glossary of medium voltage switchgear terms

7/29/2019 Glossary of Medium Voltage Switchgear Terms

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MCset - 6kV medium voltage switchgear with SF6 circuit breakers (Schneider Electric)

http ://ele ctrical- eng ine ering -p orta l.co m/glo ssa ry- of- me diu m- vo lta ge- switch ge ar- terms Fe bru ary 24, 2013

Glossary of Medium Voltage Switchgear Terms

Edvard

This glossary was published originally by Schneider Electric and it is intended to cover themeaning of many common words and phrases used in association with medium voltageswitchgear. Many terms will also be applicable to low voltage switchgear as well.

Glossary

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A

Air Blast Circuit Breaker

A circuit breaker using compressed air to force the ext inction of the arc through an arc-chut esystem. An obsolete technology. See also Circuit Breaker. IndexA

Air Magnetic Circuit Breaker
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A circuit breaker that uses air as its insulating medium, and an arc-chute system for dissipatingthe arc, a method of generat ing a magnetic f ield which fo rces the arc into the arc chute, andwith possible assistance from a air puffer system to blow t he arc into the arc chutes. Anobsolete technology. See also Circuit Breaker. IndexA

ANSI Device Function Numbers

Defined in ANSI standard C37.2, the Device Function Numbers are a world-wide standardizedsystem for describing the funct ion o f many prot ection and control components withinswitchgear.

They are commonly used to describe the type of protection applied to a circuit . For example,the most common form of protection is overcurrent, usually designated as 50/51. Thisdesignation actually covers two types of overcurrent protection, 50 which is instantaneousprotection, and 51 which is a time overcurrent protection.

Various suffixes can be used, which are sometimes context sensitive. For example, 87represents dif ferential prot ection in general, while 87T is specif ically t ransformer dif ferentialprotection and 87G is specif ically generator differential protection. However, 50G refers toinstantaneous overcurrent protection on the ground circuit (in this case the G refers to ground,not generator). Refer to the ANSI standard for a full explanation. IndexA

ANSI Standards

The American Nat ional Standards Institute control the National Standards of the United States.These standards are of course used in the US and some are used by certain ot her countries aswell, including Canada. The electrical engineering standards of ANSI are of ten writt en for themby IEEE, the Inst itute of Electrical and Electronics Engineers. IndexA

ANSI Switchgear

Not a def initive term, but used to generally describe switchgear built in accordance with ANSIstandards, as opposed to IEC standards (IEC Switchgear). IndexA

Anti-single Phase Tripping Device

A device that operates to open all phases of a circuit by means of a polyphase switchingdevice, in response to the interruption o f the current in one phase. For example, the striker pinof an expulsion fuse can be used to actuate a common t ripping bar which causes a three phaseload break switch to open. IndexA

Anti-pumping Relay

Anti pumping relay is device used in association with a circuit breaker to prevent the circuitbreaker from reclosing after an opening operation, as long as the device initiating the reclosingis maintained in the position for closing. For example, as long as the opening coil is energized,then any attempt to close the circuit breaker will not work. IndexA

Arc-chute

A structure used as part of a current interrupting device, af fording a conf ined space orpassage, lined with arc-resisting material, through which an arc is directed in order to cause it toextinguish.

In medium volta e switch ear, this was the main current interru tin means in the air-ma netic
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type of circuit breakers, but the same concept does not exist in vacuum or SF6 designs. Arc-chutes are common in low voltage circuit breakers. IndexA

Arc-resistant Switchgear

A type of switchgear design which is designed to withstand the effects of an internal arcingfault , without causing harm to personnel who are located in defined areas. It is not intended towithstand these internal arcing fault without possibly causing physical damage to t he structure

and/or components, but of ten the physical damage is less with an arc-resistant design.

There are three classes of protection:

Type A eliminates the emission of gases and part icles from the f ront of the switchgearduring an internal arcing fault,

Type B eliminates the emission of gases and part icles from the front and sides of theswitchgear during an internal arcing fault,

Type C eliminates the emission of gases and part icles from the front and sides of theswitchgear, from between compartments within the same cell, and between adjacent cellsduring an internal arcing fault.

Arc-resistant switchgear has traditionally been metal-clad, but the basic concept could also beapplied to other types of switchgear as well. See also Proo f, Resistant. IndexA

Arc-proof Switchgear

An incorrect t erm. Please refer to Arc-resistant Switchgear, Proof, Resistant. IndexA

Arcing Contacts

The contacts of a switching device upon which the arc is deliberately drawn after the maincontacts have parted. See also Main Contacts. IndexA

Arcing Time

The amount of t ime between the f irst init iat ion of the arc and the instant of f inal arc ext inctionin all phases. See also Opening T ime, Closing Time. IndexA

Asymmetrical Current

The current which appears in a circuit during fault condit ions, composed of the to tal of the ACcomponent of the fault current and the DC of fset component. IndexA

Auxiliary Circuits

All the cont rol, indicating and measuring circuits in the switchgear. When speaking of the circuitbreaker, these are often called the secondary circuits (with the main current path called theprimary circuit). IndexA

Auxiliary Relay

A relay whose funct ion is to assist another relay or control device in performing a generalfunction by supplying supplementary actions. Some examples are a relay used to provide seal-in functions, to increase the number of available cont acts, to provide circuit opening instead ofcircuit closing contacts or vice-versa etc. IndexA
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B

Backfeed

To energize a section of a power network, supplied from a source other than its normal source.This may be a desired state, or may be non-desired if the circuit is thought to be deenergizedwith t he main incoming device turned of f , but is actually being backfed, for example, throughthe secondary of a transformer.

It is known that workers are sometimes injured when they t ry to work on a system they think isdeenergized but is actually backfed from a source they did not know about. IndexB

BIL (Basic Impulse Insulation Level)

A reference impulse insulat ion strength expressed in terms of the crest value of the withstand

voltage of a standard full impulse voltage wave. It is commonly used to define the overallinsulat ion strength of the design, with other test levels (e.g. applied tests) related in a standardmethod to the BIL.

Typically 5 kV class switchgear is 60 kV BIL, 15 kV class switchgear is 95 kV BIL, 25 kV classswitchgear is 125 kV BIL (although 150 kV BIL is not uncommon at 27.6 kV), and 38 kV classswitchgear is 150 kV BIL (with some requests for 170 or 200 kV BIL at t imes). IndexB

Bifurcated Feeder

A feeder connect ion where two loads are connected to one power source. Higher number of

connect ions are also possible, such as trifurcated feeders for three loads etc. IndexB

Bolted Fault

A short circuit condition that assumes zero impedance exists at t he point of the fault, as if aheavy bus bar was solidly bolted across the point o f the fault. This assumption is typically usedto calculate the worst-case fault current levels.

This term can also imply a fault that does not burn away in any reasonable t ime, but must beactually removed or repaired. IndexB

Burden (of a Relay)

The load impedance imposed by a relay on its source, expressed in ohms and phase angle atspecif ied conditions.

Modern solid state protective relays have very small burdens, while some olderelectromechanical relays had signif icant burdens, requiring the use o f high output instrumenttransformers to drive them. IndexB

Bus

A conductor, or group of conductors, which serve as a common connect ion for two or morecircuits. In practice, bus generally refers to the main power bus in the switchgear, but it couldalso refer to a data bus, or ground bus etc. IndexB
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Bus Duct

See Metal-enclosed Bus, Isolated Phase Bus Duct. IndexB

Bushing

An insulat ing structure including a through conduct or, or a passageway for such a conduct or,with provision for mount ing on a barrier (conducting or insulating), used for the purpose of

insulat ing the conductor from the barrier and conducting current f rom one side of the barrier tothe other.

Bushings may use porcelain, epoxy or ot her type of insulat ing material in the structure, besmoot h surfaced or with sheds to increase the creepage distance. In metal-clad switchgear,bushing is somet imes the term used to describe the assembly of the primary disconnect system(the t erm spouts is also used). IndexB

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C

Cartridge Fuse

A low voltage fuse consist ing of a current-responsive element inside of a fuse tube withconnect ion terminals on both ends. Commonly used to protect control circuits in theswitchgear LV cont rol cabinet. IndexC

Cassette

An assembly which provides all the necessary mechanical and electrical interlocks, circuitbreaker supports, shutter mechanism, racking mechanism etc. for a withdrawable circuitbreaker, and which comprises nearly the ent ire circuit breaker compartment. It st ill requires thefront door to be added, and the other switchgear compartments as well (low voltage, main bus,cable etc.).

Typically supplied to OEM switchgear manufacturers when t hey purchase a circuit breaker froma primary manufacturer so that they do not have to design all of these componentsthemselves. See also Cradle. IndexC

Closing Time

The amount of t ime from the init iat ion of the closing operation and the instant when metalliccont inuity is established in all phases. See also Opening Time, Arcing Time. IndexC

Circuit Breaker

A mechanical switching device, capable of making, carrying and interrupting currents undernormal circuit conditions, and also making and carrying for specified time and interruptingcurrent under abnormal circuit conditions such as those occurring during short circuits.

Many diff erent methods of achieving this functionality have been developed over the years,such as Air-blast, Air-magnetic, Minimum Oil, Bulk Oil, etc., but most modern medium voltageapplications today use either SF6 or Vacuum methods. Circuit breakers are usually intended tooperate infrequently, although some types are made to operate frequently. IndexC
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Circuit Switcher

A mechanical switching device with an integral interrupter, suitable for making, carrying andinterrupting currents under normal circuit conditions. It can also interrupt specified short circuitcurrents which may be less than its close and latch, momentary and short t ime current ratings. Itis usually less expensive than a circuit breaker, but does not achieve the same performancelevel. IndexC

Clearing Time

For a circuit breaker, it is the interval between the t ime the actuating quantity in the main circuitreaches the value causing actuation of the release and the instant o f f inal arc extinction on allpoles of the primary arcing contacts. It is numerically equal to t he sum of the contact partingtime plus the arcing time.

For a fuse, it is the interval between a specif ied overcurrent to the f inal circuit interruption atrated maximum voltage. It is equal to the sum of the melt ing time plus the arcing t ime. IndexC

Close And Latch

The rat ing of the ability of a circuit breaker or other switching device to close against theelectromagnetic forces (which are t rying to force the contacts open), to latch into the closedposition, and to carry a specif ied current through the device under specified conditions. This ismost ly related to ensuring the operat ing mechanism has suff icient mechanical energy toovercome the electromagnetic forces. IndexC

Close-Open Operation

A close operation of the switching device, followed immediately by an opening operat ion,

without introducing any deliberate t ime delay. It is commonly shown as CO.

Closing Coil

A coil in the electromagnet that supplies power fo r closing the device, such as the circuitbreaker. It must be specif ied to have the correct control voltage rating for the installat ion. IndexC

Compartment

Inside of a cell of metal-clad switchgear, there are dif ferent and separate compartments usedto segregate diff erent components and funct ions from one another, to improve reliability andimprove safety. Each compartment is separated by a grounded metal barrier. Standardcompartments are low voltage, circuit breaker, main bus and cable, with o ther compartmentspossible as the design demands. IndexC

Conformance Tests

Those tests made to demonstrate compliance with the applicable standards, such as CSA,ANSI or IEC. These tests are generally performed af ter the test item has been subjected to allof the required production tests. Demonstration o f margin (capability) beyond t he requirements

of the standards is not required. See also Design Tests, Product ion Tests. IndexC

Connected Position (of a Circuit Breaker)

That position of the removable element in which both the primary and secondary disconnecting
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devices are in full contact. See also Disconnected Posit ion, Test Posit ion. IndexC

Contactor

A cont actor is (usually) a magnetically operated switch for repeatedly establishing andinterrupting an electric power circuit. It may also include a mechanical latching system to holdthe contactor closed without requiring a continuous power supply.

A typical applicat ion is mot or start ing. It differs from a circuit breaker in that it will withstand avery high number of operat ions (300,000 or more compared to 10,000 for a circuit breaker), hasgenerally lower continuous current ratings than ANSI circuit breakers and has lower interruptingand making capacit ies. It is not intended to interrupt fault current on its own and is thereforeof ten used in association with f uses or a circuit breaker. See also Circuit Breaker. IndexC

Control Power Transformer (CPT)

A transformer used to supply cont rol power to switchgear. It is connected to the mainswitchgear bus as its power source, is usually single phase only, and is usually 120 volts out put(other voltages are possible). It is of ten a VT located in a standard VT drawer, but may be asmall power t ransformer if a suff icient ly high power output is required. IndexC

Control Relay

An auxiliary relay whose funct ion is to initiate or permit the next desired operat ion in a controlsequence. IndexC

Control Voltage

The voltage of the power supply used to energize a device in the switchgear. Typical voltages

are 24, 48, 60, 110, 125, 220 or 250 vo lts DC, and 120, 127, 220 or 250 volts AC. IndexC

Control Wiring

See Secondary and Cont rol Wiring. IndexC

Conversion (of Switchgear)

A general term pertaining to the process of altering existing power switchgear equipment.Conversion can involve as litt le as replacing the main or arcing contact t ips with other than theoriginal design material, to replacing the ent ire arc-interrupting st ructure.

Changeover of an existing air-magnetic circuit breaker to an SF6 or vacuum design is properlycalled a conversion (not retrof it as is commonly used). See also Recondit ioning. IndexC

Corona

An electrical discharge in air that only partially bridges the gap between two or moreelectrodes. Corona leads to power loss (although this is fairly minor at medium voltage) andcauses radio interference, as well as leading to physical damage of the insulat ion over time if itis severe enough.

Corona can be tested by measuring the discharge magnitude using a radio receiver, and this isthen referred to as a Radio Influence Voltage (RIV) Test. See also Partial Discharge. IndexC

CPT
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See Control Power Transformer. IndexC

Cradle

An assembly which provides all the necessary mechanical and electrical interlocks, circuitbreaker supports, shutter mechanism, racking mechanism etc. for a withdrawable circuitbreaker, which is intended in turn to be installed inside of a switchgear cell manufactured byothers.

Typically supplied to OEM switchgear manufacturers when t hey purchase a circuit breaker froma primary manufacturer so that they do not have to design all of these componentsthemselves. See also Cassette. IndexC

Creepage Distance

The shortest distance along the surface of an insulato r from one conducting part t o another,which may be energized or at ground potential. When designing the device, the CreepageDistance must be long enough that it will withstand the highest voltage foreseen for the

installat ion within the specif ied conditions (such as pollut ion or moisture). See also StrikeDistance. IndexC

CSA Standards

CSA is the acronym for the Canadian Standards Associat ion. The National Standards ofCanada are published and cont rolled by the Canadian Standards Association. CSA has bothmandatory standards and performance standards.

There are mandatory standards published relat ing to switchgear, which are safety related. IndexC

Current Injection

A synthet ic test method where current is injected into a circuit in order to determine theoperat ing characteristics of o ther components connected to it. For example, it can be used toinject primary current into a current t ransformer, to monitor what happens to secondarydevices attached to the current transformer secondary. IndexC

Current Limiting Fuse (CLF)

A fuse unit that when it is melted by a current within its specif ied current limit ing range, abruptlyintroduces a high resistance to reduce the current magnitude and duration. CLFs are used tolimit damage to equipment by dramatically reducing the energy let-though of the fuse. IndexC

Current Transformer

A transformer specially designed to accurately represent t he primary current in a secondarycircuit. Typically in North America, current transformers are standardized to be 5 amperes whenrated current is passed through the primary circuit , although other values are possible. IndexC

Current t ransformers can be used to power or provide information to such equipment as

metering devices, or protective relays.

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D

Dead-tank

A switching device in which a vessel at ground pot ent ial surrounds and cont ains the interrupterand insulat ing medium. Metal-enclosed and metal-clad switchgear are obviously of dead f rontdesign, as are station breakers. IndexD

Dead Time

Usually referencing a circuit breaker on a reclosing operation, Dead Time refers to the intervalbetween interruption on all poles on the opening stroke, and the re-establishment of the circuiton the reclosing stroke. IndexD

Definite Time Relay

A relay in which the operat ing is substant ially constant regardless of the input quant ity. SeeInverse Time. IndexD

Derating Factors

There are a number of derating factors possible for switchgear. The two most common arerelated to alt itude above sea level. At high elevations, the breakdown strength of air decreases,and its ability to remove heat also decreases.

Other derat ing factors may involve high ambient temperatures, high harmonics in the powersystem etc.:

Voltage: At and below 1000 meters, the normal rating applies. At 1500 meters elevation,

the voltage must be reduced to 95% or clearances increased accordingly. At 3000 meters,the voltage must be reduced to 80% or clearances increased accordingly. Intermediatevalues may be obtained by interpolation. Surge arresters are strongly recommended atany elevation above 1000 meters.

Current: At and below 1000 meters, the normal rating applies. At 1500 meters elevation,the current must be reduced to 99% or conductor cross sections increased accordingly.

At 3000 meters, the current must be reduced to 96% or conductor cross sectionsincreased accordingly. Intermediate values may be obtained by interpolation.

IndexD

Design Tests

Those tests made to determine the adequacy of a part icular type, style or model of equipment,with all of its component parts, to meet its assigned ratings and to operate sat isfactorily underits normal service conditions, or under special service conditions if specified.

These tests are usually performed on representative apparatus, and are not intended to beperformed on a regular production basis. These tests are sometimes called Type Tests. Seealso Conformance Tests, Product ion Tests. IndexD

Differential Protection

A method of prot ect ing apparatus in which an internal fault is ident ified by comparing theelectrical conditions on the terminals of the apparatus. In the case of busbar dif ferentialprotection, it is very straight forward (the current that goes in must exactly equal the current
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that goes out).

In the case of motors or transformers, then special consideration must be given to inrushcurrents, and in the case of t ransformers, to t he turns ratio and winding connections as well.IndexD

Direct Acting Overcurrent Trip Device

A release or t ripping system that is completely self contained on a circuit breaker and requiresno external power or control circuits to cause it to f unct ion. IndexD

Disconnected Position (of a Circuit Breaker)

That position in which the primary and secondary disconnecting devices of the removableelement are separated by a safe distance from the stationary element contacts. SeeConnected Position, Test Position. IndexD

Disconnecting Switch

Same as Isolating Switch. A mechanical switching device used for changing the connect ions in acircuit , or fo r isolating a circuit or equipment from the source of power.

It is required to carry normal load current continuously, as well as abnormal or short circuitcurrents for short t imes as specif ied. It is required to open or close circuits when negligiblecurrent is interrupted or made. IndexD

Dual Element Fuse

A fuse having responsive elements of two dif ferent f using characteristics in a single series of

fuse. See also Single Element Fuse. IndexD

Duty Cycle

A prescribed sequence of operations for a specific time, with specif ic t ime intervals betweensequences. For example, a motor may only be allowed to have a certain number of starts perhour, due to the overheating effect of the high starting current.

The motor must be given time to cool of f between starts in order to maintain the normal lifeexpectancy. IndexD

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E

Earth Fault Protection

Same as Ground Fault Protection. Earth is typically the terminology used in Europe, Groundbeing used in North America. IndexE

Electrically Trip Free

A term applied to electrically operated switching devices such as circuit breakers, indicat ing thatthe opening release can open the device even though the closing release circuit is energized.
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Electrically trip free devices are also usually designed with an anti-pumping relay, so that theclosing mechanism will not reclose t he switching device after opening until the closing controlcircuit is opened and again closed. IndexE

Electromagnetic Compatibility (EMC)

A measure of the tolerance of equipment to external electromagnet ic f ields. It is very importantfor electronic devices (for example) that are used in power equipment t o have a high tolerance

to electromagnetic fields. IndexE

Expulsion Fuse

A vented fuse or fuse unit in which the expulsion effect of gases produced by the arc and thelining of the fuse holder, either alone or aided by a spring, extinguishes the arc. IndexE

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FFeeder

A section of a switchgear assembly which supplies a load. A simple switchgear assemblytypically consists of an incomer (supplying power to the switchgear), a number of feeders(supplying power to various loads so that they can be individually switched and/or protected),and perhaps a tie (to connect t o another switchgear assembly). IndexF

Ferroresonance

An electrical resonant condition associated with the saturation of a ferromagnet ic device, suchas a transformer, through capacitance.

Ferroresonance can arise, for example, when a weak source is isolated with a lightly loadedfeeder cont aining power cables or power facto r correction capacitors. Ferroresonance canproduce dangerously high voltages which can cause equipment damage or failure. IndexF

Fuse

An overcurrent prot ective device with a circuit opening fusible part that is heated and severed

by the passage of current through it.

The term fuse comprises all the parts that form a unit capable of performing the prescribedfunction, but it may or may not be the complete device necessary to connect it into an electriccircuit. See Cartridge Fuse, Current Limiting Fuse, Fuse Link, Expulsion Fuse.

Fuse Link

A replaceable part or assembly, comprised entirely or principally of the conduct ing element,required to be replaced after each circuit interruption t o restore the fuse to operatingconditions. IndexF

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G
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GIS Gas Insulated Substation

GIS equipment is a type of metal-clad switchgear const ruction, where all the switchgear powercomponents are located inside of a sealed metal envelope filled with (usually) SF6 gas.

GIS equipment is of ten supplied into high voltage applications, but there are now of ferings atmedium voltage. GIS has small space requirements, is impervious to atmospheric contaminantsand no derating is required for high elevations. IndexG

Ground and Test Device (GATD)

A term applied to a switchgear accessory device that can be inserted in place of the drawoutcircuit breaker for t he purpose of either grounding the main bus and/or the external circuitsconnected to the switchgear assembly, and/or primary circuit testing.

A normal GATD can carry the same full fault current of the switchgear assembly for a specifiedt ime. Opt ionally, a modified circuit breaker can be used to provide the GATD functions and inaddit ion allow full fault switching capability. IndexG

Ground Bus

A bus to which the grounds f rom individual pieces of equipment are connected, and that in turnis connected to ground at one or more points. Typically in metal-enclosed switchgear, theground bus is not insulated, and is extended into every cell and every compartment in each cell(except the main bus compartment). IndexG

Ground Fault Protection

A method of prot ect ion in which faults to ground within the protected equipment are detectedand action taken (alarm, trip etc.). Ground faults are usually detected using zero-sequencecurrent t ransformers as the sensors.

Grounding Switch

A mechanical switching device by means of which a circuit or piece of apparatus may beelectrically connected to ground.

The switch may or may not have fault making capabilities. Integrated grounding switches arecommon in European switchgear designs, but are rare in North American designs, although their

popularity is slowlygrowing. IndexG

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H

Handling Device

That accessory used with metal-clad switchgear, used for the removal, replacement ortransportation of the removable element (e.g. circuit breaker). Not all metal-clad designs requirethe use of a Handling Device to move the circuit breakers, but many do. IndexH

High Speed Relay
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In current pract ice, a relay that operates in less than 50 ms (milliseconds), which is less thanthree cycles on a 60 Hz system. IndexH

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I

IEC Standards

The acronym for t he International Electrotechnical Commission.

This series of standards is produced by the IEC, and are typically used in most count ries of theworld, but not widely in North America. Each country may adapt IEC standards to their ownneeds, especially in the past, but it is now preferred that the basic standard be applied as is,where possible. When it comes to switchgear and circuit breaker design and rat ings, the twomain global standards are IEC and ANSI.

The world is very slowly moving towards adopt ing IEC standards, but ANSI and IEC are diff erentenough that t his is not easy to do. IndexI

Interlock

A device actuated by the operation of some other device with which it is directly associated, togovern succeeding operations of the same or allied devices. Interlock systems are a series ofinterlocks applied to associated equipment in such a manner as to prevent or allow operat ionof the equipment only in a prearranged sequence.

There are three main types o f interlocks, namely mechanical, electrical and key. Mechanical and

electrical interlocks are always present in some way in switchgear to provide for safe operationof the basic equipment. Key interlocks are typically more of an opt ional accessory used toprovide customer prescribed operation sequences. IndexI

Interrupter Switch

An air insulated switch, equipped with an interrupter, for making or interrupting specifiedcurrents, or both. The nature of the current being made or interrupted is indicated by a suitableprefix, such as load interrupter switch, fault interrupter switch, capacitor current interrupterswitch, etc. IndexI

Inverse-time Relay

A relay in which the input quant ity and operating t ime are inversely related throughout at least asubstant ial port ion of the performance range. Various adjectives are commonly used toillustrate the level of inverseness of the operating characteristics, such as standard, very,extremely, ultra etc. See also Definite Time Relay. IndexI

Isolated Phase Bus

A bus in which each phase conductor is enclosed by an individual metal housing separated from

adjacent conductor housings by an air space. The bus may be self cooled or may be forcedcooled, using gas or liquid. Isolated phase bus is commonly used at very high current rat ings,such as the output o f large generators. IndexI

Isolating Switch
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Same as Disconnect ing Switch. A mechanical switching device used for changing theconnect ions in a circuit , or for isolating a circuit or equipment from the source of power. It isrequired to carry normal load current cont inuously, as well as abnormal or short circuit currentsfor short times as specified. It is required to open or close circuits when negligible current isinterrupted or made. IndexI

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K

Key Interlock

See Interlock. IndexK

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L

Load Break Switch

See Load Interrupter Switch. IndexL

Load Interrupter Switch

An interrupter switch designed to interrupt currents which are not in excess of the cont inuouscurrent rating of the switch. It may be designed to close and carry abnormal or short circuit

currents as specified. Also colloquially referred to as a Load Break Switch. IndexL

Load Shedding

The process of deliberately removing preselected loads from a power system in response toabnormal conditions, in order to maintain the integrity of the power system.

Load shedding can also be used to reduce demand in a specif ied part of a power distributionsystem for economic reasons e.g. an industrial customer may shed specif ic loads t o keep theirkilowat t demand below a price increase threshold (kW demand is of ten priced based on thepeak usage in a calendar year, so keeping the demand below a certain level can aff ect a whole

years electrical billing). IndexL

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M

Main Contacts

The main continuous current carrying contacts in a current interruption device. Typically, the

interruption device is designed so t hat the main cont acts never see arcing themselves, thatfunction being provided by arcing contacts specif ically designed to do so. Main contacts aresometimes referred to as the Primary Cont acts. See also Arcing Contacts. IndexM

Main Connections
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Those that electrically connect toget her devices in the main circuit , or connect them to t he bus,or both. Also referred to as the Primary Connect ions. IndexM

Maintenance Interval

The period, which can be defined in terms of real t ime, operating t ime, number of operat ingcycles or a combination of these, during which satisfactory performance is expected withoutmaintenance or adjustment. IndexM

Margin

The dif ference between the demonstrated capability of the equipment and that required in-service for specific conditions.

Medium Voltage

The term medium voltage refers to voltages above 1,000 volts and below 69,000 volts. Thehighest voltage metal enclosed switchgear in general use in North America is generally 38 kV

class, although there is a small amount of 44 kV. Usually at 44 kV and above, switching isachieved by the use of air-insulated substat ions with discreet components. IndexM

Metal-clad Switchgear

Metal-clad switchgear is a specific type of metal-enclosed switchgear. While all metal-cladswitchgear is metal-enclosed, not all metal-enclosed switchgear can be classified as metal-clad.

Metal-clad is characterized by always having the following feat ures:

The main switching and interrupting device is removable (drawout), with a mechanism to

move it between connected and disconnected positions, and equipped with self-aligningand self-coupling primary disconnecting devices and disconnectable control wiringconnections.

Major parts o f the primary circuit are completely enclosed by grounded metal barriers thathave no intentional openings between compartments. A metal barrier shall be included infront of or part of the circuit interrupt ing device to ensure that when in the connectedposition, no primary circuit components are exposed by the opening of a door.

All live parts are enclosed within grounded metal compartments.

Automatic shut ters that cover primary circuit elements when the removable element is in

the disconnected, test or connected posit ions.

Primary bus conductors and connections are covered with insulating material throughout.

Mechanical interlocks are provided for proper operating sequence under normal operatingconditions.

Instruments, meters, relays etc. and their wiring are isolated by grounded metal barriersfrom all primary circuit elements (short lengths of wire such as CT secondary leads areexcepted).

The door through which the removable element is inserted may serve as an instrument or

relay panel, and may also provide access to t he cont rol compartment within the housing.

Note the proper spelling of metal-clad, with a dash and a lower case c.

IndexM
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Metal-enclosed Switchgear

Switchgear which is surrounded by a metal case or housing, usually grounded. Note the properspelling of metal-enclosed, with a dash and a lower case e. IndexM

Metal-enclosed Bus

An assembly of conductors with their associated connect ions, joints and insulating supports

within a grounded metal enclosure.

The conductors may be rigid or flexible (if the flexible conductors are individual cables, then itmay be called cable bus). There are three types of metal-enclosed bus :

Nonsegregated-phase bus, in which all phase conductors are in a common metalenclosure without barriers between phases. If associated with metal-clad switchgear, thenthe primary bus and connections shall be covered with insulating material equivalent tothe switchgear insulat ion system.

Segregated-phase bus, in which all phase conductors are in a common metal enclosure

but are segregated by metal barriers between phases.

Isolated-phase bus, in which each phase conduct or is enclosed by an individual metalhousing separated f rom adjacent conductor housings by an air space. The bus may beself cooled or may be forced cooled, using gas or liquid.

IndexM

Minimum Oil Circuit Breaker

A circuit breaker using a relatively small quant ity of mineral oil for

its arc-interruption method. A low cost but now obsolete technology. See also Circuit Breaker.IndexM

MOC

Mechanism Operated Contacts. A set of auxiliary switches generally cont aining both NO andNC contacts, operated by t he operat ing mechanism of a circuit breaker. They are used to signalthe current state and the change of state of the circuit breaker operating mechanism.IndexM

MTB

Mean Time Between Failures. The t ime interval, usually expressed in hours, that may beexpected between failures of a piece of operat ing equipment. IndexM

MTTR

Mean Time To Repair. The time interval, usually expressed in hours, that may be expected toreturn failed equipment to proper operat ion. IndexM

Mimic Bus

A single line diagram of the main connect ions of a system, const ructed on the face of aswitchgear or control panel or assembly. IndexM

Momentary Current
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The current f lowing in a device, an assembly or a bus at the major peak of the maximum cycleas determined by the envelope of the current wave. IndexM

Multi-pole Operation

A descript ive term indicat ing that all poles of the device are mechanically or electrically linkedsuch that they change state (open or close) substant ially simultaneously. IndexM

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N

Natural Frequency

A mechanical term, used to describe the f requency at which a body vibrates due to its ownphysical characterist ics (mass, shape) and electric restoring forces brought into play when t hebody is distorted in a specif ic direction and then released while restrained or supported at

specif ic points.

In general switchgear usage, generally important only fo r seismic analysis. IndexN

Nominal System Voltage

A nominal value assigned to designate a system of a given voltage class. For example, in the 15kV voltage class, a Nominal System Voltage may be 13.8 kV, or 12.47 kV, or other. IndexN

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O

Octave

The interval between two frequencies that have a frequency rat io of two. For example, 1 to 2Hz, 4 to 8 Hz, 64 to 128 Hz.

May be used in discussion of harmonics, or sound level spectrum analysis. IndexO

Opening Time

The amount of t ime between the moment t he actuating quantity of the release circuit reachesits operating value, and the instant the primary contacts have parted. Typically, any intentionalt ime delays which may be a part of normal operat ion are reduced to zero for the purposes ofdetermining opening time. IndexO

Operating Mechanism

As part of a switching device, the part of the mechanism that actuates all the main circuitcontacts of the switching device either directly or by the use of a pole-unit mechanism.

It must provide suff icient mechanical fo rce for the performance specified, and is designed tomeet t he specif ic requirements of the switching device (fo r example, for vacuum circuitbreakers, the operating mechanism is expected to provide only a small contact t ravel distance,but very high acceleration and deceleration). IndexO
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P

Partial Discharge

A localized electric discharge resulting from ionizat ion in an insulat ion system when the voltagestress exceeds the critical value. This discharge partially bridges the insulation betweenelectrodes.

It is commonly used as a test t o monitor the quality of manufacture of moulded or castcomponents (such as current or voltage t ransformers, or stand-o ff insulators). If the dischargeoccurs in air, then it may also be referred to as corona discharge. IndexP

Pickup (of a relay)

The action o f a relay as it makes designated response to progressive increase of input. As a

qualifying term, it is the state of a relay when all response to progressive increases of input hasbeen completed.

Pickup is also used to ident ify t he minimum value of an input quant ity reached by progressiveincreases that will cause the relay to reach the pickup stat e from reset. Pickup is also used todenote contact operat ion on relays having mult iple inputs, but in this case the pickup value ofany input in meaningful only when related to all other inputs. IndexP

Pilot Protection

A form of line protect ion that uses some sort of a communications channel as a means tocompare electrical conditions at the terminals of a line. The communications channel may bemetallic conductor, f iber opt ic cable, microwaves etc. IndexP

Pilot Wire Protection

Pilot protection in which a metallic circuit is used for t he communicating means between relaysat the circuit terminals. IndexP

Plating (of Joints)

Copper (or aluminium) forms an oxide layer which will commonly cause a joint t o have increasedresistance and a higher temperature rise. To obt ain sat isfactory lifet ime performance, the highpower joints in the main bus system are plated with another metal, usually silver. Silver is themost conduct ive of all metals, and provides good service with only a very thin plat ing thicknessrequired (f lash plat ing).

For very polluted environments, especially those containing high levels of sulphur, tin plating isused, as the silver will rapidly tarnish. While t in is very stable in most atmospheres, it naturallyprovides a higher resistance joint t han silver, and therefore it is only used where its non-corroding propert ies are really required. IndexP

Pole

That portion o f the device associated with one electrically separated conduct ing path o f themain circuit of the device. The poles may be separate for each phase, or may be combined intoa unified enclosure. IndexP
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Potential Transformer

An obsolete name for Voltage Transformer. See also Voltage Transformer.

Prestrike

The init iat ion of current between the contacts during a closing operation before the contacts

have mechanically touched. See also Restrike. IndexP

Primary

Used as an adject ive, primary can mean:

the f irst to operate e.g. primary arcing cont acts

the f irst in preference e.g. primary prot ection

the main circuit as opposed to auxiliary or cont rol circuits e.g. primary disconnect ing device

the energy input side of a transformer (not necessarily the high voltage side)

IndexP

Production Tests

Those tests made to check the quality and uniformity of the workmanship and materials usedin the manufacture of switchgear or its components. Perfo rmed on each switchgear that hasbeen manufactured. See also Conformance tests, Design Tests. IndexP

Proof

So constructed, protected or t reated so that successful operat ion is not interfered with whenthe device is subjected to t he specif ied material or condit ion.

For example, a sprinkler-proof switchgear can continue to operate if t he fire prot ectionsprinklers are operat ing. As anot her example, the reason that the term arc-proof switchgear isnot acceptable as a term, is that in the case of an internal arcing fault , the switchgear cannotusually continue to be used until clean-up and repairs are completed. See also Resistant. IndexP

Protection Relay

A relay whose funct ion is to det ect defect ive lines or apparatus, other power systemconditions of an abnormal or dangerous nature and to init iate appropriate control circuit action.

A prot ect ive relay may be described according to its input quant ities (e.g. negat ive phasesequence relay), operating principles (e.g. differential protection) or performance characteristics.IndexP

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Q

Quick-break
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A term used to describe a device that has a high cont act opening speed which is independentof the operator. IndexQ

Quick-make

A term used to describe a device that has a high cont act closing speed which is independent ofthe operator. IndexQ

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R

Racking (a Circuit Breaker)

The process of moving a circuit breaker or ot her removable element f rom the disconnectedposit ion to the test position (if the test position is a dif ferent physical position) and into theconnected posit ion, or vice-versa. Racking may be performed manually with the door open (rare

in a modern switchgear design), manually with the door closed (referred to as through-the-doorracking) or electrically with the door closed.

For safety reasons, through-the-door racking systems are preferred, and are essentiallyrequired for arc-resistant designs. IndexR

Rated Operation Sequence

A standard operat ing sequence that a circuit breaker has been tested in accordance with.Under ANSI testing rules, the Rated Operat ion Sequence is a close-open operat ion, followedby a 15 second delay to allow the operat ing mechanism to recharge, and then a second close

open operation. This is represented by CO 15s -CO. Note that a fully tested ANSI circuitbreaker has been tested for many other operation sequences as well.

Under IEC test ing rules, there are three common Rated Operation Sequences. The f irst is anopen operation, fo llowed by three minutes for cooling of the contacts, fo llowed by a COoperat ion, then 3 minutes, then a f inal CO (O 3 min CO 3 min CO). The second is a O 0.3s -CO 15 s CO. The third is O 0.3 s -CO 3 min CO. IndexR

Reconditioning (of switchgear)

A general term covering the process of maintaining exist ing switchgear equipment in operatingcondition as recommended by the manufacturers instructions, using only the originalmanufacturers recommended replacement parts, without altering the original design. See alsoConversion (of Switchgear). IndexR

Recovery Voltage

The voltage that appears across the terminals of a pole of a circuit interrupting device uponinterruption o f the current. See also RRRV.IndexR

Refurbishment (of Switchgear)

See Reconditioning (of Switchgear). IndexR

Removable Element (of a Switchgear Assembly)
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The port ion that normally carries the circuit switching and circuit interrupting devices and theremovable part of the primary and secondary disconnecting devices. In metal-clad switchgear,generally the circuit breaker is the removable element . IndexR

Renewal Parts

Those parts that must be replaced during maintenance as a result o f wear. IndexR

Resistant (used as suff ix)

So constructed, prot ected or t reated that damage will not occur readily when the device issubjected t o the specif ied material or condition. See Arc-resistant Switchgear, Proof. IndexR

Restricted Ground Fault

Also called Restricted Earth Fault in Europe. Restricted refers to the zone that is beingprot ected, just as diff erential protection operates over a defined prot ection zone. It is of tenused to protect a specif ic machine such as a motor. See also Zone of Prot ection. IndexR

Restrike

A resumption of current bet ween the cont acts of a switching device during an openingoperat ion after an interval of zero current of 1/4 cycle at normal f requency or longer. It ispreferable that restrikes do not occur as they can stress the at tached equipment by causinghigh recovery voltages. See also Prestrike. IndexR

RIV

Radio Inf luence Voltage. Please refer to Corona, Partial Discharge. IndexR

Rowgowski Coil

Essentially an air cored current t ransformer which, because it does not have a magnetic core,has a linear response f rom zero current to very high values (well beyond what can be seen inany short circuit fault current in a power system) and will not saturate.

Rowgowski coils provide a voltage output directly related to the current passing through theprimary (e.g. 1 mV/A). Rowgowski coils measure only the AC component of the current, not theDC component . The manufacture of a good Rowgowski coil requires very precise winding ofthe coil. If the coil is wound properly, then the output does not depend on the posit ion of thecentral conductor in the coil window.

Generally, the output of a Rowgowski coil is connected to an integrator circuit which providesan output waveform. IndexR

Routine Tests

See Production Tests.

RRRV

Rate of Rise of Recovery Voltage. When current is interrupted quickly, a recovery voltage isgenerated, and the faster the rate of change of current, the higher the recovery voltagebecomes. It is important for a switching device to reform the dielectric breakdown st rength ofits interrupting medium quickly following the breaking of the current, in order to be able to
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withstand the recovery voltage generated.

A switching device that can withstand a high recovery voltage will be less likely to rest rike. Seealso Recovery Voltage, Restrike.IndexR

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S

Seal-in Relay

An auxiliary relay that remains picked up through one of its own contacts, which bypasses theinit iating circuit until deenergized by some other device.

For example, if a pressure switch is temporarily closed due to a momentary pressure surge, aseal-in relay can be used to show it had tripped though an annunciator panel, until the operatormanually resets it. IndexS

SCADA

An acronym for Supervisory Control And Data Acquisit ion. A system operat ing with codedsignals over communication channels so as t o provide control of remot e equipment, typicallyusing one communications channel per remote station.

The supervisory system may be combined with a dat a acquisit ion system by adding the use ofcoded signals over communication channels to acquire information about the status of theremot e equipment for display or recording functions.

Typical supervisory control functions are :

Alarm funct ions

Analogue funct ions

Cont rol functions

Indication or status funct ions

Sequence of events funct ion

IndexS

Secondary

Used as an adjective, secondary can mean :

operates af ter the primary device e.g. secondary arcing contacts

second in preference

referring to auxiliary or control circuits as contrasted with the main circuit e.g. secondarydisconnecting devices, secondary and control wiring

referring to the energy output side of t ransformers (which does not have to be the lowvoltage side)

IndexS
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Secondary and Control Wiring

Wire used with switchgear assemblies for control circuits and for connections betweeninstrument transformer secondaries, instruments, meters, relays, or other equipment. IndexS

Secondary Disconnecting Device

Self-coupling separable contacts provided to connect and disconnect the auxiliary and control

circuits between the removable element and the housing. IndexS

Sequence Currents

The set of currents that when combined, accurately and fully represent t he to tal current in apolyphase system. Consists of positive sequence, negative sequence and zero sequencecurrents. IndexS

SF6

Sulphur Hexaflouride, a gaseous dielectric for high and medium voltage power application, usedas an insulator and/or interrupting medium. The highly stable SF6 molecule consists of onecentral atom of sulphur (S) surrounded by 6 atoms of Flourine (F). IndexS

Short Circuit

An abnormal connect ion, including an arc, of relatively low impedance, whether madeaccidentally or deliberately, between two or more points of dif ferent potential. IndexS

Short Line Fault

A fault which occurs close to the terminals of the circuit switching device may cause a dif ferentrecovery voltage waveform to be generated, essent ially adding a high frequency sawtoo thwaveform to the voltage waveform. As the distance to the fault increases, the amplitude of thesawtoot h component increases, but t he rate of rise decreases, and the fault currentdecreases.

The increased amplitude of the voltage negatively affects the interrupting capability of thecircuit switching device while the decrease in rate of rise and the decrease in current makesinterruption easier. Since the effects are not proportional, a distance is reached where theaffect on interruption is most severe, even though the current is less than for a terminal fault .The critical distance depends on the type of interrupting medium used (air, oil, SF6 or vacuum)and with t he particular design of device.

At higher voltages, the critical distance may be on the order of 1.5 km, while at mediumvoltages the distance is less. See also Terminal Fault. IndexS

Shunt Release

A release, typically used to t rip a circuit breaker or switch, which is energized by a source ofvoltage, which may be derived f rom either the main circuit or f rom an independent source. Thevoltage is applied only when t ripping is desired. See also Undervoltage Release. IndexS

Shutter

A device that is automatically operated to completely cover the stat ionary port ion of theprimary disconnecting devices when the removable element is either in the disconnected
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position, the t est posit ion, or has been completely removed. Grounded metal shutters areusually preferred for safety reasons, but insulating materials may also be used in someapplications. IndexS

Single Element Fuse

A fuse having a current responsive element comprising one or more parts with a single fusingcharacteristic. See also Dual Element Fuse. IndexS

Sound Level

A weighted Sound Pressure Level obtained through the use of a metering characterist ic and aweight ing algorithm.

Typically, the A weighting is used which represents the sensitivity of an average human ear,based on the hearing ability of a young adult. Other weighting scales (B and C) are available butare not of ten used.

Essentially, a weighting of C is the same as the Sound Pressure Level itself. The designation of

the weighting scale used is indicated following the decibel symbol e.g. weighting scale A isindicated as dBA. IndexS

Sound Pressure Level (SPL)

Twenty times the logarithm to the base 10 of the ratio of the pressure of a sound to thereference sound pressure of 20 micro Pascals (mPa), expressed as an rms value. The unit isdecibel (dB). See also Sound Level. IndexS

Spouts

The insulator/bushing system at the back of the circuit breaker compartment in drawoutswitchgear, which allow t he main (primary) connections of the circuit breaker to be plugged intothe bus system (the main bus on one connection, the feeder bus on the other). Sometimesreferred to as the primary disconnects. See also Bushing. IndexS

Station Ground

A ground grid or any equivalent system of grounding electrodes buried beneath or adjacent toa substation that determines the rise of ground voltage level relative to remote earth andcontrols the distribution o f voltage gradients within the substat ion during a fault .

The characterist ics of the Station Ground are linked to t he type, shape, number andcomposit ion of the ground electrodes, the type of soil, the moisture content of the soil etc.IndexS

Stationary Contact

A conduct ing part having a cont act surface that remains substant ially stat ionary during use.IndexS

Strike Distance

The shortest distance measured though the insulating medium between parts of differentpolarit ies. These parts may be conduct ing or insulat ing. The insulat ing medium is commonly air,but could be oil or SF6 etc. See also Creepage Distance. IndexS
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Sulphur Hexaf louride (SF6)

See SF6. IndexS

SF6 Circuit Breaker

A circuit breaker that uses SF6 gas for its interrupting method. See also Circuit Breaker. IndexS

Supervisory Control and Data Acquisition (SCADA)

See SCADA. IndexS

Surge Arrester

A device used to limit t he severity of a voltage surge in a power system. In its simplest form, itcould be a spark gap, however a more repeatable (and less stress-inducing) method ispreferred. The f irst modern style of surge arrester used silicon carbide (SiC) discs in series witha small spark gap.

When the voltage became too high, the spark gap flashed over and the SiC discs conductedand absorbed excess energy due to their inverse resistance (as the voltage across themincreased, their resistance decreases).

In the early 1980s, modern zinc oxide (ZnO) discs were introduced and as have such anextreme inverse characteristic, no spark gap is usually required. ZnO surge arresters aresometimes referred to as metal-oxide type. IndexS

Switchboard

A type of switchgear assembly that consists of one or more panels with electric devicesmounted thereon, and an associated framework. IndexS

Switchgear

A general term which covers switching and interrupting devices and their combinat ion withassociated control, metering, prot ective and regulating devices.

Also, assemblies of these devices with their associated interconnections, accessories,enclosures and support ing st ructures used primarily in connection with the generation,transmission, distribution and conversion of electric power. IndexS

glossary of medium voltage switchgear terms

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