TM5-811-1/AFJMAN
32-1080
power to be supplied to the installation during
operate to clear the fault. Therefore, permission of
periods of maintenance on the single incoming line
the utility company must be obtained for the use
circuit breaker. The possibility of high transient
of fault-interrupter switches when the circuit
recovery voltages should be coordinated with the
b r e a k e r or protective element ahead of the
manufacturer and ANSI C37.011.
switches is under the exclusive control of the
b. Power fuse disconnecting units. Power fuse
supplying utility, and the available fault current
disconnecting units may be used in conjunction
exceeds the interrupting rating of the fault-
with incoming line switches where circuits have a
interrupter switches. These types of switches
nominal voltage class of 46 kV through 169 kV
should not be used when the station is supplied by
and where required interrupting and continuous
only one incoming line when the switches are to
current ratings of fuses are not larger than those
be used for the opening and closing of the line, as
available. ANSI C37.46 lists maximum voltage
opposed to the protection of transformers and
and maximum interrupting ratings. Power fuses
separate line switching equipment. The interrupt-
will be selected on the basis of maximum line-to-
ing element of the switches is an SF, unit. Single
line voltage, regardless of whether the fuse will be
SF, interrupter models are available at the follow-
applied to a grounded or ungrounded neutral
ing nominal voltage levels: 34.5, 46, 69, 115, 138,
system. Electrical clearances and spacings will
and 161 kV. Since there is considerable variation
comply with the clearances listed in ANSI C37.46.
in interrupting ratings from manufacturer to man-
Selection and application of power fuse disconnect-
ufacturer, fault-interrupter switches should be
ing units will be in accordance with IEEE Std
carefully specified only after the short circuit
C37.48.
study has been completed.
c. Metal-enclosed interrupter switchgear. Metal-
e. Manual and motor-operated disconnect
switches. Manual or motor-driven, group-operated
enclosed interrupter switchgear is more economi-
cal than circuit breakers in the initial cost, and
disconnect switches may be used for line switch-
may be used when more expensive switching and
ing, as well as isolation of station elements, under
protective equipment cannot be justified. The use
no-load conditions. This requires the use of such
of such switchgear is to be limited to stations
switches in conjunction with other circuit protec-
supplied by incoming lines rated 35 kV or below,
tive and switching equipment. Disconnect switches
and should be used primarily for unit substations
must be interlocked with other equipment to en-
and industrial or power plant applications. Metal-
sure operation only under no-load conditions. For
enclosed interrupter switchgear may be provided
manual switches this may require key interlocking
for primary protection of power transformers and
and for motor-driven switches this may require
electrical interlocking, with the transformer's
for incoming line ties which are a part of a
primary unit substation, when circuits have a
main secondary breaker. Serving utility practice
nominal voltage class of 35 kV or lower and
and Using Agency operating requirements at each
required interrupting and continuous current rat-
installation should be reviewed before specifying a
manual or motor-driven disconnect switch. If there
ings of fuses are not larger than those available.
Interrupter switchgear may also be used as a
is an operating requirement for automatic opening
substitute for metal-clad switchgear equipped with
of the disconnect switch, then the motor-driven
power circuit breakers in certain instances. Since
variety should be specified. Where rapid and defi-
metal-enclosed interrupter switchgear is provided
nite switch operation is required, for reasons of
as a less expensive alternative, the increase in cost
personnel or equipment safety (especially for
to provide motor operation or walk-in aisle space is
larger switches at high-voltage levels), specify a
rarely justified. Preferred ratings are given in
motor-driven switch. Without a specific reason for
IEEE Std C37.20.3 and ANSI C 37.32. Selection
a motor-driven disconnect switch, the less costly
and application of metal-enclosed interrupter
manual switch should be specified.
f. Sulphur hexafluoride (SF6 ) equipment. Guid-
switchgear will be in accordance with IEEE Std
ance for gas-insulated substation equipment is
C37.20.3 and IEEE Std C37.32.
d. Fault-interrupter switches. Fault-interrupter
contained in IEEE C37.123. Each SF, interrupter,
if located inside a structure, must be located in a
switches may be used for line switching and fault
room with direct outdoor ventilation and sensor
protection of station elements as a less costly
unit which activates the room vent fan and a room
substitute for circuit breakers. The interrupting
entry alarm when the oxygen level in the room is
ability of the switches is limited and their opera-
above 19.5 percent. This requirement is to pre-
tion must be blocked if fault currents exceed the
clude jeopardizing personnel life or health. The
interrupting rating of the switches. This requires
entry alarm will be automatically silenced when
that a circuit breaker on the line side of the switch
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