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Distribution Systems: Protective Equipment

Protective Equipment
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
Figure 1. Substation bus lightning arresters
 
Figure 2. Fused cut-out
Figure 2. Fused cut-out
 
Figure 3. Substation disconnect switch
Figure 3. Substation disconnect switch
Figure 4. Pole mounted type - lightning arrester
Figure 4. Pole mounted type - lightning arrester
 
Figure 5. Air-break isolator switch
Figure 5. Air-break isolator switch
Figure 6. Non load-break fuse
Figure 6. Non load-break fuse
Figure 7. Load-break fuse
Figure 7. Load-break fuse

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