TM5-811-1/AFJMAN
32-1080
Table 4-2. Minimum Metering for Metal-Clad Switchgear
ANSI
abbreviation
Type of meter
Circuit metered
Ammeter and 3-phase
AM
On all mains. On feeders only when transformer unit capacity exceeds 2.5
switch
MVA
Voltmeter and 3-phase
VM
On all mains
switch
WM
On all mains. On feeders only when transformer unit capacity exceeds 10
MVA
Varmeter
VARM
On all mains
WHDM
Watthour
On all mains. (Demand period to correspond to the utility demand period)
demand meter
Table 4-3. Minimum Relaying for Metal-Clad Switchgear
ANSI
device
number
Circuit breaker application
Relay function
51 and 51G or 51N
On all mains when short circuit current can flow in only one direction. The
Nondirectional
overcurrent,
51G relay should be used when a CT can be installed in the main transformer
phase and ground
neutral-to-ground connection, otherwise the residually connect 51N should be
used. "50" relays should not be used since coordination with downstream feed-
ers is impossible.
Nondirectional overcur-
On all feeders when short circuit current can flow in only one direction. The
50/51 or 51 and 50GS
50N, or 51N
rent, phase and ground
50/51 relay should be used when there is no down-stream protective device.
Use the 51 device when there is downstream protection. Where possible and
where there is no downstream protection, use a zero-sequence ground-sensor,
donut CT for sensitive 50GS protection. Where instantaneous protection is re-
quired but a ground-sensor cannot be used, specify the residually connected
50N device. Use the 51N where there is down-stream protection.
67 and 67N
On all mains and tie lines when short circuit current can flow in both
Directional
overcurrent,
directions.
phase and ground
79
Feeders serving long overhead lines, except that the 79 relay may be installed
Automatic circuit
on the main instead of the feeders when the transformer base rating is 2.5
reclosing
MVA or lower.
87B
Bus differential
On all circuit breakers connected to the main supply station/substation bus.
Studies indicate that between 75 and 90 percent of
for monitoring energy demand and consumption
the faults on aerial lines are temporary and
for the purpose of demand limiting or energy
self-clearing, and are most commonly caused by
reduction required for separately specified energy
lightning, "brushing" by tree limbs, "galloping"
monitoring and control system equipment. Such
provisions will be limited to empty raceways ex-
conductors, birds, and other external causes of a
momentary nature. Such external causes are not
tended from beneath switchgear units to a point
common to underground cable systems. Therefore,
two feet external to the switchgear foundation; to
the use of automatic circuit reclosing relays or
providing instrument transformers; and to furnish-
other devices cannot be justified for underground
ing transmitting devices.
(a) Automatic circuit reclosing relays. Auto-
feeders. For overhead feeders fed from metal-clad
switchgear, a reclosing relay (device 79) may be
matic circuit reclosing relays should be specified
added in accordance with table 4-3. In applying
for use in conjunction with aerial lines. Reclosing
reclosers, consideration must be given to the con-
relays should be considered jointly with sectional-
tinuous and short-circuit-interrupting ratings and
izing switches which should be installed to mini-
to the selection of reclosing sequence. When two or
mize the duration of outages of power to other
more reclosing devices are connected in series,
facilities served by the same aerial line, as a
proper coordination is required between pick-up
result of sustained faults. The use of sectionalizing
settings and reclosing sequences. Automatic reclos-
switches should be considered in relation to the
ing should not be used on tie lines where there is
a second source or on lines with an on-line genera-
the installation as well as the nature of the loads.
4-10
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