Power Quality Glossary

I pulled this together quickly, if you feel I missed anything important email me and I will get you a definition. 

Alternating Current (AC or ac) -An electrical current that reverses direction periodically, usually many times per second. 

 Ampere (A or Amp) -The quantitative unit measurement of electrical current.  A=(W/V)

 Automatic Transfer Switch – A switch that transfers the electrical loads automatically to an alternate power source

 Autotransformer – A transformer use for stepping the voltage down or up.  The primary and secondary windings share common turns, so there is no isolation present.


 Backup time -Time during which the UPS can supply the rated load with nominal-quality power while the mains are down. This time depends on the battery and the efficiency of the UPS. Typical backup ranges from five minutes to several hours.

 Battery –A group of cells connected to deliver more voltage and/or more voltage than a single cell.

 Blackout -The total loss of AC utility power for one cycle or longer (in excess of 8.35 milliseconds).

 Boost –The input of additional power to the utility power before it reaches the attached devices

 Brownout -Normally a planned voltage reduction by utilities to counter excessive load demands on their generation/distribution system.

 Buck –The stepping down of overly high voltage utility power before it reaches attached devices

 Building Entry Ground – The main ground where the power enters the building.  This is also called the Building Common Electrode.

 Charger –Also known as a rectifier.  The charger is a device that keeps the battery ready for any power events

 Circuit Breaker – A device that automatically interrupts the power to a circuit whenever the current surpasses the rated value of the breaker.  The device is resetable.

 Critical load –Equipment that must have an uninterrupted power input to preserve data integrity and to protect important systems.

 Dip – A short-term (less than a few seconds) drop in power-line voltage. Typically in the 70-85% range.  This is the opposite of a Spike.

 Direct current (DC) –Electrical current, which flows consistently in one direction

 Electro-Magnetic Interference (EMI) – The noise and transients that come into the facility through the power lines.  This is also referred to as Interference, Noise, and RFI.

 Emergency Power Off (EPO) – A system that allows the manual shutoff of all equipment on the circuit.  See REPO (Remote Emergency Power Off)

 Ferroresonance – A saturated Iron Core design that is commonly used as a Voltage Regulator in industrial situations.

 Filter – A device that allows only certain frequencies to pass through.  These are used to trap high-frequency noise, harmonics, and interference.  The filter does lower efficiency.

 Ground – The electrical reference point from where measurements are made.  The ground wire is traditionally, hooked to a grounding rod that is driven into the ground near the power entry into the building.  Alternately the cold water pipe and steel framework of a building can be used as a ground.

 Harmonic – A frequency that is the multiple of the desired frequency.  Harmonics are considered undesirable and are the byproduct of the power generation and distribution process.

 Harmonic Distortion – The presence of harmonics that change the waveform of an AC voltage from sinusoidal to complex.

 Hertz (Hz) –A measurement of the frequency of the AC power.  Means cycles per second.  The US standard is 60 Hz or 60 AC cycles per second

 Inrush Current – An initial surge current demand before the load resistance or impedance increases to normal operating values.  This is usually less than a sub-cycle in length.

 Interference – The encroachment of unwanted electrical, electromagnetic, rf signals, or noise into a desired signal.  This is usually RFI or EMI.  Interference is usually regular, man-caused verses noise which is irregular and nature caused.

 Inverter -UPS subassembly that recomposes a sine wave output (regulated and without breaks) using the DC current supplied by the rectifier-charger or the battery. The primary elements of the inverter are the DC/AC converter, a regulation system and an output filter.

 Isolation Transformer – The secondary windings of an iso-transformer have no physical electrical connection to the primary windings. 

 Joule -A quantitative measurement unit of energy.  The surge suppression capability is measured in Joules.  A bigger number is better.

 Lightning Arrester – A device that is used to shunt to ground any high-voltage transients caused by lightning hitting a power line.  All discharges are bypassed to a safety ground designed to handle the force of the lightning discharge.

 Line conditioning –The processing of power through a transformer to ensure proper voltage is sent to attached devices

 Line Hit – An electrical surge, dip, or transient that causes bad signals in a circuit.

 Line interactive –An ups design which uses a “buck & boost” filter to clean the utility power before passing it to the attached load.  This is a less expensive type of UPS which has less filtering capability than true online systems.  This is a better option than the offline UPS’s.

 Load shedding –(Powershare) A procedure by which the UPS switches off selected devices to increase run time of critical loads.

 National Electrical Code (NEC) – The rules and regulations and recommended practices that is published by the National Fire Protection Agency (NFPA).  These standards cover generation, distribution, and the installation of electrical equipment.

 Neutral – The active circuit conductor that is grounded.  This circuit should only carry voltage under fault conditions.  This is also called the return leg of a circuit since it takes the unbalanced voltage from the system.

 Noise –The distortions in the utility sine wave from lightning, load switching, and other sources of interference.

 Off-line (stand-by) -UPS configuration in which the unit will switch to battery on any power event.  This is for non-critical loads because it results in an interruption lasting up to 10 ms during transfer and the unit does not filter inrush currents.

 On-line -UPS configuration in which the inverter is in series mounted between the mains and the load. All power drawn by the load passes via the inverter. This is the only configuration used for high outputs.

 Outage – see Blackout.  This means the complete loss of electrical power.

 Overvoltage – This means that there is higher than normal voltage on the power lines.

 Power event –Any event which causes the utility power to go outside its normal operating range of 120V at 60Hz.  Surges, spikes, brownout, and blackout.

 Power Factor – W/VA The ration of actual power to apparent power.  This is either leading or lagging.

 Reflected Harmonics – Harmonic Distortion that is reflected back onto the power line from equipment that is attached to the circuit.

 Remote Emergency Power Off (REPO) – this is a separate piece of equipment that allows the user to fully shut down equipment from a remote location.  Many building codes require this for data center and lab space.

 rms – root-mean-square.  This is the most common method for measuring AC voltage and current.

 rms Current – The square root of the average of the squares of all the instantaneous current amplitudes in one cycle.

 rms Voltage – The square root of the mean value of the square of the peak voltage.  It is .707 times the peak voltage, or .3535 of the peak to peak voltage.

 Sag – a short term (less than 30 seconds) drop in line voltage (no more than 70%).  Sags can cause power supplies to overheat as the  voltage drops.

 Sine Wave – The basic waveform of AC voltage and current that the power company generates.

 Spike –A quick and dramatic voltage increase caused by a lightning strike or when power is restored after a blackout.  A spike can cause severe damage to any connected devices.

 Surge –A voltage increase that typically lasts at least .008 seconds.  A surge can cause premature wear on any connected loads.

 Volt –A measurement used to express the type of power.  V=(W/A)

 Volt-amperes (VA) – The VA is reached by multiplying volts by amps.  The VA rating is used to indicate a UPS’s output.  This is also a way to express the power draw of connected devices.  A larger number is means a unit can support a larger load.  VA=(VxA)



Keep your eyes open for our new cooling and data center sections to our website.

Home | About SBS | PQ Products | PQ 101 | PQ Links | People | Contact Us

This site was last updated 09/28/10

Send mail to webadmin@sbspowerquality.com with questions or comments about this web site.
Copyright © 2001-2010 South Bay Supply, Inc.