UPS Technical Glossary

If you’re trying to specify a UPS system, you’ll find yourself confronted, as with any technology, by a fair amount of terminology – which could be confusing if you’re not familiar with it. Our UPS Technical Glossary below is intended to clear away any such confusion.


AC Ripple

AC Ripple refers to an unwelcome AC component that can be superimposed on the DC output from a UPS rectifier/charger circuit. AC ripple should be prevented from reaching the battery because it causes internal heating and deterioration of the battery poles.


AMF automatic Mains Failure Detection is a useful function for standby power generators. A generator fitted with AMF can detect and respond to a mains failure by automatically starting up and becoming a ‘mains replacement’ for the UPS.

Automatic Battery Test

Automatic Battery Test is a scheduled test designed to identify any battery weakness before it can cause an online battery failure and system crash. It comprises short simulated and actual discharges of the battery and alarm generation if the battery voltage falls below a preset level during the test.


Autonomy also known as back up or discharge time, battery autonomy is a measure of the time for which the battery will support the critical load during a mains failure. Autonomy is a function of battery charge state, capacity and load size.

Availability (A)

Availability (A) is a useful measure of downtime per year for systems subject to failure and repair. It is defined as the probability of a system being operational at any given time during its working life;

  • A = MTBF/(MTBF + MTTR)
  • Where MTBF = Mean Time Between Failures and MTTR = Mean Time To Repair


Belly tank

Belly tank is a space efficient way to store fuel for a generator


Biodiesel is the common name for Fatty Acid Methyl Ester (FAME) fuel made from sources such as vegetable oil. Whilst suitable for motive applications where it will be quickly used its tendency to degrade after around 6 months of storage and form deposits that can block supply pipes and filters make it unsuitable for standby power use except where it is added in a small concentration to conventional fossil-fuel diesel.


Blackout a total loss of electrical power

Blade server friendly

Blade server friendly blade servers are increasingly popular in data centres due to their high processing density; however they impose a leading power factor on their power supply. Legacy transformer based UPS derate significantly with a leading power load, whereas modern transformerless systems can handle it more efficiently. They are, therefore, referred to as ‘blade server friendly’.

Examples include the KOHLER PW 9250DPA and KOHLER PW 6000


BMS UPS systems are often required to integrate with operators’ BMS or Building Management Systems so that they can be supervised and managed as part of an overall building management strategy.


Brownout a condition in which supply power remains at a low voltage for an extended period of time.


Bypass a power path around a UPS system. An automatic bypass is used by the UPS to switch its load to the mains if it experiences an overload or internal failure. A manual, maintenance or service bypass allows an engineer to isolate, maintain or remove the UPS without interrupting power to the load.


Capacity System

Capacity System is a parallel system where the total capacity of the UPS modules is enough to fully support the load, but with no redundant provision. The failure of any one module will therefore cause severe overloading on the rest. Alternatively, the load may be switched to raw mains.

CPA (Centralised Parallel Architecture)

CPA (Centralised Parallel Architecture) using this system, UPS modules supply their load through a central static switch (CSS) which represents a single point of failure.

Centralised power supply systems of emergency lighting

Centralised power supply systems of emergency lighting utilise a centralised battery and static inverter, easing maintenance and allowing higher light levels to be achieved.

Circuit Breaker

Circuit Breaker a protective device that interrupts the flow of current when it exceeds a specified value.

Coefficient of capacity; Coefficient of redundancy

Coefficient of capacity; Coefficient of redundancy in an N+n parallel redundant system, N is the coefficient of capacity and n is the coefficient of redundancy. N represents the number of modules needed to meet the critical load and n is the number of extra, redundant modules.

Competent engineer

Competent engineer is someone who has the experience, knowledge and training, appropriate tools and information from suppliers to support a Responsible Person with emergency lighting regulatory compliance.

Crest factor

Crest factor the ratio between the crest (peak or maximum) value and the root-mean-square (RMS) value of an alternating current. Some computer power supplies draw current with a crest factor of between two and three.

Current Limit

Current Limit the function of a circuit or system that maintains a current within its prescribed limits. UPS systems have an electrical current limit that regulates the output current to a value within the UPS limits. Current limiting may occur when a load demanding high inrush current is turned on.


DALI (Digital Addressable Lighting Interface)

DALI (Digital Addressable Lighting Interface), is a 2-way communications protocol used to provide control over and communications between component in a lighting system.

DPA (Decentralised Parallel Architecture)

DPA (Decentralised Parallel Architecture) all of the UPS modules in a Decentralised Parallel Architecture UPS feed the critical load directly. The system’s static switch is the sum of the individual UPS modules’ static switches and the system’s maintenance bypass is typically integrated into the system’s switchgear.

Products with decentralised parallel architecture:





Defined Locations

Defined Locations in emergency lighting are those which according to BS 5266-1 6.6.3 have specific emergency lighting requirements. These include lifts, escalators, toilets, plant rooms and car parks.


Dip a transient voltage decrease – also called a sag.

Double Conversion

Double Conversion or On-line UPS system receives mains AC power, rectifies it into DC for conditioning and battery charging, and then inverts it into clean AC to supply the critical load. In the event of mains overvoltage or failure the UPS continues to supply the load from its battery with no transfer delay. Provided the mains power disturbance duration is less than the battery autonomy, the event remains invisible to the load.

Double Conversion Efficiency

Double Conversion Efficiency the double conversion efficiency figure for a UPS is obtained by comparing the output power to the load with the input mains power applied to the UPS, where both figures are in kW. This efficiency improves as the load approaches the maximum UPS capacity, so efficiency can be maximised by running the UPS as near to full load as possible.


Emergency Escape Lighting

Emergency Escape Lighting is the part of emergency lighting provided to enable safe exit in the event of failure of the normal supply.

Emergency Safety Lighting

Emergency Safety Lighting gives additional protection for occupants who stay in a building during a supply failure.

EoL (End-of-Life)

EoL (End-of-Life), in a UPS battery system is the amount of power that can be provided at the end of its typical service life. This is usually 80% of the output when new.

Escape Route Lighting

Escape Route Lighting is the part of emergency lighting provided to enable safe exit for building occupants by providing appropriate visual conditions and direction finding on escape routes.

EWA (Extended Warranty Agreement)

EWA (Extended Warranty Agreement) is a service plan offered by KUP with call-out response time options adapted to the critical needs and budget of each customer.


Float charging

Float charging a battery charging scheme suitable for UPS batteries, designed to maximise battery life.

Future Proofing

Future Proofing a key design feature of the KUP and other modular UPS models is their scalable or future proof design. They can be closely sized to an initial critical load without the investment or inefficiency of over capacity. If demand grows over time then extra ‘hot swap’ modules can be plugged in without interruption of power to the critical load.


Galvanic isolation

Galvanic isolation two circuits are ‘galvanically’ isolated if their only connection is a transformer coupling. A standard transformer-based UPS design typically does not have galvanic isolation because the neutral line passes from the input via the static bypass switch to the output.


Generator a standby generator is a back-up electrical system that operates automatically. Within seconds of a utility outage an automatic transfer switch senses the power loss, commands the generator to start and then transfers the electrical load to the generator. The standby generator begins supplying power to the circuits. After utility power returns, the automatic transfer switch transfers the electrical load back to the utility and signals the standby generator to shut off. It then returns to standby mode where it awaits the next outage. To ensure a proper response to an outage, a standby generator runs weekly self-tests. Most legacy units run on diesel, natural gas or liquid propane gas but newer models can run on renewable diesel (see HVO fuel).


Harmonic Distortion

Harmonic Distortion unconditioned mains power can be distorted by high frequency components which are multiples, or ‘harmonics’ of the fundamental 50Hz frequency. This harmonic distortion can cause overheating or damage to modern equipment power supplies.

High Risk Task Area Lighting

High Risk Task Area Lighting is the part of emergency lighting provided to ensure the safety of people involved in a potentially dangerous process or situation and to enable proper shut down procedures to be carried out.

Hot swap

Hot swap within UPS installations, the term ‘hot swap’ applies to any UPS module or equipment that can be added to or removed from the UPS system with no interruption of conditioned power to the critical load.

Products with ‘hot-swappable’ modules include:






HVO (Hydrogen-treated Vegetable Oil)

HVO (Hydrogen-treated Vegetable Oil) fuel, often referred to as Renewable Diesel is a fuel made from vegetable oils such as sunflower, rapeseed or palm oil. The lowest environmental impact variant is made from used cooking oil (UCO) and has a CO2e impact up to 94% lower than fossil fuel diesel per MJ of energy produced. HVO fuels have long storage lives, eg 10 years, making them ideal for standby power use, especially as they are often considered a drop in replacement for fossil fuel diesel – this is the case for all KOHLER industrial generators produced from 2022 but users should always check for compatibility with the manufacturer before changing fuels.


Inrush current

Inrush current the current drawn by any electrical device when power is initially applied. Computer equipment typically draws an inrush current of three to ten times the nominal operating value.


Inverter part of the UPS system that converts DC power back to AC


Isolation the degree to which a device like a UPS can electrically separate its input from its output.


Line Interactive UPS

Line Interactive UPS a hybrid design that attempts to offer a higher level of performance than a conventional off-line system by adding voltage regulation features to the bypass line. Frequency variations however remain uncorrected.

Lithium-ion batteries

Lithium-ion batteries for UPS systems offer significantly reduced weight and volume compared to VRLA battery systems, and are somewhat less temperature sensitive, though they are also significantly more expensive and recycling infrastructure is presently poor for lithium-ion batteries (compared to VRLA batteries which are typically 95% recyclable via an established infrastructure).


Load any electrical device connected to a power source is a ‘load’. For a UPS, the load is the amount of current/power required by the attached electronic equipment.

Load bank test

Load bank test load bank testing uses a simulated load to test the integrity of the entire UPS system, including the battery. It is designed to reveal any weaknesses before they can cause an online failure.


Maintenance bypass

Maintenance bypass a maintenance bypass is a means of switching the load to the unprotected mains supply, while the UPS equipment is isolated and made safe for servicing or repair.

Maintained lamps

Maintained lamps in emergency lighting systems are on when the mains supply is on and remain on when the supply fails.


Modular UPS systems such as the KOHLER PW 9250DPA Series are configured by plugging up to five modules into a 19” rack to meet the capacity and redundancy requirement of the application load. This provides more flexibility and scalability than a complete cabinet containing a single standalone system.


Module a UPS module is a unit that contains all the hardware and software necessary for full system operation. Modules can be paralleled into a redundant solution with no single points of failure.


MTBF Mean Time Between Failures (MTBF) is a measure of the average time a device will function before failing. MTBF ratings are measured in hours and indicate the reliability of hardware devices such as UPS equipment.


MTTR Mean Time To Repair (MTTR) measures the total time in hours from when a device fails until when it is restored to full operation. MTTR includes fault diagnosis time, and any time necessary to obtain replacement parts, as well as the actual repair work time.


N+n redundancy

N+n redundancy describes the configuration and redundant capacity of a parallel redundant system. N represents the number of modules needed to meet the critical load and n is the number of extra, redundant modules, referred to as the coefficient of redundancy.

Noise (Electrical)

Noise (Electrical) any undesirable electrical signal.

Non-maintained lamps

Non-maintained lamps in emergency lighting systems are not illuminated when the mains supply is healthy but come on when the mains supply fails.


Off-line System

Off-line System in an off-line system, the critical load is powered from the raw mains via the bypass line during normal operation. After an unavoidable but short break, the load is transferred to the UPS inverter output if the mains fails or transgresses preset voltage limits.

On-line System

On-line System in an on-line system, the critical load is fed by conditioned power from the UPS inverter output during normal operation. If the mains power fails, the inverter continues to feed the load using DC power from the battery, with no switching interruption. If the battery autonomy is exceeded, or the UPS fails, the load can be automatically switched to an alternative mains supply using a static switch.

Open Area (or Anti-Panic Area)

Open Area (or Anti-Panic Area) lighting that part of emergency escape lighting provided to reduce the likelihood of panic and to enable safe movement of occupants towards escape routes.


Overvoltage an abnormally high voltage sustained for an extended period.


Points of emphasis

Points of emphasis in emergency lighting are specific locations that highlight potential hazards and act as beacons highlighting dangers, points of congestion and safety equipment. These include changes in level, changes in direction, first aid points, fire fighting equipment and exterior routes to safety.

Power Density

Power Density the unit of power density is Watts/m². The power density of a UPS system is found by dividing its power output in Watts by the floor area it covers, in square metres. A high power density figure is an important feature for UPS systems.

Power Factor

Power Factor is a ratio defining the relationship between useable power in Watts and total supplied power in VA (Volt Amperes). The power factor imposed by a load on a UPS system can be either lagging or leading. Traditional data centre loads have been lagging, but blade servers, which impose a leading power factor, are becoming increasingly popular because of their dense and flexible processing power. The closer the power factor is to unity, the greater is the power efficiency of the UPS operation.

Power Management Display

Power Management Display the Power Management Display, visible on the front panel of UPS equipment, allows operators local monitoring and control access to the UPS. It comprises a mimic diagram with LEDs showing the status of the UPS, control keys allowing an operator to manage the UPS, and an LCD display providing more detailed information about UPS events and status. The display also shows information during start-up, shutdown and diagnostic activities



Rectifier/Charger the part of the UPS system that converts AC input power into DC power for feeding the conditioning and inverter components, and charging the back-up battery.


Redundancy for highly critical loads, it is essential to make the UPS system resilient to the failure of any single module. This can be achieved by connecting a number of UPS modules in parallel to a common critical load bus. Redundancy exists if the number of modules actually connected exceeds the number of modules needed for the load. This means that if any one module fails, the remainder can continue to support the critical load without interruption.

Redundant Critical Circuits

Redundant Critical Circuits some UPS systems such as the KOHLER PW 6000 Series are designed with redundant critical circuits. If a redundant critical circuit fails, its partner UPS module/s can continue to support the full load without interruption. This redundancy within a unit is in addition to any system level redundancy achieved by an N+n parallel configuration of units.


Regulation describes the amount by which the voltage of an AC power source changes. A UPS has poor regulation when its average voltage varies or drifts, or if the voltage varies when a load is applied.

Remote Signalling Panel

Remote Signalling Panel this is an option for KOHLER PW UPS systems. It provides a simple way of providing UPS status information to a location up to 100m from the system, using dry relay contacts.

Renewable diesel

Renewable diesel see HVO.


Resilience the resilience of a redundant UPS system describes its ability to continue delivering uninterrupted conditioned power to its critical load even after it has suffered a module failure. Although the UPS has failed and must be repaired, the system resilience has shielded the critical load from the failure event.

RFI — Radio Frequency Interference

RFI — Radio Frequency Interference electrical noise resulting from some parts of the equipment or wiring acting as a radio antenna. The noise may be large enough to disrupt communications or cause computing errors.


Rightsizing modular systems such as the KOHLER PW 9250DPA series can be more accurately matched than traditional standalone UPS to their critical load requirement. This feature is known as ‘Rightsizing’ and is possible for two reasons. Firstly, the modules are available in relatively small capacities, so installing one module more than the load requirement will provide redundancy with minimum excess capacity. Secondly, UPS capacity can be incrementally increased to handle growing load requirements, obviating any need for initial oversizing.

Ripple free battery charging

Ripple free battery charging without adequate protection, a UPS battery can be subjected to AC ripple generated by the rectifier/charger or inverter. This will lead to internal battery temperature increase and deterioration of battery poles. Therefore, KOHLER PW UPS include built in protection from AC ripple to prolong battery life.


RS-232 a serial communications protocol. It may be used between a UPS and a computer to communicate alarm, status or control signals and instructions.



Scalability modular systems such as the KOHLER PW 9250DPA series have a power capacity that can be easily incremented — or decremented — to meet load demands that change over time. This flexibility is known as the UPS’s scalability. Because these systems are modular, capacity changes can be effected by plugging modules into or removing them from the UPS rack. This is known as vertical scalability. If the load demand grows beyond the capacity of a fully populated rack, then a second rack can be added in parallel to the first — this is known as horizontal scalability.

SCR (Selective Catalytic Reduction)

SCR (Selective Catalytic Reduction) systems spray a solution of urea, sometimes called DEF or AdBlue, into the exhaust gas of an engine to reduce emissions of nitrogen oxides (NOx). These systems are available on many KOHLER generators.


Self-Contained power supply systems of emergency lighting utilise batteries within each luminaire, easing initial installation by reducing wiring but increasing the difficulty of ongoing maintenance as each luminaire needs to be accessed individually when batteries require changing. There are also limits on the power available from the smaller batteries used in self-contained systems, potentially limited lighting output and duration.

Six Nines availability

Six Nines availability the availability of a UPS system is an important figure of merit. Availability (A) is defined as:


It describes the total time for which a system is available for operation as a proportion of the total time for which it is needed. Modern systems can achieve ‘Six Nines’ availability, which means 0.999999 or 99.9999%.


SNMP is a worldwide standard communications protocol. It stands for Simple Network Management Protocol and is used in network management systems to monitor network-attached devices for conditions that warrant administrative attention. KOHLER PW UPS systems can be given SNMP connectivity with either an internal plug in card or an external adapter. KUP‘s PowerREPORTER software can then be used to monitor and manage the UPS from a remote computer either on the enterprise network or across the Internet. A single internal or external SNMP UPS interface can manage multiple modules in a parallel redundant system.


Spike high voltage spikes are a sudden, rapid voltage peak of up to 6,000 volts with duration of 100mS to 1/2cycle. These spikes are usually the result of nearby lightning strikes, but there can be other causes as well. The effects on vulnerable electronic systems can include loss of data and burned circuit boards.

Static Bypass

Static Bypass see Bypass


Surge a power surge is when the voltage rises to 110% above the rated RMS voltage for one or more cycles. The most common cause is heavy electrical equipment being turned off. Under this condition, computer systems unprotected by a UPS may experience memory loss, data errors, flickering lights and equipment shutoff.



TCO is an abbreviation for Total Cost of Ownership. This is an important consideration because although the initial purchase price of a modular UPS system may be more than a standalone system of similar capacity, its total cost over an operational life of many years will be lower. This can be shown by comparing costs for installation, power consumption, cooling, repairs and spare part stock over the total operational period.


THDi stands for the Total Harmonic Distortion of the input current waveform. It is generally accepted that the THDi should be kept low to avoid excessive current distortion at the point of common coupling within a building due to the cumulative effect of all connected equipment.

Transformerless Design

Transformerless Design UPS are now possible, due to advances in power semiconductor technology and the introduction of the Insulated Gate Bipolar Transistor (IGBT) device. Transformerless design allows smaller, lighter and more efficient UPS implementation. Other advantages include a higher input power factor, lower THDi, reduced capital and operating costs, lower audible noise and enhanced battery life.



UPS a UPS or Uninterruptible Power Supply is best defined as a backup power supply that, in case of a power failure, allows enough time for an orderly shutdown of a computer or other electronic equipment, or for a generator to take over.  A UPS therefore protects computers and other electronic equipment from mains failures and power problems. If the mains voltage falls below a minimum level or fails entirely, the UPS battery can maintain power to the load until either the mains is restored or an orderly shutdown sequence is performed. Load protection from mains borne spikes, surges and noise is also provided.



VFI stands for Voltage and Frequency Independent, where the output of the UPS is independent of any fluctuations in the power voltage (mains) and frequency variations are maintained within the limits prescribed by ENV 61000-2-2 (CEI 110-10). An on-line double conversation UPS has VFI capability.

VRLA Battery

VRLA Battery VRLA or Valve Regulated Acid Batteries emit virtually no gas, require no topping up and need no special ventilation apart from that required by local building codes.

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