offer advantages in terms of a wider operating temperature range (-40°C to +55°C) and extended design life.  

Cross section image of VRLA battery
  • Open Vented (VLA) 

Also known as Flooded, these batteries have plates that are flooded with electrolyte acid. They have a long design life (up to 20 years) and are typically used in large installations needing a high ampere-hour (Ah) rating.  

Because they aren’t sealed, any hydrogen generated escapes directly into the environment. This means installations using VLA batteries require more powerful ventilation systems and can pose a greater safety hazard.  

To overcome these risks, VLA batteries must be placed in a dedicated room with wash-down facilities in case of acid leaks. Because they are top-vented, they must also be kept upright, with the water levels manually topped up.  

They can’t be used in cabinets or racks, which means they aren’t suitable for office environments or data centre installations. VLA batteries are also more expensive than the VRLA alternative.  

Nickel-Cadmium UPS Batteries  

Nickel-Cadmium (NiCd) batteries used to be a popular option for telecoms installations, while they are still used for UPS applications in locations with very high ambient temperatures, particularly the Middle East. 

The battery electrodes are made up of nickel hydroxide on the positive plate and cadmium hydroxide on the negative plate. 

NiCds offer the advantages of a 20-year design life, the ability to handle a wide ambient temperature range (-20°C to +40°C), a high cycle life and tolerance to deep discharges. 

On the other hand, NiCd UPS batteries are far more expensive than the more traditional VRLA variety. And because nickel and cadmium are toxic materials, this makes the disposal and recycling processes at the end of service life prohibitively expensive.

This is particularly true in countries with strict environmental policies and regulations, such as the UK. 

Lithium-Ion UPS Batteries 

Lithium-Ion (Li-Ion) batteries have long been used in electronic devices such as laptops and smartphones, while they are now core elements in the growth of electric vehicles. But recently they are becoming an increasingly viable option for uninterruptible power supplies and other energy storage systems, such as harnessing the power from renewable energy technologies like wind or solar, too.  

The advantages of Li-Ion include higher reliability than traditional VRLA/SLA batteries because of built-in battery monitoring and management systems, which check every individual cell for any change in performance.  

Another benefit of Lithium-Ion UPS batteries is that they are significantly smaller and lighter because of their significantly higher power density. They also have faster charge times, longer cycles, and at least double the service life compared to VRLA/SLA.  

Despite the cost of Li-Ion UPS batteries decreasing over recent years, it is still a far more expensive initial choice than the other options.  

However, the longer service life does balance out the higher upfront capital costs. Li-Ion generates less heat and can operate at higher temperatures, meaning they don’t require as much air conditioning, which can reduce cooling costs.  

Several Riello UPS products offer Li-Ion battery compatibility, including the Multi Power and NextEnergy

UPS Maintenance

Capacitors in an uninterruptible power supply help to smooth, filter and store energy. A UPS includes dozens of different capacitors in both the power section and the printed circuit board level (PCB). 

Capacitors contain a pair of conducting surfaces, usually electrodes or metallic plates, enclosed in aluminium or chromium-plated cylinders ranging in size from a miniature drink can through to a tube of Pringles. A third element – the dielectric medium – separates the conducting surfaces.  

The charge a capacitor can store is measured in farads – after the famous physicist Michael Faraday – which is determined by the thinness of the dielectric layer and the surface area of the aluminium. 

What Are The Different Types Of UPS Capacitor? 

In the main power section of a UPS system, the capacitors are divided into the following categories: 

  • AC input capacitors: form part of the UPS input filter and/or the power factor correction stage. These capacitors smooth out input transients and reduce harmonic distortion 
  • AC output capacitors: form part of the UPS’s output filter. These connect to the critical load output, controlling the waveform of the UPS output voltage 
  • DC capacitors: form part of the rectification system and energy storage, smoothing out any voltage fluctuations (also known as supply voltage filtering).  

As well as batteries, capacitors are the UPS components most prone to failure. They age over time, with the electrolyte, paper and aluminium foil inside degrading over time. Factors such as excessive heat or current can speed up the rate of deterioration.  

Depending on the manufacturer rating, capacitors can deliver up to 10 years of service life with the most favourable operating conditions. However, generally accepted industry best practice recommends capacitors are proactively replaced between years 4-8 of service life to reduce the risk of serious failure. 

Proactively replacing the capacitors and fans is often referred to as a UPS Overhaul.

How Does A Failing Capacitor Affect A UPS? 

A single failure may not have too much of an impact as the remainder will be able to pick up the slack, although this places them under increased strain.  

Ultimately, capacitor failure does have a negative bearing on a UPS’s performance. Filtering ability will suffer and there will be more issues with harmonics and electrical noise. In addition, energy storage volume will decrease, and it can even damage battery strings.   

The worst-case scenario of a serious capacitor failure will trigger the UPS to bypass mode, leaving the critical load unprotected.  

Can I Reduce The Risk Of Capacitor Failure? 

The most effective way to minimise the threat is by proactive replacement ahead of their rated service life as part of a UPS Overhaul.  

In addition, ensure the UPS sticks to recommended ambient operating temperatures and humidity levels, as well as keeping air filters clean so cooling air can flow freely. 

In terms of a UPS system, a warranty provides ‘best endeavour’ protection against mechanical faults or failures for a set period of time, typically 1 or 2 years from initial purchase, while UPS maintenance contracts offer ongoing support with the additional peace of mind of guaranteed emergency response times for engineer call-outs. 

Uninterruptible power supplies are complex devices containing parts that can fail and consumables, such as fans and capacitors, that need regularly replacing. Warranties and ongoing maintenance contracts both help to reduce unnecessary downtime caused by UPS failure.  

Typically, UPS systems come with 1 or 2-year warranties, although Riello UPS offers a 5 year extended warranty as standard on all power supply systems up to and including 3 kVA.  

In the main, warranties for smaller UPS (3 kVA and below) are on a ‘Return to Base’ basis where the faulty unit is sent to the supplier and then either repaired or swapped out. For larger, hardwired UPS systems an onsite warranty is more common, with most suppliers offering a next working-day response. However, even this can be an unacceptably long period without the protection of a UPS for many mission-critical 24/7/365 sites.  

Warranties are valuable but can only ever offer ‘best endeavours’. Ongoing UPS maintenance plans provide more comprehensive cover including guaranteed emergency response times defined in either working or clock hours. These response times are clearly spelled out in a Service Level Agreement (SLA). Most providers tend to offer a choice of 12 working, 8 working or 4 clock hours.  

A typical maintenance plan can cover parts, labour and carriage costs. Most exclude battery replacement unless it is specifically requested, although some contracts will include battery labour.  

Another key service provided in the majority of UPS maintenance plans is provision for an annual (or even biannual) Preventive Maintenance Visit (PMV), where a certified service engineer will undertake a detailed inspection and test of the UPS. This enables early detection of potential issues and gives the engineer the opportunity to install the latest software and firmware updates.  

UPS maintenance plans can cover uninterruptible power supplies under or outside of warranty.  

Crash kits are handy collections of the spare parts and consumables in a UPS system that require repairing or replacing the most. These crash kits are held in secure and easily transportable flight case-style storage containers for fast access. 

UPS crash kits can either be kept onsite for the most mission-critical environments, meaning they are ready and waiting when an engineer arrives to fix a fault, or quickly despatched by courier whenever the need arises. 

Onsite crash kits are most common for sites and environments where downtime isn’t an option, such as data centres, hospitals, and industrial processing plants.  

A crash kit includes everything that a service engineer needs to quickly get a faulty UPS system back online, such as: 

  • Fans 
  • Capacitors 
  • Manufacturer-specific circuit boards (i.e. rectifier, inverter, static switch, system board) 
  • Communications cards 
  • Cables 
  • Sensors 
  • Fuses  

Preventive Maintenance Visits (PMVs) are one of the key services included in an ongoing UPS maintenance plan. It is recommended that a UPS system has a minimum of one PMV every year.  

The purpose of a PMV is to provide a detailed inspection of the unit, which helps to identify and rectify any problems. It also enables service engineers to upgrade software and perform system tweaks to optimise operational performance and efficiency. 

A PMV incorporates a visual test of the UPS, plus a physical check of all electrical connections. Many UPS maintenance companies now use sophisticated thermal imaging technology that can pick up more faults than the human eye or touch. 

Particular attention is paid to circuit breakers, contactors, fuses, cabling, transformers, printed circuit boards, capacitors, fans and communications slots. Batteries are inspected for signs of damage, corrosion or leaks, with all terminal connections checked to ensure they are at the correct torque setting.  

Engineers also carry out several mechanical tests on the functionality of the UPS, recording the system’s operating state and vital electrical measurements. 

In addition, a PMV includes a download and full review of the UPS performance history logs, as well as ensuring the unit has the latest firmware installed. 

The final steps in a Preventive Maintenance Visit for UPS will see the engineer complete the maintenance register, provide a full report of any faults found during the visit, and highlight any recommended remedial actions.  

A PMV also acts as a “health check” on UPS systems that are from third-party manufacturers or for units that are outside of warranty and want to be included on a UPS maintenance plan.  

Uninterruptible power supply fans play a crucial role in keeping electronic components such as the inverter and rectifier cool enough to operate safely. If the internal UPS fans don’t work properly, these components run in much higher temperatures, which makes them deteriorate far quicker. 

Fans are found throughout the UPS in specific places to maintain effective component cooling. In general, the bigger the UPS, the more (and bigger) fans you’ll find. 

UPS fans are mechanical by their nature and subject to wear and tear. Sealed bearings inside fans contain grease which dissipates over time, slowing the fan speed, which in turn creates additional heat and noise. Dust in the atmosphere forces the fans to work harder to keep air flowing, while component switching inside the UPS generates significant amounts of heat too. 

How Long Do UPS Fans Last For?

A UPS fan has a typical service life of 5-7 years if operating in optimum environmental conditions. However, it is advised to proactively replace fans with new before this end of service life to minimise the risk of serious failure.  

As they have a similar service lifespan, many UPS manufacturers recommend replacing the fans and capacitors at the same time. This is commonly known as a UPS overhaul and it minimises the likelihood of multiple fans in the UPS reaching their end of service life at the same time. 

Individual fan failures can lead to the UPS overheating. In the worst-case scenario, this could cause a unit to transfer to bypass mode, which leaves the critical load unprotected.  

Many Riello UPS solutions incorporate smart cooling, where the fan speed automatically adjusts according to the current load – products with this feature include Sentinel Tower, Sentinel Pro, Sentinel Rack, and NextEnergy.

Impedance testing for batteries is a non-intrusive way of preventing battery failure by identifying early signs of weakness or general deterioration in individual cells.  

An AC current is applied to each battery through probes attached to the block terminals, with the internal impedance measured and recorded in milliohms. This broadly indicates the general condition the batteries are in, without placing any undue stress on their functionality. 

Battery problems evolve over time and as they get older, their internal impedance increases until it reaches a manufacturer-defined threshold where replacement is recommended to reduce the likelihood of complete failure.  

Regular impedance testing, every 6-to-12 months for example, enables you to build up a history of internal battery condition for every cell. Comparing batteries against each other makes it easier to accurately forecast end-of-working-life predictions and calculate when proactive replacement is necessary. 

Impedance testing is non-intrusive, so can be undertaken in-situ on a live working battery system and completed in a few hours. As a comparison, another type of battery testing called battery discharge testing often requires operations to be shut down for several days at a time while the test takes place. 

The main drawback of impedance testing is that it only gives a broad indication of the batteries’ condition. For a more detailed analysis, more intrusive testing is required.

Emergency Response Times when there’s a fault or failure with an uninterruptible power supply should be clearly spelled out in the Service Level Agreement (SLA) of any ongoing UPS maintenance contract.  

The response time is triggered as soon as an issue has been reported, booked in by the service team and arrangements made for an engineer to visit the site and carry out the necessary repairs.  

Emergency response times categorised as Working Hours revolve around the working day, typically 8:30am to 5pm Monday to Friday. Any incident requiring a response outside of these core working hours, for example at weekends, will likely incur additional charges. 

In many cases, the decision for out of hours work depends on whether the UPS has maintenance bypass facilities or has been installed in a parallel-redundant configuration that can support a module being serviced without taking the entire UPS system offline.  

The majority of UPS maintenance plans offering a working hours response are categorised as 12 or 8 hours. In effect, the provision for both these sorts of coverage is a “next working day” response as a best case scenario.  

As the name suggests, Clock Hours response covers an incident at any time, not just those that occur in standard Monday-Friday office hours. Most UPS manufacturers provide emergency response on a 24/7/365 basis, with a response within 4 clock hours the norm. 

For the most mission-critical of sites, even faster response times (i.e. 1 or 2 hours) are achievable, particularly if crash kits of spare parts and common components are held onsite or at a nearby local service depot.  

With smaller UPS (i.e. below 3 kVA) a “crash kit” may actually constitute a full replacement of the unit, minus the batteries, as power supplies of this size only tend to have a couple of large fully-integrated printed circuit boards (PCBs).  

It is more cost-effective to supply in a replacement UPS chassis as a ready-made working model than to replace in the existing frame. 

Battery maintenance and testing is crucial to the continued performance of a UPS system. There are a variety of common battery tests including impedance testing and discharge testing, more commonly known as load bank testing. 

Most uninterruptible power supplies have built-in functionality that automatically tests their batteries regularly, typically every 24 hours, and will alarm if it detects a battery fault.  

Such tests place a load onto the battery set and monitor the discharge performance. However, this only offers a general indication of the overall set. It doesn’t provide an individual cell level report. 

Similarly, just measuring the float voltage across a battery set doesn’t provide a true indication of its condition. 

The best way to assess the true state of a large battery set is through external battery testing. For large battery sets, individual block testing can be more reliable.  

External battery testing should form part of a planned preventive UPS maintenance regime, although it can also be provided as a standalone service.  

Impedance Testing 

This is a non-intrusive test designed to build up a performance history of each battery cell. It is typically undertaken annually as this enables performance to be tracked over time. This makes it easier to identify any signs of deterioration or any cells with high internal impedance that might require replacing. 

Impedance testing involves applying an AC current to each battery via probes attached to the block terminals. Impedance is measured and recorded in milliohms.  

It gives a general indication of the batteries’ status without placing them under too much stress or needing to take them offline.  

Electro-Chemical Testing 

Another non-invasive check that compares the data from batteries to algorithms for common battery conditions, such as sulphation and electrolyte dry-out.  

Traditionally, these tests were carried out in laboratories to predict battery failure on satellites and space vehicles. Now tests can be conducted using portable hand-held units. 

This process uses probes on the terminals to measure the frequency response to voltage and current signals passed into the battery. These results are cross-referenced to performance data for “healthy” batteries. 

Because electro-chemical testing measures sulphation and electrolyte dry-out rather than just impedance, it is said to provide a more detailed summary of battery condition. Failing blocks can either be recharged at a higher rate to reduce sulphation or replaced completely. 

Load Bank Testing (Discharge Testing) 

This is the most comprehensive battery test and the only true examination that determines the actual capacity of the battery string.  

Load bank testing audits the batteries under both normal and peak load conditions. This demonstrates which cells hold the charge and highlights which might be approaching their end of service life. 

The Institute of Electrical and Electronics Engineers (IEEE) recommends performing discharge testing at the time of installation, then repeating the test every year. 

The main drawback of load bank testing is that the UPS batteries must be taken out of service during the test. Usually the batteries are available again within 24 hours, although in the worst-case scenario this can last for several days. 

Partial Discharge Testing

As the name suggests, this offers something of a middle ground. Partial discharge testing involves discharging the batteries up to a maximum of 80%. While this takes them out of action similar to load bank testing, they should be available again inside 8 hours.  

If there’s a fault condition that requires the UPS to run off its batteries it can do so, although they will only have 20% of their full capacity.  

UPS Battery Monitoring 

In addition to battery testing, dedicated battery monitoring systems also measure UPS battery performance. It is advised all monitoring systems incorporate the parameters outlined by the globally-recognised IEEE 1491 standard, including: 

  • String and cell float voltage 
  • String and cell change and discharge voltage 
  • AC ripple voltage 
  • AC ripple current 
  • String charge current 
  • String discharge current 
  • Ambient and cell temperature 
  • Cell internal resistance 
  • Cycles. 

There are two ways in which you can monitor your UPS remotely. One option is to install a wall mounted Remote Panel (MultiPanel) that connects to the UPS to a serial port on the UPS and offers the advantage of full digital metering (available for all UPS models – 400 m cable max and required 240 Vac UPS backed mains supply). The panel incorporate a multi function audible alarm with mute facilities and lamp test.

The alternative option is to utilise the computer network with our NetMAN network adaptors to provide full UPS status and measurement values via a web-browser with facilities to send alarms using email, SMS messaging, SNMP traps or directly to an existing BMS system. This device can also offer temperature monitoring and unattended server shutdown when combined with optional modules.

High ambient temperature is the most important factor that influences UPS battery ageing and can cause premature battery failure.

Higher temperatures mean a faster chemical reaction inside the battery, which increases water loss and corrosion.

Valve Regulated Lead-Acid (VRLA) batteries have a rated design life capacity based on an optimum operating temperature of 20-25°C.

For every 10°C constant increase in temperature above this recommendation, it is generally accepted that battery service life will halve (reduce by 50%).

See the below image for an indication how rising ambient temperature can impact service life.

Short-term fluctuations in ambient temperature have relatively little effect on UPS battery lifespan. While adjusting the float voltage according to the ambient temperature can mitigate the impact of higher temperatures, but only marginally.

Without effective control of the ambient temperature around a battery set, the likelihood of a premature battery failure increases significantly. This could result in both a costly battery replacement and system downtime.

A UPS battery failure due to over-temperature can also lead to a complete system failure that only becomes apparent when the battery set is placed under load during a mains failure.

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