1 General
Switchgear is a commonly used name for metalenclosed distribution apparatus
of modular, cubicletype construction. Despite this commonly used name, there
are technical and physical distinctions between various classes of switchgear
assemblies. The American National Standards Institute (ANSI) and National Electrical
Manufacturers Association (NEMA) have published standards for electrical equipment.
These standards are followed by most manufacturers of electrical switchgear.
The ANSI lists switchgear assemblies into three main categories, which are
further classified into subcategories as shown in Table 1.
2 MediumVoltage Switchgear
Mediumvoltage (MV) metalenclosed power switchgear is defined as a switch
gear assembly completely enclosed on all sides and top with sheet metal (except
for ventilating and inspection openings) containing primary power circuit switching
and/or interrupting devices with buses and connections, and may include control
and auxiliary devices. The rated voltage and insulation levels for MVclass
metalenclosed switchgear assemblies are shown in Table 2.
2.1 Construction Features
Switchgear is a general term used to define switching (and/or interrupting),
protective, regulating, and metering devices, including all associated controls
and interconnections, and accessories used for generation, transmission, and
distribution of electrical power. As shown in Table 1, switchgear equipment
comes in various forms and ratings that can be used to perform particular functions.
There are some fundamental differences among the various types of equipment
available in the MV class. These differences are important from a maintenance
and operation point of view and are discussed in the following sections.
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TABLE 1 ANSI Classification of Switchgear Assemblies
 MetalEnclosed Power Switchgear Metal clad Metalenclosed interrupter Stationtype
cubicle Lowvoltage power circuit breaker
 MetalEnclosed Bus Nonsegregated
Segregated Isolated phase
 Switchboards
 Control
 Enclosed and dual duplex
 Vertical panel
 Benchboard
 Control desk
 Dual benchboard
 Duplex benchboard
Power
 Enclosed
 Dead front
 Live front
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TABLE 2 MetalEnclosed Switchgear Assemblies Voltage and Insulation Ratings
TABLE 3 Power Circuit Breaker Ratings and Characteristics, ANSI C37.061971
TABLE 4 Power Circuit Breaker Rating and Characteristics, ANSI C37.61964
2.1.1 MetalClad Switchgear
Metalclad switchgear consists of indoor and outdoor types with power circuit
breakers rated from 4.16 to 13.8 kV, 1200 to 3000 A, 75 to 1000 MVA interrupting
capacity as shown in Tables 7.3 and 7.4. Metalclad switchgear has the following
additional features:
• The interrupting and switching device (i.e., breaker) is removable and can
be physically moved into disconnect, test, or operating position.
• The primary bus conductors and connections are insulated.
• All live parts are enclosed within grounded metal compartments.
• Automatic shutters close off and prevent exposure of the primary circuit
elements when the circuit breaker is removed from operating position.
• The primary and secondary contacts are selfaligning and selfcoupling.
• Mechanical interlocks are provided to ensure a proper and safe operation.
The circuit breaker housing cell door may be used to mount relays, instruments,
and wiring. The relays, control devices, and associated wiring are isolated
by grounded metal barriers from primary conductors.
The elements of primary circuits such as circuit breakers, buses, and potential
transformers are completely enclosed by grounded metal barriers.
2.1.2 MetalEnclosed Interrupter Switchgear
Metalenclosed interrupter switchgear consists of indoor and outdoor types
with or without power fuses, voltage rating from 4.16 to 34.5 kV, and current
rating of up to 1200 A. This switchgear is characterized by the following features:
 The primary buses and connections are uninsulated.
 The disconnecting device is an interrupter switch that may be removable
or stationary.
 This switchgear may have instrument transformers, control wiring, and accessory
devices.
2.1.3 StationType Cubicle
The stationtype cubicle switchgear consists of indoor and outdoor types with
power circuit breakers rated from 14.4 to 34.5 kV, 1200 to 5000 A, 1500 to
2500 MVA interrupting capacity. This switchgear has the following features:
 Poweroperated stationary circuit breaker
 Bare buses having continuous carrying capacity equal to the service required
 Bare connections having currentcarrying capacity equal to that of the
power circuit breaker
 Each phase of primary is segregated and enclosed by metal
 Threepole, singlethrow, groupoperated disconnect switches that are interlocked
between power circuit breaker and front door giving access to primary compartment
 A set of instrument transformers
 Control wiring, terminal blocks, ground bus, and accessory devices
2.2 ShortCircuit Considerations and Power Circuit Breaker Ratings
To understand the rating basis of power circuit breakers, it is important
to understand how the circuit breaker will perform under conditions where the
shortcircuit current varies with time. The rating structure of a power circuit
breaker is complicated because of the opening time of the circuit breaker during
a shortcircuit condition. The total operating time of the circuit breaker
is based on the following:
 Protective relay operation time
 Circuit breaker operation time
The protective relay operation time is a function of relay type and its setting.
The types of relays and their operating characteristics are discussed in Section
9. The breaker operation time (i.e., mechanical time) consists of the following:
 Circuit breaker trip coil to energize its operating mechanism
 Circuit breaker contact parting time
 Circuit breaker to quench the arc in the arc chamber (or in the vacuum
bottle in case of vacuum interrupters)
High mechanical stresses are produced instantaneously in the circuit breaker
during the interruption of a short circuit. These stresses vary as the square
of the current and are greatest at maximum current. The fault current magnitude
varies from shortcircuit inception to the time when it reaches a steadystate
condition. This variation depends on the instantaneous value of the system
voltages at the instant the fault occurs, also known as the closing angle;
the dynamic change in alternating current (AC) impedance as the energy balance
changes and the decay in the direct current (DC) component of the fault current.
Consequently, the circuit breaker interrupts the fault current at some time
(usually a few cycles) after the short circuit occurred. Therefore, power circuit
breaker ratings are established on two bases:
 Momentary rating, that is, circuit breaker ability to close and latch on
the maximum shortcircuit current available without mechanical damage.
 Interrupting rating, that is, circuit breaker ability to interrupt the
flow of fault current without mechanical damage.
The fault current is highly asymmetrical from the time of fault inception
to several cycles later. It becomes symmetrical after it reaches steadystate
conditions. To understand fully the varying phenomena of shortcircuit current,
let us briefly review shortcircuit definitions and the kinds of current avail
able in a fault.
Rootmeansquare (rms) (effective) value: This is an effective value of AC
and is usually expressed as 0.707Im, where Im is the AC peak value. This rms
current value is shown in FIG. 1.
FIG. 1
Peak value (crest): This is the maximum value of the AC wave peak.
Average value: The average value of an AC wave is zero because the positive
and negative loops have the same area. However, the average value of the positive
loop of a symmetrical wave can be expressed in terms of peak value. For sine
wave the average value is expressed as 0.636Im, where Im is the peak value.
Instantaneous (momentary) value: It is difficult to manipulate analytically
the instantaneous value of alternating wave forms. In general, for shortcircuit
considerations the instantaneous value (or momentary value) is the peak value
of the sine wave occurring at first halfcycle.
Symmetrical current: A symmetrical current wave is symmetrical about the time
axis (zero axis) of the wave. This is shown in FIG. 2.
FIG. 2 Symmetrical current wave.
FIG. 3 Asymmetrical current wave, fully offset.
Asymmetrical current: An asymmetrical current wave is not symmetrical about
the time axis. The axis of symmetry is displaced or offset from the time axis.
This is shown in FIG. 3. An asymmetrical wave can be partially offset or fully
offset. Offset waves are sometimes called displaced waves.
DC component: The axis of symmetry of an offset wave resembles a DC, and symmetrical
currents can be readily handled if considered to have an AC and a DC component.
Both of these components are theoretical. The DC component is generated within
the AC system and has no external source.
FIG. 4 shows a fully offset asymmetrical current with a steady DC component
as its axis of symmetry. The symmetrical component has the zero axis as its
axis of symmetry. If the rms or effective value of the symmetrical current
is 1, the peak of the symmetrical current is 1.41. This is also the effective
value of the DC component. We can add these two effective currents together
by the square root of the sum of the squares and get the effective or rms value
of asymmetrical current.
The rms value of a fully offset asymmetrical current is 1.73 times the symmetrical
rms current. It is readily apparent that the peak asymmetrical current is twice
the peak symmetrical current: 2 × 1.41 = 2.82.
Total current: The term total current is used to express the total or sum
of the AC and DC components of an asymmetrical current. Total current and total
asymmetrical current have the same meaning and may be expressed in peak or
rms amperes.
FIG. 4 Fully offset asymmetrical wave with DC component.
FIG. 5 DC and AC component currents.
FIG. 6 Asymmetrical current wave when fault occurs at zero voltage.
FIG. 7 Symmetrical current wave when fault occurs at maximum voltage.
Decay: Unfortunately, fault currents are neither symmetrical nor fully asymmetrical
but somewhere in between. The DC component is usually shortlived and decays
after several cycles. In FIG. 5, the DC component decays to zero in about four
cycles. The rate of decay is called decrement, and depends upon the circuit
constants. The DC component would never decay in a circuit having reactance
but zero resistance and would remain constant forever. In a circuit having
resistance but zero reactance, the component would decay instantly. These are
theoretical conditions; all practical circuits have some resistance and reactance,
and the DC component disappears in a few cycles. In practice when performing
short circuit analyses the X/R ratio is computed to give a practical estimate
for determining how quickly the DC component will decay. For more detail, see
definition of X/R in this Section.
Closing angle: A shortcircuit fault can occur at any point on the voltage
wave. So far we have avoided discussing voltage characteristics, but the volt
age wave resembles the current wave. The two waves may be in phase or out of
phase, and the magnitude and symmetry of the current wave of a short circuit
depends on the point on the voltage wave at which the fault occurs.
This is known as the closing angle of the fault.
Random closing: In real life, faults occur at any and every point on the volt
age wave. In a laboratory, this can be duplicated by closing the circuit at
random. This is known as random closing. The following is true of a short circuit
having negligible resistance:
If the fault occurs at zero voltage, the current wave is fully asymmetrical,
as shown in FIG. 6 If the fault occurs at maximum voltage, the current wave
is completely symmetrical, as shown in FIG. 7
FIG. 8 First halfcycle current.
FIG. 9 Currentlimiting fuse action.
Most natural faults occur somewhere in between these two extremes.
Available short circuit (first halfcycle current): What is the available
short circuit current value of a wave that is neither symmetrical nor asymmetrical?
Referring to FIG. 8, the current wave is symmetrical about four cycles after
the DC component becomes zero. We can also determine the total rms asymmetrical
current at one, two, or three cycles or at any other time after the short circuit
started. The accepted practice is to use the current that is available one
half cycle after the short circuit starts. For a fully offset wave, the maximum
current does occur at the end of the first halfcycle of time. Because this
is the worst case, we should determine the peak and rms currents at this point.
As already mentioned, the rate of decay depends upon the circuit constants.
A study of actual circuits of 600 V or less indicates that the proper halfcycle
value for rms asymmetrical current is 1.4 times the rms symmetrical current,
and the peak instantaneous current is 1.7 times the rms asymmetrical current.
Halfcycle available current is 1.7 × 1.4 = 2.4 rms symmetrical current.
Current limitation: Currentlimiting fuses and circuit breakers do not allow
the shortcircuit current to reach the full available value. They interrupt
the circuit in less than one halfcycle before the current builds up to the
maxi mum value. The various times associated with fuses are the following:
Melting time: Time required to melt the fusible link
Arcing time: Time required for the arc to burn back the fusible link and reduce
the current to zero
Total clearing time: Sum of the melting and arcing times, or the time from
fault initiation to extinction
Letthrough current: The maximum instantaneous or peak current that passes
through the fuse is called the letthrough current. This can be expressed in
rms amperes, also.
Triangular wave: The rise and fall of the current through a current limiting
fuse resembles an isosceles triangle, and can be assumed to be a triangle without
introducing appreciable error. Since this is not a sine wave, the rms value
of the letthrough current cannot be determined by taking 0.707 of the peak
value as is done for a sine wave. The effective or rms value of a triangular
wave is equal to the peak value divided by 3.
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peak rms 1.73 3 IFII
The letthrough current of a currentlimiting fuse varies with the design,
ampere rating, and available shortcircuit current. Fuse manufacturers furnish
letthrough curves for the various types of currentlimiting fuses.
Threephase short circuit: Threephase shortcircuit currents can be determined
in exactly the same way as singlephase currents if we assume the three phases
are symmetrical. The three phases have different current values at any instant.
Only one can be fully asymmetrical at a given time.
This is called the maximum or worst phase, and its rms current value can be
found by multiplying the symmetrical rms current by the proper factor.
The currents in three phases can be averaged, and the average threephase
rms amperes can be determined by multiplying the symmetrical rms current by
the proper factor, which is determined by the X/R ratio of the power system.
X/R ratio: Every practical circuit contains resistance (R) and inductive reactance
(X). These are electrically in series. Their combined effect is called impedance
(Z). When current flows through an inductance (coil), the voltage leads the
current by 90°, and when current flows through a resistance, the voltage and
current are in phase. This means that X and R must be combined vectorially
to obtain impedance. The impedance triangle relating X, R, and Z is shown in
FIG. 10.
FIG. 10 Impedance triangle.
FIG. 11 Power triangle.
The resultant angle q is the angle between the voltage and current waves and
is called the phase angle. The voltage leads the current or the current lags
the voltage by an amount equal to the phase angle. The asymmetrical current
may be obtained from known symmetrical current by multiplying with a multiplying
factor for a known X/R ratio. The multiplying factors for commercial and industrial
systems vary between 1.0 and 1.6 depending on the X/R ratio.
Power factor: Power factor is defined as the ratio of real power (kW) to apparent
power (kVA).
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Real power kW PF kVA apparent power Kilowatts are measured with a wattmeter.
Kilovoltamperes are determined with a voltmeter and an ammeter, and the voltage
and current waves may be in phase or out of phase. The relationship of kilowatts,
kilovars, and kilovolt amperes is shown in FIG. 11.
Power circuit breaker rating: The ANSI ratings for power circuit breakers
are expressed by two rating structures. The older standard ANSI C37.6 expresses
the interrupting rating of AC highvoltage (HV) breakers megavoltamperes (MVA)
based on total or asymmetrical current at the time of contact parting.
The newer standard ANSI C37.06, which was introduced in 1964, expresses the
interrupting of AC HV breakers based on total rms symmetrical currents at the
time of contact parting. The rating and characteristics of the ANSI C37.06
and ANSI C37.6 are shown in Tables 7.3 and 7.4, respectively.
The standards define that a circuit breaker shall be capable of performing
the following in succession:
1. Close and immediately latch at normal frequency current, which does not
exceed its momentary capability
2. Carry its maximum rated symmetrical current at specified operating voltage
for duration of its rated permissible tripping delay
3. Interrupt all currents not greater than its rated symmetrical interrupting
current at a specified operating voltage and its related asymmetrical current
based on its rated contact parting time
The typical shortcircuit current consists
of the following:
1. Symmetrical AC wave shape
2. DC component
3. Total current
Let us now compare the rating structure of the two standards by making calculations
for a 13.8 kV, 1200 A, 500 MVA rated breaker. The calculations for shortcircuit
interrupting MVA capability using ANSI C37.06 and C37.6 are made for power
circuit breakers operating at minimum, nominal, and maximum voltages. These
values are shown in Table 5.
It is interesting to note from Table 5 that the maximum interrupting MVA calculated
based on ANSI C37.06 is less than the MVA rating based on ANSI C37.6. The MVA
based on ANSI C37.06 also varies for different operating voltages as compared
to ANSI C37.6 MVA values. The standards committee developing new standards
developed a more stringent basis for the short circuit ratings as stated in
ANSI C37.06, 2000. The emphasis under the ANSI C37.06 is to rate the circuit
breakers in rms amperes rather than MVA, as is the case in the ANSI C37.6.
The rms symmetrical rating system is based on the symmetrical component of
the shortcircuit current at the time of contact opening. Since the shortcircuit
current is varying from fault inception until it reaches steadystate conditions,
some fixed relationship must be defined (or calculated) between symmetrical
and asymmetrical currents based on breaker opening time. This fixed relationship
is defined in ANSI C37.044.5.2.2 by the S curve, which is shown in FIG. 12.
In summary, the ANSI C37.06 standard requires that the proper breaker should
be selected by calculating the total symmetrical rms current at contact parting
time. The reason for selecting the contact parting time as the basis of fault
interrupting rating is that the breaker should be able to withstand the high
mechanical stresses imposed upon it by the first halfcycle fault current,
if the breaker is closed in on a fault. The contact parting time of a circuit
breaker is the sum of one halfcycle tripping delay plus the operating time
of the circuit breaker. For breakers rated at eight, five, three, and two cycles
of interrupting time, the related standard contact parting times are four,
three, two, and one and a half cycles, respectively.
TABLE 5 Comparison of Calculated MVA Values Based on ANSI Standards
FIG. 12 S curve per ANSI C37.044.
2.3 Selection and Application of Power Circuit Breakers
The application of power switchgear is relatively a simple procedure in most
cases whether the switchgear is metalclad or fused interrupter switches. The
following steps are usually considered in applying this equipment:
 Selection of switching scheme
 Calculation of available fault current for breaker or fused interrupter
selection
 Main bus continuous current rating selection
 Current and potential transformer selection
 Protective relay selection
 Circuit breaker control power selection
Other special considerations
Many different switching schemes are available in power switchgear to meet
the desired reliability or operational flexibility. A choice should be made
based on system requirements, maintenance considerations, reliability, and
future expansion.
The selection of the power circuit breaker or fused interrupter switch involves
the calculation of available fault current for interrupting duty purposes.
Insofar as the interrupting of the circuit breaker is concerned, the following
limits should not be exceeded:
 Interrupting rms symmetrical amperes between the operating voltage limits.
 Total fault current that is available at contact parting time in terms
of symmetrical rms amperes.
 Momentary current that is available during the first halfcycle. The standards
allow the momentary rating of circuit breakers to be 1.6 times the rms interrupting
rating at an X/R ratio of 10. In general industrial systems, where X/R ratios
can approach 1520, the next higher (standard) interrupting duty may have
to be applied even though the RMS sym metrical currents may be similar to
those where the X/R is 10 or less.
 The continuous current that the breaker is rated to carry.
 The operating voltage limits, that is, the minimum and maximum design voltages
of the circuit breaker.
The breakers used for reclosing or repetitive duty should be derated in accordance
with NEMA SG 42005 standards in order to be applied properly.
