Power Glossary

November, 2003

Are you confused about all of the terms and acronyms used by power contractors and vendors?  This simplified glossary may help. (Also see Backup Power Systems)

AC: Alternating Current.

Alternating Current (AC): Current that alternates from positive to negative.  AC is the normal current used to power most devices in homes and businesses.  (Compare to Direct Current.)

Ampere (amp): A measurement of current flow, often abbreviated as amps.

Arrester: A device to suppress voltage spikes, sometimes called suppressor or protector.  (See Suppressor.)

Battery: An electrical storage device that produces direct current.

Blackout: A loss of alternating current (AC).

Brownout: A low voltage situation.  Some equipment will not work in a brownout, whereas others will, depending on the severity.  In either case, a brownout is bad for electrical equipment.  (See Undervoltage.)

Circuit Breaker: A resettable device that interrupts current flow when it exceeds a preset level, thereby preventing damage to circuit elements.  (Compare to Fuse.)

Current: The flow of electricity in a circuit; it is measured in amperes (which is often shortened to amps).

DC: Direct Current.

Dip: A short brownout, often caused by motors turning on and off, such as on large air conditioners.

Direct Current (DC): Electricity where current flows in one direction, such as from a battery.  (Compare to Alternating Current.)

Dropout Voltage: The voltage at which a device fails to operate properly or safely.  Computer systems will reboot, reset, or lose data when line voltage falls below approximately 95-100 Volts AC.  (See Brownout and Undervoltage.)

Efficiency: The ratio of output energy to input energy.  For uninterruptible power supplies (UPSs), it is the amount of energy lost in the form of heat during DC to AC inverter operation.

Filter: An electronic device that blocks certain frequencies while allowing other frequencies, such as 60 hertz (Hz), to pass.

Frequency: The number of cycles of current flow in one second.  In North America, the frequency of commercial and residential service is 60 cycles per second or 60 Hz.

Fuse: A device that breaks, thereby opening the circuit and protecting the equipment connected to it, when the current passing through it exceeds its rated value.  A fuse can be used only once, whereas a circuit breaker, which accomplishes the same thing, can be reused. (Compare to Circuit Breaker.)

Generator: A gasoline, natural gas, or propane powered engine that produces electricity.  Generators are often used with uninterruptible power supply (UPS) systems.  Since generators require a few minutes to stabilize after being started, UPSs are often used for short duration outages and generators reserved for longer power interruptions.  Some less sophisticated generators may have a problem with frequency regulation, never appearing to the UPS as a stable power source.

Ground: A connection, either intentional or inadvertent, between an electrical circuit and the earth.

Hertz (Hz): A measure of the frequency of alternating cycles in an AC current per second.  (See Frequency.)

Hot Swappable Battery: A feature that allows an uninterruptible power supply (UPS) battery to be changed for maintenance without taking the UPS and the equipment connected to it, off-line.

Hz: Abbreviation for hertz.  (See Frequency.)

Inverter: A device that converts DC power to AC power.

Isolation Transformer: A transformer inserted to reduce noise and isolate a circuit.  These are used in many larger uninterruptible power supply (UPS) systems.

kVA: An acronym for kilovolt-amps (See VA) that is a measure of apparent power; 1 kVA = 1000 VA.

kW: An acronym for kiloWatt (See Watt) that is a measure of real power; 1 kW = 1000 Watts.

Line Conditioner: A device that optimizes the quality of commercial power.  By definition, an uninterruptible power supply (UPS) system is a line conditioner, but a line conditioner isn't always a UPS.

Load: The electrical devices that use power.

Outage: A long-term loss of power.

Overvoltage: By definition, a voltage that is at least 10% above normal.  (See Spike and Surge.)

Peak Voltage: Also called peak-to-peak voltage, it is a measure of an AC waveform of the highest peak-to-peak voltage present on the waveform.  A proper 120V AC waveform will have a peak voltage of about 170 volts.  (Compare to RMS voltage.)

Response Time: The time it takes for a UPS, conditioner, suppressor, fuse, or circuit breaker to sense and react to a surge or spike.

RMS: Acronym for Root-Mean-Square, a formula used to calculate effective voltage.  The RMS value of normal AC power is 120V.  (Compare to Peak Voltage.)

Rolling Blackouts/Rolling Brownouts: A condition where power utilities purposely impose blackouts or brownouts over a portion of their service area to free up capacity so that the remaining service area can continue to be served.  After a time, another portion of the service area has a blackout or brownout imposed on it, so that power can be restored in the original area.

Sag: See Undervoltage.

Sine Wave: The shape of an ideal AC waveform.  Most high-end uninterruptible power supplies (UPSs) provide an accurate sine wave.  Some less sophistical UPSs produce a square wave, which may cause some devices to not work properly or to shorten their life span.

Spike: A high voltage condition which lasts for a short duration, usually less than 1 millisecond.  A spike can be thousands of volts.  (See Overvoltage and Surge.)

Square Wave: It is an undesirable waveform for critical computing applications, as it is harder on equipment and may cause undesired side effects.  (Compare to Sine Wave.)

Standby UPS: An uninterruptible power supply (UPS) that passes line power straight through to the output when conditions are stable, but switches to battery power when line voltage drops

Suppressor: A device that diverts voltage surges to an electrical ground.  (See Arrester.)

Surge: A voltage increase lasting for several cycles or more.  (See Overvoltage and Spike.)

Transfer Time: The time it takes to switch from AC line power to battery power.

Transformer: A device used for changing AC voltage or isolating a circuit from its power source.

Undervoltage: By definition, it is voltage at 90% or less of the normal level for more than one minute.  Sometimes it is called a brownout.

UPS (Uninterruptible Power Supply): A device that provides continuous, acceptable ("clean") power to its load, regardless if the commercial utility's power is present or of proper quality.

VA: See volt-amp.

Volt: A unit of measure for voltage, which is electrical pressure that forces current to flow.

Volt-amp: A unit of measurement of apparent power.  Most UPS systems are rated in volt-amps; the actual wattage is about 60-70% of the VAs.

Watt: A measurement of true power consumption.  Watts = VA divided by Power Factor.

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