Glossary of Medium Voltage Switchgear Terms.pdf
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Application Note
Glossary of Medium Voltage
Switchgear Terms
This glossary is intended to cover the meaning of many com-
mon words and phrases used in association with medium
voltage switchgear. Many terms will also be applicable to low
voltage switchgear as well.
A
Air Blast Circuit Breaker
A circuit breaker using compressed air to force the extinction of the
arc through an arc-chute system. An obsolete technology. See
also Circuit Breaker.
Air Magnetic Circuit Breaker
A circuit breaker that uses air as its insulating medium, and an arc-
chute system for dissipating the arc, a method of generating a mag-
netic field which forces the arc into the arc chute, and with possible
assistance from a air puffer system to blow the arc into the arc
chutes. An obsolete technology. See also Circuit Breaker.
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 over-
current, usually designated as 50/51. This designation actually
covers two types of overcurrent protection, 50 which is instanta-
neous protection, and 51 which is a time overcurrent protection.
Various suffixes can be used, which are sometimes context sensi-
tive. For example, 87 represents differential protection in general,
while 87T is specifically transformer differential protection and 87G
is specifically generator differential protection. However, 50G re-
fers 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.
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 Switchgear
Not a definitive term, but used to generally describe switchgear
built in accordance with ANSI standards, as opposed to IEC stan-
dards (IEC Switchgear). Switchgear manufactured in Canada by
Schneider Canada can be accurately said to be ANSI switchgear.
Anti-single Phase Tripping Device
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.
Anti-pumping Relay
A 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.
Arc-chute
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 interrupt-
ing means in the air-magnetic type of circuit breakers, but the same
concept does not exist in vacuum or SF
designs. Arc-chutes are
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common in low voltage circuit breakers.
Arc-resistant Switchgear
A type of switchgear design which is designed to withstand the ef-
fects 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 dam-
age to the structure and/or components, but often the physical
damage is less with an arc-resistant design. There are three class-
es of protection - Type A eliminates the emission of gases and par-
ticles 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
Arc-proof Switchgear
An incorrect term. Please refer to Arc-resistant Switchgear, Proof,
Resistant.
Arcing Contacts
The contacts of a switching device upon which the arc is deliber-
ately drawn after the main contacts have parted. See also Main
Contacts.
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Arcing Time
The amount of time between the first initiation of the arc and the in-
stant of final arc extinction in all phases. See also Opening Time,
Closing Time
Asymmetrical Current
The current which appears in a circuit during fault conditions, com-
posed of the total of the AC component of the fault current and the
DC offset component.
Auxiliary Circuits
All the control, indicating and measuring circuits in the switchgear.
When speaking of the circuit breaker, these are often called the
secondary circuits (with the main current path called the primary
circuit).
Auxiliary Relay
A relay whose function is to assist another relay or control device
in performing a general function by supplying supplementary ac-
tions. Some examples are a relay used to provide seal-in functions,
to increase the number of available contacts, to provide circuit
opening instead of circuit closing contacts or vice-versa etc.
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 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.
BIL (Basic Impulse Insulation Level)
A reference impulse insulation strength expressed in terms of the
crest value of the withstand voltage of a standard full impulse volt-
age wave. It is commonly used to define the overall insulation
strength of the design, with other test levels (e.g. applied tests) re-
lated in a standard method to the BIL. Typically 5 kV class switch-
gear 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 re-
quests for 170 or 200 kV BIL at times)
Bifurcated Feeder
A feeder connection where two loads are connected to one power
source. Higher number of connections are also possible, such as
trifurcated feeders for three loads etc.
Bolted Fault
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.
Burden (of a Relay)
The load impedance imposed by a relay on its source, expressed
in ohms and phase angle at specified conditions. Modern solid
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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.
Bus
A conductor, or group of conductors, which serve as a common
connection for two or more circuits. In practice, "bus" generally re-
fers to the main power bus in the switchgear, but it could also refer
to a data bus, or ground bus etc.
Bus Duct
See Metal-enclosed Bus, Isolated Phase Bus Duct.
Bushing
An insulating structure including a through conductor, or a pas-
sageway 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 sur-
faced 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
Cartridge Fuse
A low voltage fuse consisting of a current-responsive element in-
side of a fuse tube with connection terminals on both ends. Com-
monly used to protect control circuits in the switchgear LV control
cabinet.
Cassette
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.). 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 Cra-
dle.
Closing Time
The amount of time from the initiation of the closing operation and
the instant when metallic continuity is established in all phases.
See also Opening Time, Arcing Time
Circuit Breaker
A mechanical switching device, capable of making, carrying and in-
terrupting currents under normal circuit conditions, and also mak-
ing 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 ap-
plications today use either SF
or Vacuum methods. Circuit break-
ers are usually intended to operate infrequently, although some
types are made to operate frequently.
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Circuit Switcher
A mechanical switching device with an integral interrupter, suitable
for making, carrying and interrupting currents under normal circuit
conditions. It can also interrupt specified short circuit currents
which may be less than its close and latch, momentary and short
time current ratings. It is usually less expensive than a circuit
breaker, but does not achieve the same performance level.
Clearing Time
For a circuit breaker, it is the interval between the time the actuat-
ing 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.
Close And Latch
The rating of the ability of a circuit breaker or other switching device
to close against the electromagnetic forces (which are trying to
force the contacts open), to latch into the closed position, and to
carry a specified current through the device under specified condi-
tions. This is mostly related to ensuring the operating mechanism
has sufficient mechanical energy to overcome the electromagnetic
forces.
Close-Open Operation
A close operation of the switching device, followed immediately by
an opening operation, without introducing any deliberate time de-
lay. It is commonly shown as CO.
Closing Coil
A coil in the electromagnet that supplies power for closing the de-
vice, such as the circuit breaker. It must be specified to have the
correct control voltage rating for the installation.
Compartment
Inside of a cell of metal-clad switchgear, there are different and
separate compartments used to segregate different components
and functions from one another, to improve reliability and improve
safety. Each compartment is separated by a grounded metal barri-
er. Standard compartments are low voltage, circuit breaker, main
bus and cable, with other compartments possible as the design de-
mands.
Conformance Tests
Those tests made to demonstrate compliance with the applicable
standards, such as CSA, ANSI or IEC. These tests are generally
performed after the test item has been subjected to all of the re-
quired production tests. Demonstration of margin (capability) be-
yond the requirements of the standards is not required. See also
Design Tests, Production Tests.
Connected Position (of a Circuit Breaker)
That position of the removable element in which both the primary
and secondary disconnecting devices are in full contact. See also
Disconnected Position, Test Position.
Contactor
A contactor is (usually) a magnetically operated switch for repeat-
edly 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 ap-
plication is motor starting. It differs from a circuit breaker in that it
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