Electrical Glossary

Electrical Glossary

 

A

 
AC or ac:  Abbreviation for alternating current.
 
Alternating Current:  Electrical current that periodically reverses direction, usually several times per second.
 
Ampere or Amp:  The measurement unit for electrical current.
 
ATS:  Abbreviation for Automatic Transfer Switch.
 
Automatic Transfer Switch:  A switch that automatically transfers electrical loads to alternate  or emergency-standby power sources.
 

B

 
Backup Time:  Time during which the UPS can supply the rated load with nominal-quality power while the normal source of power is unavailable. This time depends on the battery and the efficiency of the UPS. Typical backup times range from five minutes to several hours.
 
Battery on Shelves:  Battery cell installation system whereby the cells are placed on several vertically stacked shelves or racks.
 
Battery (Recombination):  Battery with a gas recombination rate at least equal to 95%, so that no water need be added over battery life, usually called "maintenance free."
 
Battery (Tier-Mounted):  Battery cell installation system whereby the cells are placed on tiers.
 
Battery (Vented):  The battery cells are equipped with a filling port for distilled, demineralized water used to top off the free electrolyte.
 
Battery Cells:  The interconnected battery elements that supply electrical power created by electrolytic reaction.
 
Battery Circuit Breaker:  See Circuit Breaker.
 
Battery Monitor:  Battery monitoring and protection system that incorporates software to calculate the real available backup time, predicts when batteries need replacement and acts as a protection system against excessive discharges.
 
BEM (Building and Energy Management) System:  System used for control/monitoring of all building utilities and systems. It is generally composed of sensors, actuators and programmable controllers connected to a central computer or several computers equipped with specific software.
 
Blackout:  A complete loss of power lasting for more than one cycle. A blackout can damage electronics, corrupt or destroy data, or cause a system shutdown. Blackouts can result from any of a number of problems, ranging from acts of God (hurricanes or other high winds, ice storms, lightning, trees falling on power lines, floods, geomagnetic storms triggered by sunspots and solar flares, etc.) to situations such as cables being cut during excavation, equipment failures at the utility, vandalism, corrosion, etc. - also known as an outage.
 
Brownout:  Prolonged sag, occurring when incoming power is reduced for an extended period. Usually caused when demand is at its peak and the line becomes overloaded.
 
Bypass (Manual):  Manually operated switch used to supply the load via direct connection to utility power during servicing of the UPS system.
 
Bypass (Automatic):  In the event of an overload or an unlikely UPS problem, your application is still powered thanks to the automatic bypass module.
 

C

 
Capacitor:  Any AC circuit element possessing the property of capacitance (i.e., the ability to store a charge). Normally, a capacitor is a dedicated device, designed for the prime purpose of exhibiting the property of capacitance (as opposed to inductive devices, in which inductance is used by the device to produce other results, such as turning a motor shaft).
 
Charger:  Device associated with the rectifier and used to supply the battery with the electrical power (DC Current) required for recharging and/or float charging the battery, thus ensuring the rated backup time.
 
Circuit Breaker (Battery):  DC circuit breaker that protects the battery of a UPS.
 
Critical Load:  Equipment that must have an uninterrupted power input to prevent damage or loss to a facility or to itself, or to prevent danger of injury to operating personnel.
 
Current:  The flow of electricity in a circuit. The term current refers to the quantity, volume or intensity of electrical flow, as opposed to voltage, which refers to the force or "pressure" causing the current flow. Current may be either direct (DC) or alternating (AC). Direct current refers to current whose voltage causes it to flow in only one direction. Common direct current sources are batteries. Alternating current refers to current whose voltage causes it to flow first in one direction, then the other, reversing direction periodically, usually several times a second. A common alternating current source is commercial/household power. This current reverses direction 120 times each second, thus passing through 60 complete cycles each second for a frequency of 60 Hertz.
 
Current (Float):  DC current that maintains the battery at nominal charge, corresponding to the float voltage. This current compensates open circuit losses.
 
Current (Inrush):  Temporary current observed in a network when electrical devices are first energized, generally due to the magnetic circuits of the devices. The effect is measured by the current’s maximum peak value and the RMS current value it generates.
 
Current Harmonics:  See Harmonics (Current and Voltage).
 

D

 
DC or dc:  Abbreviation for direct current.
 
Direct Current:  Electrical current which flows consistently in one direction.
 
Distortion (Individual):  Ratio between the RMS value of an nth order harmonic and the RMS value of the fundamental.
 
Distortion (Total):  Ratio between the RMS value of all harmonics of a non-sinusoidal alternating periodic value and that of the fundamental. This value may also be expressed as a function of the individual distortion of each harmonic Hn = Yn /Yl.
 
Double-Conversion:  A UPS design in which the primary power path consists of a rectifier and inverter. Double-conversion isolates the output power from all input anomalies such as low voltage, surges and frequency variations by converting AC to DC to AC. Also see Online UPS.
 

E

 
Electromagnetic Compatibility:  Possibility of a device to operate normally when installed near other devices, given the disturbances emitted by each device and their mutual sensitivities.
 
EMI/RFI:  Electromagnetic/Radio Frequency Interference. These high-frequency signals are generally low level (<1V) and range from 1MHz up. EMI/RFI filters are generally not suitable for large amplitude surge suppression.
 

F

 
Float Current:  See Current (Float).
 
Floating Voltage:  See Voltage (Float).
 
Frequency:  The number of cycles (oscillation positive and negative) completed in one second. Defined as hertz (Hz). In North America, utility power completes 60 cycles per second (60 hertz).
 

G

 
Grounding Conductor:  A conductor connected between a circuit and the earth.
 

H

 
Harmonics (Current and Voltage):  All alternating current, which is not absolutely sinusoidal, is made up of a fundamental and a certain number of current harmonics, which are the cause of its deformation (distortion) when compared to the theoretical sine wave. For each current harmonic of order n and an RMS value In, there is a voltage harmonic with an RMS value Un. If Zsn is the voltage source output impedance for the harmonic of the nth order, then: Un = Zsn x In.
 
Harmonic Distortion:  A measure of the degree to which the impedance of a UPS affects the shape of the output voltage waveform. Distortion is stated as a percentage and may refer to any single harmonic or to the total waveform, in which case it is referred to as "total harmonic distortion" (THD).
 

I

 
Inrush Current:  See Current (Inrush).
 
Interference (High-Frequency):  High-frequency parasitic current that is either conducted (electrostatic origin) or radiated (electromagnetic origin) by a device.
 
Inverter:  The DC to AC power converter driven by the UPS rectifier-charger or battery via the DC bus. The inverter output drives the critical load.
 
Inverter (Off-Line or Stand-By):  UPS configuration in which the inverter is parallel-mounted to the load supply line and backs up the mains. This configuration offers a substantial cost reduction but is applicable only to low outputs, under 3 kVA, because it results in an interruption lasting up to 10 ms during transfer and does not filter inrush currents.
 
Inverter (Online):  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.
 
ISO 9000:  Standard defining procedures and systems used to attain an internationally recognized level of production quality. ISO 9000 certification is proof that the quality system effectively complies with the standard. Certification is carried out by an official organization (AFAQ), unaffiliated with either clients or suppliers or the company itself, and is valid for a three-year period with yearly audits and checks.
 
Isolation:  The separation (often through the use of an isolation transformer) of one section of a system from undesired electrical influences of other sections.
 
Isolation Transformer:  A multiple-winding transformer with physically separate primary and secondary windings. Although the two windings are physically disconnected, the magnetic field in the windings of the primary creates (induces) electrical power in the secondary winding. In this way the electrical power available at the input can be transferred to the output. An isolation transformer does not transfer unwanted noise and transients from the input circuit to the output windings. This attenuation, or reduction in amplitude, could be as high as one million to one.
 

K

 
KVA:  Abbreviation for kilovolt-amperes (1000 volt-amperes).
 
KW:  Abbreviation for kilowatt (1000 watts).
 

L

 
Line Conditioner:  A transformer that attempts to smooth out fluctuations in input voltage to provide near uniform output voltage or voltage waveform.
 
Line Disturbance Analyzer:  A tool used in analyzing problems in a facility’s incoming power.  The line disturbance analyzer is connected at the power input to measure and record incoming power, and then left in place for long enough to gather data typical of the site.
 
Line-Interactive:  A UPS containing an off-line inverter that must transfer on during a blackout, but provides faster transfer times than an off-line UPS. Power conditioning and surge suppression are provided to protect the connected load.
 
Load (Linear): Load for which voltage form and current form are similar. Voltage and current are related by Ohm's law: U(t) = Z x I(t).
 
Load (Non-Linear): Load (generally with a switched-mode power supply) generating major harmonic currents. Current waveform is different from voltage waveform. Ohm's law is not applicable. It can be used only with each harmonic.
 
Load Shedding: The ability to selectively shut off a set of UPS output receptacles, extending the capacity of the UPS battery. Some UPS models are able to shed less critical loads by turning off selected output receptacles during an extended power failure while maintaining power to the more critical load(s) on the remaining output receptacles.
 
Load Power:  Apparent power Pu that the UPS inverter supplies under given load conditions. It is less than or equal to the rated output Pn. The ratio PuiPn defines the % load of the inverter.
 

M

 
Manual Bypass:  See Bypass (Manual).
 
MOV:  Metal Oxide Varistors used to control spikes. These are common in power strips. More than two usually indicates a fairly decent power strip. They look like largish disk capacitors.
 
MTBF (Mean Time between Failures):  Mathematical calculation of the duration of normal operation of a repairable device between failures. The product, expressed in hours, is an indication on the reliability of the device.
 
MTTF (Mean Time to Failure):  Mathematical calculation of the duration of normal operation of an irreparable device, i.e. for which a MTBF is not possible. The product, expressed in hours, is an indication on the reliability of the device.
 
MTTR (Mean Time to Repair):  Mathematical calculation (or statistical average if available) of the time required repairing a device.
 

N

 
NETA:  International Electrical Testing Association.  NETA is an association of leading electrical testing companies committed to advancing the industry's standards for power systems installation and maintenance to ensure the highest level of reliability and safety.
 
Noise:  Noise is the result of distortion of the normal line power sine wave by hundreds or thousands of small increases in voltage similar to EMIIRFI, though it encompasses lower frequencies. The amplitude of this type of disturbance is less than a surge but may be as low as EMIIRFI.
 
Non-Linear Load:  See Load.
 
Normal Line Power:  Commercial electricity supplied by U.S. power utilities is generally delivered as 60-cycle (Hz) alternating current (AC).
 

0

 
Off-Line UPS:  A UPS type that feeds power to the load directly from the utility and then transfers to battery power via an inverter after utility drops below a specified voltage. The delay between utility power loss and inverter startup can be long enough to disrupt the operation of some sensitive loads. Also called a standby UPS.
 
Online UPS:  A UPS in which the inverter is on during normal operating conditions supplying conditioned power to the load through an inverter or converter that constantly controls the AC output of the UPS regardless of the utility line input. In the event of a utility power failure, there is no delay or transfer time to backup power.
 
Output (Rated):
Apparent power Pn that the UPS can deliver under given load conditions (power factor= 0.8).
 
Overall Distortion:  See Distortion (Overall)
 
Overload Capacity:  A UPS's overload capacity is its ability to respond to sudden surges in load current without allowing the output voltage level to decrease.
 

P

 
Parallel Online UPS:  Online UPS technology that provides redundant sources of conditioned backup power so that the critical load is protected even in the event of UPS component failure.
 
Partial Discharge:  Localized electrical discharge that only partially bridges the insulation between conductors.
 
Percent Load:  Ratio between the power Pu drawn by the load and the rated output Pn of a UPS system (Pu/Pn). Sometimes referred to as the load factor.
 
Power Conditioning Systems:  A broad class of equipment that includes filters, isolation transformers, and voltage regulators. Generally, these types of equipment offer no protection against power outages.
 
Power Factor (PF):  The ratio of total real power (W), to the total apparent power in volt-amperes (VA), W/VA, generally expressed in a percentage.
 
Power Source (Alternate):  Backup source used in the event of a mains failure. The connection time and the duration of the source depend on the type of source used.
 
Power Synthesizer:  Power synthesizers actually use the incoming utility power as an energy source to create a new sine wave that’s free from power disturbances. They can be as much as 99% effective against power disturbances. Types of power synthesizers include magnetic synthesizers (capable of generating a sine wave of the same frequency as the incoming power-60Hz), motor generators (which use an electric motor to drive a generator that provides electrical power), and UPS equipment.
 
PWM (Pulse Width Modulation):  Inverter high-frequency chopping technique using a means of regulation enabling rapid modification of pulse widths over a single period, thus making it possible to maintain the inverter output within tolerances even for non-linear loads.
 

R

 
Reactance (Subtransient Uscx %, for Generator):  Relative measurement (%) of the internal impedance of an AC generator during harmonic phenomena. This reactance, also called the longitudinal sub transient reactance of the generator, is sometimes identified as X"d. For most common generators, the value ranges between 15 and 20%. It can drop to 12% for optimized systems and to 6% for special devices.
 
Recombination Battery:  See Battery (Recombination).
 
Rectifier/Charger:  UPS component that draws from the mains the power required to supply the inverter and to float charge or recharge the battery. The alternating input current is rectified and then distributed to the inverter and the battery.
 
Redundancy:  Duplication of elements in a system or installation to enhance the reliability or continuity of operation.
 
Reliability:  Probability that a device will accomplish required function under given conditions over a given period of time. RMS value of AC current with harmonics. The RMS value Yrms of a non-sinusoidal alternating current may be determined on the basis of the individual harmonic currents where Y is the RMS value of the fundamental.
 
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.)
 

S

 
Sag: A momentary decrease from nominal voltage lasting one or more line cycles. Severe conditions may dictate a need for a UPS or voltage regulator. Also known as a temporary under-voltage (TUV).
 
Sine Wave: A mathematical curve that describes a smooth repetitive oscillation; the fundamental waveform from which other waveforms may be generated by combinations of various group of harmonics. The voltage and current waveforms produced from the power company generators (alternators) are basic sine waves.
 
SNMP: Simple Network Management Protocol.
 
Spike: A spike involves a sudden marked increase in voltage, which can damage electronics and corrupt or destroy data.
 
Spike/Surge Protector: These products are inexpensive solutions that provide minimal protection against surges, but no protection against sags and outages.
 
Static Bypass Switch: Power-electronics device that can be used to switch from one source to another without interruption in the supply of power. In a UPS, transfer is from Mains 1 to Mains 2 and back. Transfer without interruption is possible due to the fact that there are no mechanical parts and the ultra-fast switching capabilities of the electronic components.
 
Suppressed Voltage Ratings: Several ranges are assigned by UL for grading transient suppression voltages. For instance, a 400 volt rating indicates a maximum peak voltage between 330 and 400 volts. These ratings appear between 330 volts peak and 6000 volts peak.
 
Surge: A surge is a prolonged over-voltage condition. Surges can damage electronics and corrupt or destroy data.
 
Swell: An increase from nominal voltage lasting one or more line cycles.
 

T

 
Tan Delta (or Dissapation Factor):  The tan delta measurement is used to determine a loss factor for insulating material.  The tan delta is measured by applying an AC voltage at a given frequency and measuring the phase difference between the voltage waveform and the resulting current waveform.
 
Transfer Time:  Transfer time can refer to either the speed with which an off-line UPS transfers from utility power to battery power, or to the speed with which an on-line UPS switches from the inverter to utility power in the event of an inverter failure. In either case, the time involved must be shorter than the length of time that the computer's switching power supply has enough energy to maintain adequate output voltage. This hold-up time may range from eight to 16 milliseconds, depending on the point in the power supply's recharging cycle that the power outage occurs, and the amount of energy storage capacitance within the power supply. A transfer time of 4ms is most desirable, however, it should be noted that an oversensitive unit may make unnecessary power transfers.
 
Transient:  The fast radical change in a smooth sine wave that occurs in both voltage and current waveforms during the transition from one steady-state operating condition to another.
 
Transient Suppression Voltage (Let-Through Voltage):  The maximum peak voltage occurring within 100 microseconds after the test wave.
 
Transient Voltage Surge Suppressor (TVSS): A device used to reduce voltage surges. Products may be wired in series or in parallel with the AC electrical conductors.
 

U

 
UL:  Underwriters Laboratories - An approval organization based in the United States.
 
UPS:  Uninterruptible Power Supplies (sometimes called Uninterruptible Power Systems). A system designed to protect against short-term power outages.
 
UPS (Parallel with Redundancy):  A UPS made up of several parallel-connected UPS units with equal output ratings (P) and each equipped with its battery. If one unit fails, one or several of the others pick up the resulting excess load. If a UPS has a rated output n x P and is made up of n + k units, k is the level of redundancy for the entire set of n + k units.
 
UPS (Parallel without Redundancy):  A UPS made up of several (n) parallel-connected UPS units with equal output ratings (P) and each equipped with its battery, for large loads. The total output is equal to the number of units multiplied by their individual output (n x P). In this configuration, no UPS unit is redundant.
 
UPS (Single):  A UPS made up of one single UPS unit (rectifier/charger, inverter and bypass) and a battery.
 

V

 
VLF:  Very low frequency.  This is generally a voltage source used in testing insulation quality.  The power source frequency may range from 0.01 Hz to 60 Hz.
 
Volt (V):  The unit of measure for voltage, the electrical pressure which forces the current to flow in a conductor such as a wire.
 
Volt-Ampere (Va):  Voltage (V) multiplied by the current (ampere); apparent power. For instance, a device rated at 10 amps and 120 V has a VA rating of 1200 or 1.2 kVA.
 
Voltage:  A term referring to electrical force or potential. A technical synonym for voltage is emf or "electromotive force." Voltage is the parameter of electricity which causes current to flow when a circuit is completed. Voltage is always present in an energized line, whether or not the circuit is complete (i.e., whether or not current flows).
 
Voltage (Float):  DC voltage applied to the battery to maintain its charge level. This voltage depends on the type of battery, the number of cells, and the manufacturer's recommendations.
 
Voltage Regulator:  A device designed to regulate RMS voltage by removing swells and sags (such as an automatic tap switching transformer or Ferro resonant transformer).
 

W

 
Walk-In Time:  The time that the rectifier takes to reach rated output current after the Start-Up Delay. The slope is fixed so that a lower output current will have a shorter walk in period.
 
Watt:  The quantitative unit of measurement of actual power. Actual power in an AC circuit is the measurement of the effective energy available for doing work, and is normally less than apparent power (volt-amperes) because of power factor considerations. Watts may be measured directly, by means of a wattmeter, or may be calculated by multiplying volt-amperes by the power factor of the equipment.
 
Wye Connection:  A three-phase source of load connection, with a single common  junction  and three  phase lines out or in.