UPS Technical Glossary


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 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) 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;


Where MTBF = Mean Time Between Failures and MTTR = Mean Time To Repair


Blackout a total loss of electrical power

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 PowerWAVE 9000DPA and PowerWAVE 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 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.

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

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

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.

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 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.


Decentralised Parallel Architecture (DPA) 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:

PowerWAVE 8000DPA

PowerWAVE 9000DPA

PowerWAVE 6000

Dip a transient voltage decrease – also called a sag.

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 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.


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


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

Future Proofing a key design feature of the PowerWAVE 9000DPA UPS is its scalable or future proof design. It can be closely sized to its initial critical load without the investment or inefficiency of over capacity. If demand grows over time then extra ‘hot swap’ modules can be plugged into the PowerWAVE 9000DPA rack without interruption of power to the critical load.


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 units run on diesel, natural gas or liquid propane gas.


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.

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:

PowerWAVE 8000DPA

PowerWAVE 9000DPA


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 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.

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 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 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.

Modular UPS systems such as the PowerWAVE 9000DPA 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 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) any undesirable electrical signal


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

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.

Overvoltage an abnormally high voltage sustained for an extended period.


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 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 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 some UPS systems such as the PowerWAVE 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 this is an option for PowerWAVE 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.

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 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 PowerWAVE 9000DPA 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 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, PowerWAVE 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 PowerWAVE 9000DPA 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.

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. PowerWAVE UPS systems can be given SNMP connectivity with either an internal plug in card or an external adapter. UPS Ltd‘s WAVEMON 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

(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 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 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.


Wavemon software can be used for remote monitoring, control and start up/shutdown of UPS systems. After installation on a networked connected computer, Wavemon communicates with the UPSs under its control via an onsite LAN or the Internet, using SNMP protocol (see also SNMP). Management of UPS modules as part of a parallel redundant system is included, to avoid problems such as initiating a system shutdown if a single redundant module fails.


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