Glossary

The electrical force given to any equipment which is operating on Electric Current.

The unit of electrical force defined as the force needed to carry one ampere of current against one ohm of resistence.

This is a form of power measurement called “Volt-Amps”. A VA rating is the Volts rating multiplied by the Amps (current) rating. The VA rating can be used to indicate the output capacity of a UPS or other power source or it can be used to indicate the input power requirement of a computer or other AC load. For loads, the VA rating multiplied by the Power Factor is equal to the Watts rating. The VA rating of a load must always be greater than or equal to the Watts rating because Power Factor cannot be greater than 1.

Uninterruptible Power Supply (or System).

A momentary variation, which ultimately disappears. Most power problems are transients. Usually, it is desirable to be more specific and indicate if the transient is a surge, spike, sag, blackout, noise, or other type of transient.

A Transformer is a device that can change AC voltage and/or provide ISOLATION. A transformer that changes an AC voltage is called a step-down or step-up transformer depending on whether the output voltage is lower or higher than the input voltage. When a transformer steps-down voltage, it steps-up current in precisely the same ratio, thereby conserving the product of voltage and current, which is POWER. Transformers which have an electrical connection between the input and output voltages are called auto-transformers. Transformers with no power connection between input and output are called ISOLATION transformers. In an AC power distribution system, power is transferred through large distances at very high voltages (such as 200,000 Volts) and stepped-down by transformers at distribution substations to an intermediate voltage (typically 13,800 Volts) and finally reduced to the service utilization voltage (120V or 230V) by pole mounted transformers outside the building. Pole transformers that supply building power have multiple output connections or “taps” which allow utility service technicians to select the connection that provides the correct utilization voltage even if the substation voltage is chronically high or low. Sometimes these taps must be changed when distribution system changes cause the substation voltage to be affected (for example: changes due to new construction). Accessory transformers may be purchased and are sometimes used to change the voltage available ( for example 220V) to the voltage required by equipment that was designed for another voltage (such as 120V). See also TAP CHANGING REGULATOR.

The interval of time between the instant of initiation of transfer and the instant the load power is transferred from one source to another.

A switch used to transfer a load from one power source to another.

Transfer of the load power between two sources which are SYNCHRONIZED.

Used to denote the switching of a load power from one power source to another

Three Phase is a method of distributing AC power in the range of 2 to 5 kilowatts or greater. In this method, power is distributed using 3 wires (called the DELTA style) or 4 wires where the fourth wire is the NEUTRAL wire (the WYE style). An additional safety ground wire may be included. Equipments which require a large amount of power, such as mainframe computers or large air conditioners, are designed to use three phase power due to the fact that at high power ratings, three phase circuits are more efficient than single phase circuits. Three phase equipment is often wired DELTA style with a 4 prong plug, where the fourth prong is the safety ground (green wire). At the lower power levels of typical office or networking equipment, single phase power is used.

Since all main power distribution is three phase, single phase power must be derived from three phase power. This is accomplished within an office building in one of two ways, called line-to-line or line-to-neutral connection. In the line-to-line connection, two of the three phases are used as a single phase source. The single phase voltage provided in this case is 380 to 415 VAC in most countries, or 208 VAC in North America. In the line-to-neutral connection, one of the three phases is used along with the neutral in order to make a single phase source. The single phase voltage provided in this case is 220 to 240 VAC in most countries, or 120 VAC in North America and is found at typical office outlets. Sometimes, three phase equipment is wired with a 5 prong plug. The fifth prong is a neutral wire which is needed in applications in which the equipment needs to internally derive a low voltage single phase power source (such derivation requires a neutral wire as explained above).

Three Phase is a method of distributing AC power in the range of 2 to 5 kilowatts or greater. In this method, power is distributed using 3 wires (called the DELTA style) or 4 wires where the fourth wire is the NEUTRAL wire (the WYE style). An additional safety ground wire may be included. Equipments which require a large amount of power, such as mainframe computers or large air conditioners, are designed to use three phase power due to the fact that at high power ratings, three phase circuits are more efficient than single phase circuits. Three phase equipment is often wired DELTA style with a 4 prong plug, where the fourth prong is the safety ground (green wire). At the lower power levels of typical office or networking equipment, single phase power is used. Since all main power distribution is three phase, single phase power must be derived from three phase power. This is accomplished within an office building in one of two ways, called line-to-line or line-to-neutral connection. In the line-to-line connection, two of the three phases are used as a single phase source.

The single phase voltage provided in this case is 380 to 415 VAC in most countries, or 208 VAC in North America. In the line-to-neutral connection, one of the three phases is used along with the neutral in order to make a single phase source. The single phase voltage provided in this case is 220 to 240 VAC in most countries, or 120 VAC in North America and is found at typical office outlets. Sometimes, three phase equipment is wired with a 5 prong plug. The fifth prong is a neutral wire which is needed in applications in which the equipment needs to internally derive a low voltage single phase power source (such derivation requires a neutral wire as explained above).

A Surge Suppressor is a device used to protect equipment from transient overvoltages present on AC power, data, or telephone circuits. The suppressor may operate by absorbing the surge (the shunt type suppressor) or by blocking the surge from flowing (the series type suppressor) or by a combination of the two. The shunt type suppressor has a characteristic clamping voltage which is normally chosen to be around the maximum safe operating voltage of the circuit. The performance of a surge suppressor is measured by applying a known test surge (such as one of the surges described in the IEEE 587 standard) and measuring the maximum voltage which is let-through to the device to be protected.

A Surge is an unwanted momentary transient overvoltage which may be present on an AC power circuit, data circuit, or telephone circuit. A Surge may be as brief as a few billionth of a second or as long as a few thousandths of a second (millisecond). A transient voltage is considered a surge when its peak voltage is greater than the safe operating voltage of the circuit. For AC power circuits, surges are a voltages greater than a few hundreds of volts while in data circuits a few tens of volts constitutes a surge. Electronic equipment connected to a circuit that experiences a surge may become damaged.

The instantaneous addition or removal of electrical loads to a power source.

This is a UPS configuration in which the normal power source is filtered utility AC power and the back up source is a DC/AC converter operating from battery power.

In common utility terminology, this is a system in which the normal power source utility AC power and the back up source is an AC generator. The back up source may also be a DC/AC converter operating from battery power. The maximum transfer time for such a system is 1 minute as specified by the NFPA. Some STANDBY UPS systems are designated as SPS or STANDBY POWER SYSTEMs by their manufacturers.

There are various stabilizers – manual, automatic relay controlled, Transistor controlled and servo controlled. All of them use mechanical movements of switches, relays, motors, which generate heavy spikes. These spikes are unsuitable to sophisticated devices like computers, etc. Moreover their manual movements are prone to frequent failures.

This waveform is similar to the sinewave except that the transitions from negative maximum to positive maximum are abrupt. Most off line and line interactive UPSs supply a square wave as it is simple to generate

An impulse. A spike is an instantaneous, dramatic increase in voltage. Akin to the tidal wave, a spike can enter electronic equipment through the AC, network serial or phone lines and damage or completely destroy the components.

A Surge Suppressor is a device used to protect equipment from transient overvoltages present on AC power, data, or telephone circuits. The suppressor may operate by absorbing the surge (the shunt type suppressor) or by blocking the surge from flowing (the series type suppressor) or by a combination of the two. The shunt type suppressor has a characteristic clamping voltage which is normally chosen to be around the maximum safe operating voltage of the circuit. The performance of a surge suppressor is measured by applying a known test surge (such as one of the surges described in the IEEE 587 standard) and measuring the maximum voltage which is let-through to the device to be protected.

SMPS is an inbuilt power conditioner of all computers which basically works on pulse width modulation technology. In this system according to the input voltage variations, pulses are generated to control the output voltage and maintain a constancy. SMPS, however, does not provide protection against spikes, surges and line noises. Normally the computers are built with 150 watts/ 188VA or with 200 watts/ 250 VA SMPSs. While evaluating the power consumption of a CPU, this factor is to be taken in account for providing an UPS for the Computer System.

The rate of change of frequency in HZ per second. In a UPS, slewing occurs when the frequency of the internal INVERTER within the UPS is automatically adjusted so as to bring the INVERTER and the input utility power into SYNCHRONIZATION. Such SYNCHRONIZATION is desired so that there will be no PHASE error between the INVERTER and the AC utility power in case of any TRANSFER between these sources.

With Poweron’s outstanding control circuitry, the inverter provides clean and pure sinewave power with less than 3% total harmonic distortion under linear load and less than 5% under computer load.

A Short Circuit is a situation where two wires become connected, usually by accident, and results in a system malfunction. In a data circuit, the malfunction may be a loss of signal or information. In an AC power circuit, a short may result in large, uncontrolled current flow which might cause overheating of wiring or cause overcurrent protective devices such as FUSES or CIRCUIT BREAKERS to operate. AC power short circuits often cause BLACKOUTS or SAGS in nearby circuits.

This is a standard for ASYNCHRONOUS TRANSFER between computer equipment and accessories. Data is transmitted bit by bit in a serial fashion. The RS-232 standard defines the function and use of all 25 pins of a DB-25 type connector. In the minimal configuration, 3 pins are used, namely: Ground, Transmit Data, and Receive Data. Some systems use 6 additional lines for a total of 9, including “handshaking” signals and modem control signals. Virtually no systems require more than 9 connections, and therefore a modified RS-232 standard has been developed with uses the smaller DB-9 type connector instead of the DB-25 connector. On PCs, the RS-232 ports labelled as “serial” or “asynch” and are of either 9 or 25 pin male type.

(Root-Mean-Square) The square root of the average value of the squares of all the instantaneous values of current or voltage during one-half cycle of an alternating current. For a sine wave, the RMS value is approximately equal to 0.707 times the peak value of the waveform. RMS is also called the effective value.

Radio Frequency Interference. This is electrical noise which is present in communications or computing equipment which results from some parts of the equipment or attached wiring acting as a radio antenna. Under certain conditions the amplitude of this noise may be large enough to disrupt communications or cause computing errors.

Regulation is a term used to describe the degree to which the voltage of an AC power source varies. An AC power source has poor regulation when its average voltage varies or drifts randomly or if the voltage varies when a load is applied. Some AC sources such as small generators keep their output voltages quite constant in the steady state but exhibit large transients when loads are switched on or off; sources of this type are said to have poor dynamic regulation.

An intentional repetition of a given function – usually intended to improve reliability.

A device for converting AC into DC. An AC to DC converter.

A reset of a computer that is manually initiated or occurs due to a software crash or due to a power sag or surge. A reboot causes all the current work which was in process to be lost.

An AC load is called reactive when it draws a current from the AC receptacle which is not in PHASE with the AC voltage. Motors are examples of reactive devices. Computers are not reactive loads because even though they exhibit low POWER FACTOR they draw current that is in phase with the AC source.

This is a modified square wave, where the peaks and the RMS voltage is equal to a sinewave. Quasi sinewave is supplied by most off-line UPS System.

A characteristic of many new computer power supplies. A power factor corrected supply draws low distortion current from the AC source, typically exhibits low CREST FACTOR, and has a POWER FACTOR rating of approximately 1. A non-power factor corrected supply draws highly distorted current and is sometimes referred to as a “non linear” load. The benefit of power factor corrected supplies is that they do not overheat building wiring or distort the AC source waveform. For this reason they are required in some countries where the IEC 555 standard has been enacted into law.

This is a number between 0 and 1 which represents the portion of the VA delivered to the AC load which actually delivers energy to the AC load. With some equipment such as motors or computers, AMPS flow into the equipment without being usefully converted to energy. This happens if the current is distorted (has HARMONICS) or if the current is not in PHASE with the voltage applied to the equipment. Computers draw HARMONIC currents which cause their power factor to be less than 1. Motors draw out of phase or REACTIVE currents that cause their power factor to be less than 1.

In AC power systems, load current is drawn from a voltage source. In typical systems, the AC voltage is a sine wave. Ideally, the current drawn by the loads in the system is also a sine wave. With a simple resistive load such as an incandescent light bulb, the current sine wave is always aligned in time with the voltage sine wave. With some loads, such as motors, the current is delayed and lags behind the voltage sine wave. The amount of this lag is expressed in degrees and is called a phase difference. For REACTIVE loads, the POWER FACTOR is exactly equal to the cosine of this phase difference.

Any power protection equipment takes the Input Voltage and discharges the defined or required Output for a smooth running of the system which is protected.

An On-Line UPS is one of the many types of UPS. In this type of UPS the load is supplied from a power converter that always operates and takes its input from a DC supply which consists of a battery and a large battery charger which are connected in parallel. Under normal conditions when the input AC power is present, the output power converter draws its power from the battery charger; when the input AC power fails, the output converter draws power from that battery. Most UPS systems under 5kVA described as “on-line”: actually do not have the battery connected constantly but actually have a standby DC-DC converter that engages when power fails and backs up the UPS DC bus from a low voltage battery.

OFF-LINE UPSs monitor and regulate the incoming AC power to the load. The moment the power goes below or above the set limits, it transfers the load to the inverter, which draws its power from the in built sealed maintenance free or attached lead Acid or tubular batteries. The Off-line UPSs take a time lag between 1 to 15 milliseconds to witch over from mains to invertor. Anything over and above 8 milliseconds for switch over is not safer and may result in data losses .
Off-Line UPSs are compact and mostly been desktop models which occupy lesser space. These UPSs normally supports individual systems, thereby decentralizing the power distributions and the failure of any single UPS does not affect the job of other systems. This is the reason the Off-Line series hold the major market share.

See DIFFERENTIAL MODE

The return path/wire for the current carried by the live wire in an electrical system. It is supposed to be at the same electrical potential (voltage level) as the earth in a perfect electrical system but due to the improper wiring or excess loads or imbalances in the phase voltages etc., the same may be at a different potential than the absolute earth potential.

A Metal Oxide Varistor. A varistor is a very rugged voltage clamping device capable of absorbing very large currents without damage. By itself, a MOV is capable of holding typical power line surge voltages down to a level of approximately 330V peak (on a 120V circuit). The MOV is often compared with another clamping device, known as a SILICON AVALANCHE DIODE. The MOV is commonly thought to have a slower reaction time than the avalanche diode, but this is not true. See SILICON AVALANCHE DIODE.

More technically referred to as Electro Magnetic Interference and Radio Frequency interference- the line noise causes electrical noises which disrupt the smooth sine wave expectation from the utility power.

A type of UPS that combines some of the performance benefits of on-line UPS with some of the reliability and efficiency benefits of a standby UPS. In the line interactive UPS the INVERTER is always connected to the output. The INVERTER works in parallel with conditioned input AC power to supply power to the load, and only handles the full load power when the AC input power fails. It is this “interaction” with the input AC source or “line” that gives this topology it’s name. The INVERTER normally operates at low power and provides output voltage regulation and runs backwards to charge the battery until called upon to supply the load power in an outage. The line interactive UPS is the most commonly used UPS for network power protection.

The term Line Conditioner is not used consistently and therefore its meaning has been blurred. The term is sometimes used to describe equipment that provides some type of filtering or REGULATION to an AC power source and may be any of the following devices: SURGE SUPPRESSOR, FERRORESONANT TRANSFORMER, AC filter, or TAP CHANGING REGULATOR

The Joule is a measure of the amount of energy delivered by one WATT of power in one second or 1 million watts of power in one microsecond. The Joule rating of a surge protection device is the amount of energy that it can absorb before it becomes damaged. In comparing surge protection performance, the Joule rating of a surge suppressor is less important than the let-through voltage rating. This reflects the fact that surge suppressors may protect equipment by “deflecting” surges as well as absorbing them. There is no standard for measuring the Joule rating of surge suppressors which has resulted in wildly exaggerated claims by unscrupulous vendors.

See GALVANIC ISOLATION

An electronic converter used to convert direct current (DC) into alternating current (AC). All UPS systems incorporate an inverter.

This type of noise consists of any AC voltage, noise voltage, or surge voltage that occurs between the grounding pins of AC receptacles in different parts of a building. Ideally, no such inter-system ground noise should exist since all grounding wires are connected to each other at the SERVICE DISTRIBUTION PANEL (in most countries). However, this noise exists as a result of noise injection into the grounding wires, wiring faults, or overloaded power circuits. The case and circuitry of computers are both connected to the grounding pin of the AC receptacle attachment plug. Therefore, if inter-system ground noise exists, different computers or peripheral equipment will have their circuitry connected to different voltages. This becomes a serious problem if the equipment is interconnected using RS-232, AUI, or other direct data links because the data is then not referenced to the same voltage. Errors or even hardware damage may result. Inter-system ground noise is a much bigger problem for interconnected computers than COMMON MODE noise, which it is frequently mistaken/confused with.

A current higher than the nominal steady state value which is drawn by a device when initially energized or activated. Computing equipment commonly draws inrush currents of 3 to 10 times the nominal operating value.

The electrical force given to any equipment which is operating on Electric Current.

The amount of time that a power supply can continue to supply the load after input power is terminated. The duration of a BLACKOUT or TRANSFER TIME that a power supply can accept without any disturbance of the output. Holdup time is to be a minimum of 8 ms for business and computer equipment. The typical value specified for commercial computer power supplies is 25ms. Holdup time is increased when a power supply is lightly loaded.

In an AC power system, DISTORTION of voltage or current waveforms may be expressed as a series of harmonics. Harmonics are voltage or current signals that are not at the desired 50 or 60 Hz fundamental frequency, but rather at some multiple frequencies. For example, the fifth harmonic of 60 Hz is 300Hz. It is a characteristic of AC signals that any distortion will have components only at integer multiples of the fundamental frequency. In AC power distribution, these distortion components only occur at odd multiples of the fundamental frequency. The third harmonic voltage distortion at a typical wall outlet is about 3%. Harmonic voltages have virtually no effect on modern computers, but can cause overheating in some equipment.

The method by which computers or other devices communicate their status. In the case of RS-232 communications, handshaking signals are used by one of the communicating devices to inform the other that it is ready to accept data transmission. Hardware handshaking is when one piece of equipment signals the other through special handshaking wires. Software handshaking is when status is communicated using special information BYTES encoded into the normal data stream. Some UPS monitoring systems which use simple signalling (such as Lan Manager) communicate UPS status using RS-232 hardware handshaking lines which are then monitored by software in the computer.

Common wiring conditions where a ground current may take more than one path to return to the grounding electrode at the SERVICE PANEL. AC powered computers are connected to each other through the ground wire in common building wiring. Computers may also be connected by data communications cables. Computers are therefore frequently connected to each other through more than one path. When a multi-path connection between computer circuits exists, the resulting arrangement is known as a “ground loop”. Whenever a ground loop exists, there is a potential for damage from INTER SYSTEM GROUND NOISE.

Electrical ground in an AC power system is a wire which is actually connected to the earth, hence the name “ground”. The reason for such connection stems from the need to protect users of electrical equipment from shock hazards. Power is delivered to the utilization site using a pole mounted or other type of TRANSFORMER. The output of such a transformer consists essentially of two lead wires, with the utilization voltage available between the leads. For a variety of complicated reasons involving safety, one of these transformer lead wires is connected to the earth using a copper bar driven into the ground. From this ground connection, TWO wires are taken to the power utilization point. One of these wires is called the “safety ground” or “green” wire and the other is called the “neutral” wire. The ungrounded lead from the transformer is also taken to the utilization point and is called the “live” wire. These three wires together (live, neutral, and safety ground) make up the connections found on a typical office power receptacle. The safety ground wire appears to be redundant since the neutral wire is derived from the same point. In fact the safety ground wire is not needed as evidenced by the large number of electrical appliances which only use only two (live and neutral) prongs. In electrical equipment which has a safety ground connection (as evidenced by a three prong plug), the safety ground is always connected to any exposed metal parts of the equipment. The purpose of this connection is to prevent any exposed part of the equipment to become energized with a hazardous voltage in case of a wiring fault inside the equipment. If a fault were to occur, such as an accidental connection between the live wire and the case, then the safety ground connection causes the live connection to be directly connected to earth, which would result in the activation of protective devices such as CIRCUIT BREAKERs, shut the power.

A characteristic of a UPS or TRANSFORMER in which the output is completely electrically disconnected from the input. Power is coupled from input to output by magnetic fields in a TRANSFORMER within the UPS. A galvanically isolated output is considered to be a SEPARATELY DERIVED SOURCE and is required to be grounded, that is, the output grounding wire must be directly bonded to the input grounding wire. It is commonly but falsely believed that galvanic isolation eliminates GROUND LOOPs.

A protective device that interrupts the flow of current when the current exceeds a specified value. Fuses are calibrated when manufactured to a specific overcurrent value. Building or equipment wiring may overheat and become a fire hazard if excessive current is passed through such wiring. CIRCUIT BREAKERS or fuses are installed and coordinated with wiring by selecting the appropriate trip value so that if equipment malfunction or user error causes too much current to flow through a wire, the fuse will blow to prevent the wire from overheating. Unlike a circuit breakers, a fuse cannot be reset and must be replaced after it blows.

The battery charging technique for which sealed lead acid batteries are designed. A float charger maintains a voltage on the battery known as the “float voltage”. The float voltage is the ideal maintenance voltage for the battery which maximizes battery life. When the float voltage is applied to a battery a current known as the “float current” flows into the battery, exactly cancelling the batteries’ own internal self-discharge current. Sealed lead acid batteries require float charging at least occasionally or they will become permanently degraded by a process called “sulfation”. Maximum lifetime is obtained when a sealed lead acid battery is permanently float charged.

A special transformer which puts out regulated AC voltage even when the input voltage is variable. A ferroresonant transformer may be used by itself to correct BROWNOUTS or it may be built into a UPS. A ferroresonant transformer has an undesirable characteristic called “high output impedance” which can prevent protective devices such as circuit breakers on equipment plugged into it from functioning, resulting is possible safety hazard. Another problem is that computer loads applied to ferro based line conditioners or UPS systems cause the voltage waveform applied to the computer to be very distorted, which may result in undervoltage conditions within the computer. Ferro based UPS systems are becoming obsolete because they can become unstable and oscillate when supplying modern POWER FACTOR CORRECTED power supplies.

The current that can flow in a circuit as a result of an undesired short circuit.

A popular networking standard used to physically connect nodes on a LAN (Local Area Network). Often referred to as a contention-based topology because workstations are in competition for channel bandwidth. For this reason, ETHERNET uses a cable-sharing process called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Within each main branch of the network, Ethernet can consist of up to 1,024 workstations.

Electro-Magnetic Interference. EMI usually refers to unwanted electrical noise present on a power line. This noise may “leak” from the power lines and affect equipment that is not even connected to the power line. Such “leakage” is called a magnetic field. Magnetic fields are formed when unwanted noise voltages give rise to noise currents. Such noise signals may adversely affect electronic equipment and cause intermittent data problems. EMI protection is provided by noise filters placed on the AC power line. The filter reduces the noise voltage on the protected line, and by doing so also eliminates the magnetic fields of noise generated by the protected line. Noise signals that act over a significant distance are called RFI (Radio Frequency Interference). Equipment power cords and building wiring often act as antennas to receive RFI and convert it to EMI.

A ratio of output power to input power expressed in percent. If a UPS has a specific efficiency, such as 80%, then the difference between the efficiency rating and 100%, which in the example case would be 20%, represents the fraction of the input power wasted as heat. UPS efficiency of less than 100% causes increased energy use. It is possible for the lifetime energy expense of a UPS to greatly exceed the initial purchase cost, so efficiency should be carefully evaluated in making life-cycle cost comparisons.

When used in relation to AC power distribution, this refers to deviations between the actual AC voltage waveform delivered to the user and the ideal sine wave of voltage. Total DISTORTION is usually expressed as a percentage of desired sine wave, for example, a square wave has approximately 33% distortion. DISTORTION in AC power systems can also be resolved into a series of HARMONICS. In this case percentages for each harmonic, such as the third, fifth, seventh, etc. are provided. The square root of the sum of the squares of the individual HARMONICS is equal to the total DISTORTION.

For AC power systems, the term DIFFERENTIAL MODE may refer to either noise or surge voltage disturbances. The terms NORMAL MODE and DIFFERENTIAL MODE are interchangeable. Differential mode disturbances are those that occur between the power live and the neutral conductor. Most differential mode disturbances result from load switching within a building, with motor type loads being the biggest contributor. Surge voltages that come from outside of the building, such as surges caused by lightning, enter the building on the live wire and are therefore primarily DIFFERENTIAL MODE in nature since the neutral wire is nominally at ground voltage. Surge suppressors sometimes divert DIFFERENTIAL MODE noise and surges into the neutral wire, resulting in voltages on the neutral wire called COMMON MODE noise or surge voltages.

A wiring system for distributing and utilizing THREE PHASE electrical power. In this system, three power carrying conductors are used, possibly with a fourth safety ground wire. The voltage between any two of the three power wires is the rated distribution voltage, which is most commonly 380 to 415 VAC in most countries. The other type of three phase power distribution is called the WYE style.

db stands for decibels, which is a way to represent logarithmic ratios. Since a db number is a ratio, it must always be a ratio between two things. The db number is equal to 20 times the log of the ratio between two numbers. A ratio of 10 is 20db, a ratio of 100 is 40db, a ratio of 1000 is 60 db, etc. For example, if a filter has -40db of noise reduction, that means that the ratio between the output noise and the input noise is 40db or 100. The letters “db” are always used in lower case.

The function of a circuit or system that maintains a current within its prescribed limits. A CIRCUIT BREAKER terminates current flow when current exceeds the trip limit. Most UPS systems have an electrical subcycle current limit that regulates the output current to a value within the UPS design limits. This subcycle current limit may activate when a load demanding high inrush current (like a computer) is switched on. The activation of the subcycle current limit protects the UPS from damage but allows the output voltage to become distorted or even collapse momentarily. Most on-line UPS systems will have the subcycle current limit activated by computer load switching and use a BYPASS in order to maintain load continuity when the current limit activates. STANDBY and LINE-INTERACTIVE UPS systems can draw on the utility grid directly to supply load switching current transients and therefore do not activate the subcycle current limit or need to use the automatic bypass feature.

The crest factor is the ratio of the crest (peak, maximum) value of a current to the root-mean-square (RMS) value. A square wave of current has a crest factor of 1. A sine wave has a crest factor of 1.414. The current drawn by a typical computer power supply when powered from a typical wall outlet has a crest factor of 4. The crest factor in this case results from a complex interaction between the power supply and the utility power sine wave. The crest factor of a computer power supply is usually reduced when it is operated from a UPS. The reduction in crest factor when operating from a UPS does not adversely affect a computer power supply, and in fact actually makes it run cooler. Crest factor is always a property of the interaction between a load and a source, so it is meaningless to attribute to either a load or source independently. Factors which generally affect the ability of a UPS to supply high crest factors are: output impedance at harmonic frequencies, output distortion, and current limit. Although a high crest factor rating of a UPS has been considered to be a measure of UPS output stability and quality, differences in measurement techniques makes product comparisons on this basis useless. A preferred method is to specify the output voltage response to a step load or output voltage distortion under load.

It is a circuit which converts AC to DC.

A Transformer is a device that can change AC voltage and/or provide ISOLATION. A transformer that changes an AC voltage is called a step-down or step-up transformer depending on whether the output voltage is lower or higher than the input voltage. When a transformer steps-down voltage, it steps-up current in precisely the same ratio, thereby conserving the product of voltage and current, which is POWER. Transformers which have an electrical connection between the input and output voltages are called auto-transformers. Transformers with no power connection between input and output are called ISOLATION transformers. In an AC power distribution system, power is transferred through large distances at very high voltages (such as 200,000 Volts) and stepped-down by transformers at distribution substations to an intermediate voltage (typically 13,800 Volts) and finally reduced to the service utilization voltage (120V or 230V) by pole mounted transformers outside the building. Pole transformers that supply building power have multiple output connections or “taps” which allow utility service technicians to select the connection that provides the correct utilization voltage even if the substation voltage is chronically high or low. Sometimes these taps must be changed when distribution system changes cause the substation voltage to be affected (for example: changes due to new construction). Accessory transformers may be purchased and are sometimes used to change the voltage available (for example 220V) to the voltage required by equipment that was designed for another voltage (such as 120V). See also TAP CHANGING REGULATOR.

For AC power systems, the term COMMON MODE may refer to either noise or surge voltage disturbances. Common mode disturbances are those that occur between the power neutral and the grounding conductor. Ideally, no common mode disturbances should exist since the neutral and grounding wires are always connected at the SERVICE DISTRIBUTION PANEL in most countries. However, unwanted common mode disturbances exist as a result of noise injection into the neutral or grounding wires, wiring faults, or overloaded power circuits. Modern computers are quite immune from COMMON MODE noise. COMMON MODE noise is frequently mistakenly confused with INTER-SYSTEM GROUND NOISE, a distinct problem which frequently causes computer damage and data errors.

A protective device that interrupts the flow of current when the current exceeds a specified value. Circuit breakers are calibrated when manufactured to a specific overcurrent value. Building or equipment wiring may overheat and become a fire hazard if excessive current is passed through such wiring. Circuit breakers or fuses are installed and coordinated with wiring by selecting the appropriate trip value so that if equipment malfunction or user error causes too much current to flow through a wire, the circuit breaker will trip to prevent the wire from overheating.

A frame of eight continuous BITS, representing the smallest addressable item of information in a computer system.

An AC power path around one or more functional units of a UPS. An automatic bypass is controlled by UPS control logic and activated when some part of the UPS malfunctions or intentionally shuts down due to overload or other abnormal condition in order to maintain power to the protected load. A manual bypass is a user controlled switch on a UPS that allows a complete electrical bypass of the unit and may be engaged when there is a total UPS failure or when performing certain types of diagnostics or repair. A service bypass is a manual bypass that allows complete maintenance, or even removal, of the UPS without shutting off the load. A true service bypass is commonly a separate device from the UPS.

A condition, usually temporary, when the AC utility voltage is lower than normal value. If a brownout lasts less than about a second, then it is called a SAG. Brownouts are sometimes caused by overloaded circuits and are sometimes caused intentionally by the AC utility company in order to reduce the power drawn by users during peak demand periods. Studies have shown that brownouts of all durations make up the vast majority of power problems that affect computers.

Total loss of utility power, bringing down the operation to a standstill.

A binary digit which represents the smallest unit of computer information and indicates one of the two electrical states in a computer: off (0) or on (1).

An umbrella term used by various UPS manufacturers to describe a suite of functions related to charging, testing, and maximizing the life of a UPS battery. Battery management may include imminent battery failure diagnosis and indication , scheduled battery testing , hot swappable user replacable batteries, high speed battery charging, output regulation to reduce unnecessary battery usage, and/or special battery charging techniques.

An energy storage system which is used in a UPS. The battery used in a UPS is rechargeable. Almost all UPS batteries under 20kVA are of the “sealed, suspended electrolyte lead-calcium” type. The electrolyte in these batteries is suspended in a felt-like fabric, making the batteries spillproof, leak-proof, and usable in any orientation. The UPS converts the DC power from the battery to AC power for use by the load.

The mathematical product of the RMS current and the RMS voltage. Identical to the VA rating.

The AMPERE is a measure of current which represents the flow of electrons through a wire. In an AC power system, current (AMPS) is delivered to a load through a wire called the “hot” wire and returns through a wire called the “neutral” wire.

Alternating Current. Used to indicate that voltage or current in a circuit is alternating in polarity at some frequency as in “230 Volts AC”. In an AC power distribution system, the frequency of alternation is 50 cycles per second (Hertz or Hz) in most countries except that it is 60 Hz in North America. Typically the alternating waveform is a sine wave, but it may be another kind of wave such as a stepped approximation to a sine wave or a square wave. Batteries generate DC or Direct Current. For power distribution, a major advantage of AC transmission when compared with DC transmission is that AC power can be stepped up or down in voltage by means of TRANSFORMERs which do not work for DC. Another advantage of AC power when compared with DC power is that during every cycle the voltage, current, and power go momentarily through zero when the current is reversing, which extinguishes arcs in devices such as switches, fuses, relays, and circuit breakers; such arcs do not extinguish easily and can be a fire hazard in DC power systems.