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GE Power Management
Technical Notes
Fuse Saving Schemes in
Feeder Relays
GE Publication No. GET-8395
Copyright © 2002 GE Power Management
Selectivity between feeder breaker and downstream devices is normally expected in typical distribution systems. However, in some situations the feeder breaker is set to trip for
faults beyond a downstream branch fuse, before the fuse will blow. This practice is
known as a "fuse saving" scheme and can save expensive fuse replacement and consumer extended outage time.
If the short circuit current in a substation is too high to prevent fuse damage, the scheme
is not applicable. The fuse melting point and feeder breaker clearing must be compared
for the fault current level at the fuse location to determine if a fuse saving scheme can be
implemented. If the fuse melting time is below the sum of the protection and breaker
operating times for the calculated short circuit value at the fuse location, the fuse saving
scheme will not be applied.
The fuse saving scheme typically uses a low set instantaneous overcurrent element
which will trip the feeder breaker before the fuse branch can blow, and the breaker is
then immediately reclosed. The low set elements are automatically cut out of service
after the first tripout, so that if the fault should persist the inverse time elements will have
to operate to trip the circuit breaker. This gives time for the branch circuit fuse of the
faulty circuit to blow if the fault is beyond the fuse. In this way the cost of replacing blown
branch circuit fuses is minimized, and at the same time the branch circuit outage is also
minimized. If the breaker is not tripped within a certain time after the reclosure, the low
set instantaneous protection should be automatically returned to service.
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GET-8395: Fuse Saving Schemes in Feeder Relays
13.8 kV
Ik1 = 12000 A
. . .
52-2
52-1
52-3
F1
Ik2 = 9000A
Fault
F2
FIGURE 1. Fuse Saving Scheme
In the following graph, the fuse will melt before low-set instantaneous overcurrent operate for Ik = 9000 A at the fuse F1 location. In this case. the fuse saving scheme cannot
be applied, and therefore the low-set overcurrent should not be enabled.
T(s)
1000
100
TOC at 52-1 Bkr
Low-set
instantaneous
10
High-set
instantaneous
Ik2
Ik1
1
Maximum
clearing time
F1
0.1
I(A)
100
2
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1000
10000
100000
GET-8395: Fuse Saving Schemes in Feeder Relays
However, with a fuse of a different characteristic, the fuse saving scheme application
becomes possible. See the curves below:
T(s)
1000
100
TOC at 52-1 Bkr
Low-set
instantaneous
10
High-set
instantaneous
Ik2
Ik1
1
Maximum
clearing time F1
0.1
Low set IOC
operating time at Ik2
Fuse Melting
Time at Ik2
I(A)
100
1000
10000
100000
In many situations the fuse saving scheme may not be applicable or desirable and the
feeder protection will simply maintain coordination with all downstream devices.
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