Prefixes

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 600 Volt

A

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 American National Standards Institute, 1430 Broadway, New York, NY 10018

ANSI Standards
The American National 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 other countries as well, including Canada. The electrical engineering standards of ANSI are often written for them by IEEE, the Institute of Electrical and Electronics Engineers.

ANSI Device Function Numbers
Defined in ANSI standard C37.2, the Device Function Numbers are a world-wide standardized system for describing the function of many protection and control components within switchgear.

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. This designation actually covers two types of overcurrent protection, 50 which is instantaneous protection, and 51 which is a time overcurrent protection.

Various suffixes can be used, which are sometimes context sensitive. For example, 87 represents differential protection in general, while 87T is specifically transformer differential protection and 87G is specifically generator differential protection. However, 50G refers to instantaneous 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.

 Anti-Pumping Device

Anti-Pumping Relay

Anti pumping relay is device used in association with a circuit breaker to prevent the circuit breaker from reclosing after an opening operation, as long as the device initiating the reclosing is 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.

 A device that operates to open all phases of a circuit by means of a polyphase switching device, in response to the interruption of the current in one phase. For example, the striker pin of an expulsion fuse can be used to actuate a common tripping bar which causes a three phase load break switch to open.

 A structure used as part of a current interrupting device, affording a confined space or passage, lined with arc-resisting material, through which an arc is directed in order to cause it to extinguish.

In medium voltage switchgear, this was the main current interrupting means in the air-magnetic 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.

 A type of switchgear design which is designed to withstand the effects of an internal arcing fault, without causing harm to personnel who are located in defined areas. It is not intended to withstand these internal arcing fault without possibly causing physical damage to the structure and/or components, but often the physical damage is less with an arc-resistant design.

There are three classes of protection:
• Type A eliminates the emission of gases and particles from the front of the switchgear during an internal arcing fault,
• Type B eliminates the emission of gases and particles from the front and sides of the switchgear during an internal arcing fault,
• Type C eliminates the emission of gases and particles from the front and sides of the switchgear, from between compartments within the same cell, and between adjacent cells during an internal arcing fault.

Arc-resistant switchgear has traditionally been metal-clad, but the basic concept could also be applied to other types of switchgear as well. See also Proof, Resistant.

B

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 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 deenergized with the main incoming device turned off, but is actually being backfed, for example, through the secondary of a transformer.

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

 A reference impulse insulation 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 overall insulation strength of the design, with other test levels (e.g. applied tests) related in a standard method to the BIL.

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

 A short circuit condition that assumes zero impedance exists at the point of the fault, as if a heavy bus bar was solidly bolted across the point of the fault.  This assumption is typically used to calculate the worst-case fault current levels.

This term can also imply a fault that does not burn away in any reasonable time, but must be actually removed or repaired.

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

Modern solid state protective relays have very small burdens, while some older electromechanical relays had significant burdens, requiring the use of high output instrument transformers to drive them.

 An insulating structure including a through conductor, or a passageway for such a conductor, with provision for mounting on a barrier (conducting or insulating), used for the purpose of insulating the conductor from the barrier and conducting current from one side of the barrier to the other.

Bushings may use porcelain, epoxy or other type of insulating material in the structure, be smooth surfaced or with sheds to increase the creepage distance. In metal-clad switchgear, bushing is sometimes the term used to describe the assembly of the primary disconnect system (the term “spouts” is also used).

C

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

 A device that can be used to manually open or close a circuit, and to automatically open a circuit at a predetermined level of over current without damage to itself.

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

Many different 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 voltage applications today use either SF6 or Vacuum methods. Circuit breakers are usually intended to operate infrequently, although some types are made to operate frequently.

 For a circuit breaker, it is the interval between the time the actuating quantity in the main circuit reaches the value causing actuation of the release and the instant of final arc extinction on all poles of the primary arcing contacts. It is numerically equal to the sum of the contact parting time plus the arcing time.

For a fuse, it is the interval between a specified overcurrent to the final circuit interruption at rated maximum voltage. It is equal to the sum of the melting time plus the arcing time.

 A contactor is (usually) a magnetically operated switch for repeatedly establishing and interrupting an electric power circuit.  It may also include a mechanical latching system to hold the contactor closed without requiring a continuous power supply.

A typical application is motor starting.  It differs from a circuit breaker in that it will withstand a very high number of operations (300,000 or more compared to 10,000 for a circuit breaker), has generally lower continuous current ratings than ANSI circuit breakers and has lower interrupting and making capacities.   It is not intended to interrupt fault current on its own and is therefore often used in association with fuses or a circuit breaker.  See also Circuit Breaker.

 A general term pertaining to the process of altering existing power switchgear equipment. Conversion can involve as little as replacing the main or arcing contact tips with other than the original design material, to replacing the entire arc-interrupting structure.

Changeover of an existing air-magnetic circuit breaker to an SF6 or vacuum design is properly called a conversion (not retrofit as is commonly used). See also Reconditioning.

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

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

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

Typically supplied to OEM switchgear manufacturers when they purchase a circuit breaker from a primary manufacturer so that they do not have to design all of these components themselves. See also Cassette.

 CSA is the acronym for the Canadian Standards Association. The National Standards of Canada are published and controlled by the Canadian Standards Association.  CSA has both mandatory standards and performance standards.

There are mandatory standards published relating to switchgear, which are safety related.

 A transformer specially designed to accurately represent the primary current in a secondary circuit. Typically in North America, current transformers are standardized to be 5 amperes when rated current is passed through the primary circuit, although other values are possible.

Current transformers can be used to power or provide information to such equipment as metering devices, or protective relays.

D

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 dB

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

Other derating factors may involve high ambient temperatures, high harmonics in the power system 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. Intermediate values may be obtained by interpolation. Surge arresters are strongly recommended at any 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 sections increased accordingly. Intermediate values may be obtained by interpolation.

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

These tests are usually performed on representative apparatus, and are not intended to be performed on a regular production basis. These tests are sometimes called Type Tests. See also Conformance Tests, Production Tests.

 A method of protecting apparatus in which an internal fault is identified by comparing the electrical conditions on the terminals of the apparatus. In the case of busbar differential protection, it is very straight forward (the current that goes in must exactly equal the current that goes out).

In the case of motors or transformers, then special consideration must be given to inrush currents, and in the case of transformers, to the turns ratio and winding connections as well.

 A prescribed sequence of operations for a specific time, with specific time intervals between sequences. For example, a motor may only be allowed to have a certain number of starts per hour, due to the overheating effect of the high starting current.

The motor must be given time to cool off between starts in order to maintain the normal life expectancy.

E

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 A term applied to electrically operated switching devices such as circuit breakers, indicating that the opening release can open the device even though the closing release circuit is energized.

Electrically trip free devices are also usually designed with an anti-pumping relay, so that the closing mechanism will not reclose the switching device after opening until the closing control circuit is opened and again closed.

F

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 Fault Current

 An electrical resonant condition associated with the saturation of a ferromagnetic device, such as a transformer, through capacitance.

Ferroresonance can arise, for example, when a weak source is isolated with a lightly loaded feeder containing power cables or power factor correction capacitors. Ferroresonance can produce dangerously high voltages which can cause equipment damage or failure.

 An overcurrent protective 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 prescribed function, but it may or may not be the complete device necessary to connect it into an electric circuit. See Cartridge Fuse, Current Limiting Fuse, Fuse Link, Expulsion Fuse.

G

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

GIS equipment is often supplied into high voltage applications, but there are now offerings at medium voltage. GIS has small space requirements, is impervious to atmospheric contaminants and no derating is required for high elevations.

 Glare is the sensation produced by luminances within the visual field area sufficiently greater than the luminance to which the eyes are adapted.  This causes annoyance, discomfort or loss in visual performance and visibility. The magnitude of the sensation of glare depends upon such factors as the size of the task, relative position of the light and the viewer, the luminance of a source, the number of sources and the luminance to which the eyes are adapted.

The four most common types of glare are:
* Direct glare
* Indirect (reflected) glare
* Disability glare
* Discomfort glare

Bright areas, such as luminaires, ceilings and windows that are directly in the field of view cause direct glare.  Indirect glare is caused by light that is reflected to the eye from surfaces that are in the field of view often in the task area. Disability glare reduces visual performance and visibility.  Discomfort glare produces physical discomfort.  It is possible to experience disability glare without discomfort and conversely, discomfort without disability; however, one often accompanies the other.

 A term applied to a switchgear accessory device that can be inserted in place of the drawout circuit breaker for the purpose of either grounding the main bus and/or the external circuits connected 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 specified time. Optionally, a modified circuit breaker can be used to provide the GATD functions and in addition allow full fault switching capability.

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

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

H

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 Henry

 Hp

I

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 Acronym for the International Electrotechnical Commission.

This series of standards is produced by the IEC, and are typically used in most countries of the world, but not widely in North America. Each country may adapt IEC standards to their own needs, 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 ratings, the two main global standards are IEC and ANSI.

The world is very slowly moving towards adopting IEC standards, but ANSI and IEC are different enough that this is not easy to do.

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

There are 3 main types of interlocks, namely mechanical, electrical and key. Mechanical and electrical interlocks are always present in some way in switchgear to provide for safe operation of the basic equipment. Key interlocks are typically more of an optional accessory used to provide customer prescribed operation sequences.

J

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K

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L

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

Load shedding can also be used to reduce demand in a specified part of a power distribution system for economic reasons e.g. an industrial customer may shed specific loads to keep their kilowatt demand below a price increase threshold (kW demand is often priced based on the peak usage in a calendar year, so keeping the demand below a certain level can affect a whole year’s electrical billing).

M

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

Metal-clad is characterized by always having the following features:
• The main switching and interrupting device is removable (drawout), with a mechanism to move it between connected and disconnected positions, and equipped with self-aligning and self-coupling primary disconnecting devices and disconnectable control wiring connections.

• Major parts of the primary circuit are completely enclosed by grounded metal barriers that have no intentional openings between compartments. A metal barrier shall be included in front of or part of the circuit interrupting device to ensure that when in the connected position, no primary circuit components are exposed by the opening of a door.

• All live parts are enclosed within grounded metal compartments.

• Automatic shutters that cover primary circuit elements when the removable element is in the disconnected, test or connected positions.

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

• Mechanical interlocks are provided for proper operating sequence under normal operating conditions.

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

• The door through which the removable element is inserted may serve as an instrument or relay panel, and may also provide access to the control compartment within the housing.

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

 An assembly of conductors with their associated connections, joints and insulating supports within a grounded metal enclosure.

The conductors may be rigid or flexible (if the flexible conductors are individual cables, then it may be called “cable bus”). There are three types of metal-enclosed bus:
• Nonsegregated-phase bus, in which all phase conductors are in a common metal enclosure without barriers between phases. If associated with metal-clad switchgear, then the primary bus and connections shall be covered with insulating material equivalent to the switchgear insulation 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 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 forced cooled, using gas or liquid.

 MV

N

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 A mechanical term, used to describe the frequency at which a body vibrates due to its own physical characteristics (mass, shape) and electric restoring forces brought into play when the body is distorted in a specific direction and then released while restrained or supported at specific points.

In general switchgear usage, generally important only for seismic analysis.

 Nominal Voltage

Nominal System Voltage

 A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class. The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.

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

O

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

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

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

It must provide sufficient mechanical force for the performance specified, and is designed to meet the specific requirements of the switching device (for example, for vacuum circuit breakers, the operating mechanism is expected to provide only a small contact travel distance, but very high acceleration and deceleration).

P

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 A localized electric discharge resulting from ionization in an insulation system when the voltage stress exceeds the critical value. This discharge partially bridges the insulation between electrodes.

It is commonly used as a test to monitor the quality of manufacture of moulded or cast components (such as current or voltage transformers, or stand-off insulators). If the discharge occurs in air, then it may also be referred to as corona discharge.

 The action of 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 has been completed.

Pickup is also used to identify the minimum value of an input quantity reached by progressive increases that will cause the relay to reach the pickup state from reset. Pickup is also used to denote contact operation on relays having multiple inputs, but in this case the pickup value of any input in meaningful only when related to all other inputs.

 Copper (or aluminium) forms an oxide layer which will commonly cause a joint to have increased resistance and a higher temperature rise. To obtain satisfactory lifetime performance, the high power joints in the main bus system are plated with another metal, usually silver. Silver is the most conductive of all metals, and provides good service with only a very thin plating thickness required (“flash” plating).

For very polluted environments, especially those containing high levels of sulphur, tin plating is used, as the silver will rapidly tarnish. While tin is very stable in most atmospheres, it naturally provides a higher resistance joint than silver, and therefore it is only used where it’s non-corroding properties are really required.

 So constructed, protected or treated so that successful operation is not interfered with when the device is subjected to the specified material or condition.

For example, a sprinkler-proof switchgear can continue to operate if the fire protection sprinklers are operating. As another example, the reason that the term arc-proof switchgear is not acceptable as a term, is that in the case of an internal arcing fault, the switchgear cannot usually continue to be used until clean-up and repairs are completed. See also Resistant.

 A relay whose function is to detect defective lines or apparatus, other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action.

A protective relay may be described according to its input quantities (e.g. negative phase sequence relay), operating principles (e.g. differential protection) or performance characteristics.

Q

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R

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 The process of moving a circuit breaker or other removable element from the disconnected position to the test position (if the test position is a different physical position) and into the connected position, 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-door racking) or electrically with the door closed.

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

 A standard operating sequence that a circuit breaker has been tested in accordance with. Under ANSI testing rules, the Rated Operation Sequence is a close-open operation, followed by a 15 second delay to allow the operating mechanism to recharge, and then a second close open operation. This is represented by CO – 15s -CO. Note that a fully tested ANSI circuit breaker has been tested for many other operation sequences as well.

Under IEC testing rules, there are three common Rated Operation Sequences. The first is an open operation, followed by three minutes for cooling of the contacts, followed by a CO operation, then 3 minutes, then a final 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.

 RJ-11

 Essentially an air cored current transformer which, because it does not have a magnetic core, has a linear response from zero current to very high values (well beyond what can be seen in any 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 the primary (e.g. 1 mV/A). Rowgowski coils measure only the AC component of the current, not the DC component. The manufacture of a good Rowgowski coil requires very precise winding of the coil. If the coil is wound properly, then the output does not depend on the position of the central conductor in the coil window.

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

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

A switching device that can withstand a high recovery voltage will be less likely to restrike. See also Recovery Voltage, Restrike.

S

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 Acronym for Supervisory Control and Data Acquisition. A system operating with coded signals over communication channels so as to provide control of remote equipment, typically using one communications channel per remote station.

The supervisory system may be combined with a data acquisition system by adding the use of coded signals over communication channels to acquire information about the status of the remote equipment for display or recording functions.

Typical supervisory control functions are :
• Alarm functions
• Analogue functions
• Control functions
• Indication or status functions
• Sequence of events function

Secondary
Used as an adjective, secondary can mean :
• operates after 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. secondary disconnecting devices, secondary and control wiring
• referring to the energy output side of transformers (which does not have to be the low voltage side)

Secondary and Control Wiring
Wire used with switchgear assemblies for control circuits and for connections between instrument transformer secondaries, instruments, meters, relays, or other equipment.

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

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

 An abnormal connection, including an arc, of relatively low impedance, whether made accidentally or deliberately, between two or more points of different potential.

 (1) A load that occurs when at ungrounded conductor comes into contact with another conductor or grounded object
 (2) An abnorman connection of relatively low impedance, whether made intentionally or by accident, between two points of different potential.

 A fault which occurs close to the terminals of the circuit switching device may cause a different recovery voltage waveform to be generated, essentially adding a high frequency sawtooth waveform to the voltage waveform. As the distance to the fault increases, the amplitude of the sawtooth component increases, but the rate of rise decreases, and the fault current decreases.

The increased amplitude of the voltage negatively affects the interrupting capability of the circuit switching device while the decrease in rate of rise and the decrease in current makes interruption easier. Since the effects are not proportional, a distance is reached where the affect 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 the particular design of device.

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

 A weighted Sound Pressure Level obtained through the use of a metering characteristic and a weighting 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 but are not often 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 is indicated as dBA.

 A ground grid or any equivalent system of grounding electrodes buried beneath or adjacent to a substation that determines the rise of ground voltage level relative to remote earth and controls the distribution of voltage gradients within the substation during a fault.

The characteristics of the Station Ground are linked to the type, shape, number and composition of the ground electrodes, the type of soil, the moisture content of the soil etc.

 A device used to limit the severity of a voltage surge in a power system. In its simplest form, it could be a spark gap, however a more repeatable (and less stress-inducing) method is preferred. The first modern style of surge arrester used silicon carbide (SiC) discs in series with a small spark gap.

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

In the early 1980’s, modern zinc oxide (ZnO) discs were introduced and as have such an extreme inverse characteristic, no spark gap is usually required. ZnO surge arresters are sometimes referred to as “metal-oxide” type.

 A general term covering switching and interrupting devices and their combination with associated control, metering, protective and regulating devices.

Also, assemblies of these devices with their associated interconnections, accessories, enclosures and supporting structures used primarily in connection with the generation, transmission, distribution and conversion of electric power.

T

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 That position in which the primary disconnecting devices of the removable element are separated by a safe distance from the stationary contact elements, and some or all of the secondary circuits are in operating contact.

The test position may be the same physical position of the removable element as the disconnected position, just with the secondary circuits not connected. See also Connected Position, Disconnected Position.

 The capability of a switching device to have the moving contacts return to and remain in the open position when the open operation is initiated after the initiation of the closing operation, even if the closing force and the opening command are maintained.

It may be necessary for the contacts to momentarily reach the closed condition.

U

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 A close-coupled arrangement consisting of one or more medium voltage service entrance cells of switchgear (usually metal-enclosed type), a transformer (dry or liquid), and a line-up of typically low voltage (but perhaps medium voltage) switchgear with feeder cells.

Close coupled means that the MV service entrance switchgear is bolted directly to the transformer which is bolted directly to the LV (or MV) switchgear. If the transformer is cable connected or remote, then by definition it is NOT a unit substation.

V

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A circuit breaker which uses a vacuum bottle for its interrupting method. VCB Circuit breakers, normally applied at medium voltages, that use vacuum interrupters to extinguish the electrical arc and shut-off flowing current.

 A transformer specially designed to accurately represent the primary voltage in a secondary circuit.

Typically in North America, voltage transformers are standardized to output 120 volts when rated voltage is impressed on the primary circuit, although other values are possible. Voltage transformers can be used to power metering devices, or provide control power.

Some users still use the previous term, Potential Transformer, however the term Voltage Transformer is the correct one.

W

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X

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Y

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Z

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