Selective Tripping; Typical Time-Current Characteristics For Overcurrent Trip Devices - GE AK-2-50 Maintenance Manual

3. On drawout breakers wired for the fourth
gr· o und sensor coil, unsolder external sensor coil
leads at terminal board on female disconnect
plug. On stationary breakers unsolder twisted
shielded pair from Terminal Board (Fig. 40).
4~ Remove the four bolts connecting each
· coil as·sembly to the pole unit.
5. Remove the coil assemblies taking care
not to damage the inter-connecting coil wires.
· 6. No further breakdown of the .coil assem-
blies should be made. Do not remove the coils
from around their iron core.
7. · Replace coil assemblies in reverse order.
Sensor Coils (Al_{-3-75/100) Only (Fig. 33)
1. Disconnect control plug between Power
Supply Unit and Sensor Coils.
2.
If
present, disconnect fourth ground sensor
coil lead as described in Step 3 above.
3. Disconnect ·resistor board from back frame
by removing two nuts from studs welded to back
frame. Remove the two remaining hex head
bolts and nuts. Lift board from the two studs.
· 4. Remove collar at end of primary dis-
connects by loosening two allen head bolts and
sliding off collar.
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Low Voltage Power Circuit Breakers GEK-7303
5. Work coils toward end of stud. Starting
with one outside pol e, slip coils off stud and
onfo your right arm or properly supported cyl-
inder, now move to center pole ~d O?t s ide .Pole
until coils are removed place coils with resistor
board on clean flat surface. When removing coils
from your arm be careful not to break or f?ul
interconnecting coil wires. Also note location
of spacers. Spaces may vary in thickness their ·
purpose is to space coils out . for a firm fit.
SELECTIVE TRIPPING .
Selective tripping is obtained when the breakers
in the electrical a1str1out1on system are arranged
on the basis of a progressive series of time avd
current pickup. This will allow the breaker having
the shorter time setting and the 1ower pickup
to trip before the breaker having the longer time
setting and the higher current pickup, provided
the fault is on the · part of the line protected by
the breaker having the lower setting. Hence, if
a fault occurs in any part of the electrical system,
only the breaker nearest the fault will trip.
In
order to reduce .the possibility of damaging
the equipment and to provide maximum safety to
the operator, the overload caused by a. fault is
removed in a minimum amount of time by selec-
tive tripping. Overloads producing current up to
five or ten times the breaker rating are r emoved
in a matter of a few cycles while currents in
excess of this value are remqved instantaneously.
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Fig. 41 (215Dl80) Typical time-current characteristics for Magnet Overcurrent Trip Devices