User's Manual

7
Date Code 20170317 Instruction Manual SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System
Application Examples
Figure 6 Example Time-Overcurrent Element Coordination
Fuse-Blowing Scheme
Shortcomings
For a radial distribution system, the goal of the fuse-blowing scheme is to
minimize the number of customers exposed to an interruption. The scheme
accomplishes this by allowing a fuse to clear a given fault. The recloser only
trips for faults that are not protected by a fuse. This scheme is sometimes
called a trip-saving or fuse-blowing scheme because the recloser only trips
when absolutely necessary.
Refer to Figure 6 for the recloser control time-overcurrent element slow curve
(shown in red). This curve must coordinate with the highest-rated fuse size
present on the system, which is shown in green. An intentional coordination
margin allows for prefault load and variances in fuse construction.
For faults on sections of the feeder that are not fuse-protected, the recloser
must still implement this intentional coordination margin. The recloser control
cannot determine which downstream branch the fault is on and assumes that
the fault will be cleared by a fuse. Figure 7 shows an example of a main line
feeder and a fused lateral without a fuse. For a fault on the main line, the
recloser control will wait before using the slow curve to clear the fault (see
Figure 6). For a fault on the main line, the recloser control delays tripping
unnecessarily because there is no fuse present.
Figure 7 Fault on Unfused Tap
Recloser Control
Fast Curve
Recloser Control
Slow Curve
Fuse-Melting Curve
Fuse-Clearing Curve
0.01
0.1
1
10
100
100
Current (A)
1,000
1,000
Time (s)
R