Protective equipment in a distribution system consists
of protective relays, cutout switches, disconnect switches, lightning arresters, and fuses. These work
individually or may work in concert to open circuits whenever a short circuit, lightning
strikes or other disruptive event occurs.
When a circuit breakers opens, the entire distribution circuit is
deenergized. Since this can disrupt power to many customers, the distribution
system is often designed with many layers of redundancy. Through redundancy, power
can be shut off in portions of the system only, but not the entire system,
or can be redirected to continue to serve customers. Only in extreme events, or failure of redundant systems, does an
entire system become deenergized, shutting off power to large numbers of
customers.
The redundancy consists of the many fuses and fused cutouts
throughout the system that can disable parts of the system but not the
entire system. Lightning arresters also act locally to drain off electrical
energy from a lightning strike so that the larger circuit breakers are not
actuated.
Figure 1. Substation bus lightning arresters
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Figure 2.
Fused cut-out
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Figure 3.
Substation disconnect switch
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Figure 4.
Pole mounted type - lightning arrester
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Figure 5.
Air-break isolator switch |
Figure 6.
Non load-break fuse |
Figure 7.
Load-break fuse
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