Electrotechnology

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Line 4 tripped on zone 1, reclose failed so it's locked out, and the 86 on the transformer flagged as well.

  TranslationThe line's distance protection saw a fault in its first, undelayed zone; the automatic reclose did not hold, so the breaker stays open, and the transformer's lockout relay also operated and is showing its indicator.

• Rack it out, tag it out, prove it dead and get the earths on before I write the permit.

  TranslationWithdraw the breaker to its isolated position, hang danger tags on the isolation points, test for absence of voltage, and apply portable earthing leads before the permit to work is issued.

• That bus is soft, every time the crusher starts we sag to 0.85 pu and the drives kick.

  TranslationThe supply has high source impedance, so starting the crusher motor drops the voltage to 85 percent of nominal and the variable-speed drives trip on undervoltage.

• Megger's fine but the Doble numbers on that bushing have doubled, I'd pull it before it lets go.

  TranslationThe insulation resistance test passed, but the insulation power factor test shows the bushing's losses have risen sharply, so it should be replaced before it fails.

• Voltage THD is over 8 percent with all the drives on, and the fifth is going to ring with those caps.

  TranslationTotal harmonic distortion of the voltage is above 8 percent with every drive running, and the fifth harmonic is close to the resonant frequency of the capacitor bank.

• We're not N-1 with Westside out, so hold that switching order until after the evening peak.

  TranslationWith the Westside line out of service, losing one more element would overload the system, so planned switching should wait until demand falls.

• Squirrel on the bushing, the feeder blinked twice and then locked out.

  TranslationAn animal bridged a transformer bushing to earth; the recloser tried twice, causing two momentary outages, then stayed open because the fault persisted.

How power flows, how its quality is described and how grid operators keep supply and demand in balance.

• Active poweralso real power, true power
  The part of electrical power that does real work, averaged over each cycle and measured in watts.
• Apparent poweralso S
  Product of RMS voltage and RMS current, the figure transformers, generators and UPS units are rated on.
• Black startalso blackstart
  Restarting a power system from total shutdown using generators that can start without any outside supply.
• Brownoutalso sustained undervoltage
  Sustained drop in supply voltage, caused by overload or applied on purpose to cut demand during a shortage. Shop talk
• Busalso busbar, bus bar
  Rigid conductor or common node in a substation or switchboard where several circuits connect and share power.
• Harmonicsalso harmonic distortion
  Voltage or current components at whole multiples of the supply frequency, produced mostly by nonlinear loads.
• Islandingalso loss of mains, LOM
  Condition where part of a network stays energised by local generation after being cut off from the main grid.
• Load sheddingalso load shed, rolling blackouts
  Deliberate disconnection of blocks of customer load to stop frequency or voltage from collapsing.
• N-1 criterionalso N minus one, single contingency criterion
  Planning rule that a grid must stay within limits after losing any single major element such as a line or generator.
• Per-unit systemalso per unit, pu
  Shorthand that states voltages, currents and impedances relative to chosen base values, so 1.0 means nominal.
• Power factoralso PF
  Ratio of active power to apparent power, showing how much of the current drawn does useful work.
• Reactive poweralso vars, Q
  Power that surges back and forth between the source and the fields of inductors and capacitors, doing no net work.
• Sagalso voltage sag, voltage dip
  Brief drop in RMS supply voltage, usually to between 10 and 90 percent of nominal, lasting up to about a minute.
• Stiffalso strong
  Describes a supply with low source impedance, so its voltage barely moves when load or fault current changes. Shop talk
• Swellalso voltage swell
  Short-lived rise in RMS voltage above normal, often seen on the healthy phases during an earth fault.
• THDalso total harmonic distortion
  Total harmonic distortion: the combined size of all harmonics relative to the fundamental, as a percentage.

Fault currents, the relays and breakers that clear them, and the settings that decide which device trips first.

• 86 relayalso 86, lockout relay
  Lockout relay that latches after a serious trip and holds breakers open until someone resets it by hand. Shop talk
• Arc flash
  Explosive release of heat, light and pressure when an electric arc forms through air between energised parts.
• Auto-reclosealso autoreclose, reclosing
  Automatic closing of a breaker shortly after a fault trip, restoring supply when the fault was only transient.
• Blinkalso momentary, momentary interruption
  Momentary outage customers see when an upstream breaker or recloser trips and recloses on a passing fault. Shop talk
• Breaking capacityalso interrupting rating, interrupting capacity
  Highest fault current a circuit breaker or fuse can interrupt safely at its rated voltage.
• Differential protectionalso 87, diff
  Scheme that compares current entering and leaving a protected zone and trips when the two disagree.
• Distance protectionalso impedance protection, 21
  Line protection that measures impedance to a fault and trips fastest for faults close to the relay.
• Earth faultalso ground fault, line-to-ground fault
  Fault in which a live conductor makes unintended contact with earth or with earthed metalwork.
• Fault currentalso short-circuit current, fault level
  Abnormally high current that flows when insulation fails and a short circuit or earth fault forms.
• IDMTalso inverse time overcurrent, 51
  Inverse definite minimum time: an overcurrent curve that trips faster as current rises, down to a set minimum.
• Nuisance tripalso false trip, spurious trip
  Protective device opening with no real fault present, usually from bad settings, leakage or transients. Shop talk
• Pickupalso pick-up, operate value
  Threshold at which a protection element starts to operate, and the moment the measured value crosses it.
• Protection relayalso protective relay, relay
  Device that watches currents and voltages and orders a circuit breaker to trip when it detects a fault.
• Reachalso zone reach
  Extent of line or network a distance relay zone is set to cover, expressed as an impedance.
• Selectivityalso discrimination, coordination
  Arranging protection so only the device nearest a fault operates and the rest of the system stays in service.
• Short circuitalso short
  Unintended low-impedance path between conductors at different potentials, bypassing the load.
• Tripalso trip out, trip-out
  To open a circuit breaker automatically through its protection, or to order a breaker open by command.
• X/R ratioalso X over R
  Ratio of reactance to resistance in the fault path, which sets how large and slow the DC offset in fault current is.

Earthing systems, bonding, shock voltages and the words that separate a safe isolation from a dangerous one.

• Deadalso de-energised, de-energized
  Isolated from every source, proven free of voltage and safe to work on. Shop talk
• Earth fault loop impedancealso Zs, loop impedance
  Total impedance of the path a fault current takes from the source, through an earth fault and back again.
• Earthing systemalso earthing arrangement, system earthing
  Arrangement describing how a supply's neutral and an installation's exposed metalwork are connected to earth.
• Equipotential bondingalso bonding, protective equipotential bonding
  Connecting metal parts together so they sit at one potential and no dangerous voltage appears between them.
• Hotalso live, energized
  Shop word for live or energised, and in North America the everyday name for a phase conductor. Shop talk
• IT systemalso isolated system, unearthed system
  Earthing arrangement in which the source is isolated from earth or earthed only through a high impedance.
• Livealso energised, energized
  Connected to a source of voltage, or describing a conductor meant to be energised in normal use.
• PEN conductoralso combined neutral and earth, CNE conductor
  Single conductor that serves as both the neutral and the protective earth in a TN-C network.
• Permit to workalso PTW, permit
  Formal written authority to work on equipment that has been isolated, earthed and declared safe.
• Portable earthsalso grounds, protective grounds
  Removable earthing leads clamped onto isolated conductors to short them together and to earth during work. Shop talk
• Prove deadalso test before touch, live-dead-live
  To test isolated equipment with a verified voltage detector and confirm no voltage before anyone touches it. Shop talk
• RCDalso RCCB, GFCI
  Residual current device: trips when the current going out on the live conductors does not all come back.
• Step voltagealso step potential
  Voltage between a person's feet across the ground surface near an earth fault or an earthed structure.
• TN-C-S systemalso PME, protective multiple earthing
  Supply earthing in which a combined neutral and earth conductor splits into separate ones at the installation.
• TN system
  Earthing arrangement where the source is earthed and installation metalwork connects back to it by a protective conductor.
• Touch voltagealso touch potential, prospective touch voltage
  Voltage a person bridges between a hand on faulted metalwork and their feet or other hand during a fault.
• TT systemalso TT earthing
  Earthing arrangement where the supply and the installation each have their own separate earth electrode.

Clearances, creepage, impulse levels and the ways insulation degrades and fails.

• BILalso basic insulation level, LIWL
  Basic lightning impulse insulation level: the peak impulse voltage equipment must withstand without breakdown.
• Clearancealso air clearance, electrical clearance
  Straight-line air gap between two conductive parts, the dimension that decides whether a surge can flash across.
• Coronaalso corona discharge
  Faint glowing discharge around high-voltage conductors where the air is overstressed, causing noise and loss.
• Creepage distancealso creepage, creep distance
  Shortest route over an insulating surface from one conductive part to another, the dimension that resists tracking.
• CTIalso comparative tracking index
  Comparative tracking index: voltage an insulating material withstands before tracking under a wet contamination test.
• Flashoveralso flash-over
  Disruptive discharge over or around insulation that creates a short circuit through the air.
• Insulation coordinationalso insulation co-ordination
  Sizing insulation and surge arresters together so any flashover lands at a cheap, self-restoring point, not inside plant.
• Overvoltage categoryalso OVC, installation category
  Class from I to IV ranking equipment by its position in an installation and the surges it must survive.
• Partial dischargealso PD, discharge activity
  Tiny discharge in a void, on a surface or at a sharp edge that never spans the whole insulation, an early sign of decay.
• Pollution degreealso PD
  Grade from 1 to 4 describing how much conductive dirt and moisture can settle on insulation in a location.
• Surge arresteralso lightning arrester, arrestor
  Device that clamps transient overvoltages by diverting surge current to earth, then returns to insulating.
• Trackingalso surface tracking, carbon tracking
  Conductive carbon trail that leakage arcs through dirt and moisture gradually burn across an insulating surface.

Transformers, motors, switchgear and the instrument transformers that feed protection and metering.

• Buchholz relayalso gas relay, 63
  Gas and oil-surge relay in the pipe between a transformer tank and its conservator that detects internal faults.
• Burdenalso VA burden, secondary load
  Load connected to the secondary of an instrument transformer, stated in VA or ohms.
• CTalso current transformer
  Current transformer: an instrument transformer that scales large primary currents down to a standard secondary.
• Disconnectoralso isolator, disconnect switch
  Off-load switch that provides a visible isolating gap but is not built to make or break load current.
• Inrush currentalso inrush, switch-on surge
  Large transient current drawn when a transformer, motor or capacitive load is first switched on.
• OLTCalso on-load tap changer, load tap changer
  On-load tap changer: a transformer switch that changes the turns ratio under load to regulate voltage.
• Potheadalso cable termination
  Field name for the termination where an underground power cable meets an overhead line or equipment. Shop talk
• SF6also sulphur hexafluoride, sulfur hexafluoride
  Sulphur hexafluoride: a dense, highly insulating gas used to quench arcs in high-voltage switchgear.
• Slipalso rotor slip
  Shortfall between the speed of an induction motor's rotating field and the actual speed of its rotor.
• Switchgearalso SWGR, gear
  Assemblies of breakers, switches, fuses, busbars and enclosures that control and protect power circuits.
• Vector groupalso vector group designation, connection symbol
  Code stating how a three-phase transformer's windings are connected and the phase shift between them.
• VTalso voltage transformer, PT
  Voltage transformer: an instrument transformer that steps high voltage down to a standard level for relays and meters.

Interference, coupling and the circuit fundamentals behind electromagnetic compatibility.

• Common-modealso CM, longitudinal
  Describes signals or noise present equally, in the same phase, on both conductors of a pair relative to ground.
• Crosstalkalso XT, cross-talk
  Unwanted coupling of a signal from one conductor or channel into a neighbouring one.
• EMCalso electromagnetic compatibility
  Electromagnetic compatibility: a product neither upsets nearby equipment with its noise nor gets upset by theirs.
• EMIalso electromagnetic interference
  Electromagnetic interference: unwanted electrical noise that disturbs the operation of a circuit or system.
• ESDalso electrostatic discharge, static zap
  Electrostatic discharge: sudden flow of built-up static charge that can destroy or upset electronics.
• Ground loopalso earth loop
  Unintended current path formed when connected equipment is earthed at points of different potential.
• Impedancealso Z
  Total opposition a circuit offers to alternating current, combining resistance and reactance, in ohms.
• Skin effect
  Tendency of alternating current to crowd toward the surface of a conductor as frequency rises.

Units, meters and field tests used to prove equipment healthy before and after it goes into service.

• Clamp meteralso clamp-on ammeter, amp clamp
  Handheld meter whose jaws close around a conductor to measure current without breaking the circuit.
• Doble testalso power factor test, tan delta test
  Insulation power factor test on transformers, bushings and breakers, named after a well-known test set maker. Shop talk
• Ductoralso micro-ohmmeter, DLRO
  Shop name for a micro-ohmmeter used to measure very low contact and joint resistances. Shop talk
• Hipotalso hi-pot, high-potential test
  High-potential test: applying a voltage well above working level to prove insulation will not break down. Shop talk
• Insulation resistance testalso IR test, megger test
  DC test that applies a high voltage between conductors and earth and reads leakage resistance in megohms.
• Meggeralso megohmmeter, IR tester
  To test insulation resistance with a high-voltage DC tester; also the jobsite name for the tester itself. Shop talk
• RMSalso root mean square, effective value
  Root mean square: the effective value of an alternating quantity, giving the same heating as that DC value.
• Thumperalso surge generator, cable thumper
  Field nickname for a surge generator that pulses a faulted cable so the fault can be heard and pinpointed. Shop talk
• True RMSalso TRMS
  Describes a meter or reading that computes the real RMS of any waveform, not one scaled from a sine-wave average.
• varalso VAr, volt-ampere reactive
  Unit of reactive power, the volt-ampere reactive, written lower case and usually scaled to kvar or Mvar.

Radio calls, switching language and yard slang from grid control rooms and substations.

• Backfeedalso back-feed, reverse feed
  Power arriving at a circuit from an unexpected direction, such as a generator or second supply feeding back. Shop talk
• Bumpalso bump test, jog
  To energise a motor for a split second to check its direction of rotation before running it properly. Shop talk
• Chatteralso contact chatter, relay chatter
  Rapid repeated opening and closing of relay or contactor contacts, usually from marginal voltage or vibration. Shop talk
• DC groundalso battery earth fault, DC earth fault
  Unwanted earth fault on a substation battery or DC control supply, which normally floats free of earth. Shop talk
• Dead busalso dead bar, dead busbar
  Busbar with no voltage on it, the condition that lets a breaker close without synchronising checks. Shop talk
• Flagalso target
  Indicator on a protection relay showing which element or phase operated; also the verb for it showing. Shop talk
• Gallopingalso conductor galloping, dancing
  Large, slow vertical swinging of overhead line conductors driven by wind on ice-coated wires.
• Hot stickalso hotstick, operating pole
  Insulated fibreglass pole used to operate, test or earth high-voltage equipment from a safe distance. Shop talk
• Kickalso kick out, pop
  Shop verb for a breaker, overload or relay tripping, as in the breaker kicked. Shop talk
• Magic smokealso let the smoke out
  Joking name for the smoke a burnt-out component releases, as if the smoke were what made it work. Shop talk
• Rack outalso rack-out, withdraw
  To wind a withdrawable breaker out of its connected position in metal-clad switchgear to isolate it. Shop talk
• Ring outalso buzz out, bell out
  To check continuity and identity of every core of a cable end to end before termination or after a fault. Shop talk
• SCADAalso supervisory, telecontrol
  Supervisory control and data acquisition: the system that lets a control room watch and switch remote plant.
• Squirrelalso animal contact, squirrel outage
  Control-room shorthand for an animal-caused fault, typically a squirrel or bird bridging live parts. Shop talk
• Switching orderalso switching programme, switching program
  Written step-by-step sequence of operations used to isolate or restore equipment safely.
• Tag outalso LOTO, lock out tag out
  To hang a danger tag on an isolation point so nobody operates it while people work downstream. Shop talk

Frequently asked questions

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