Publication date: 2018-09-20 18:34
Sometimes there are physical limitations to how close a circuit protection device can be placed to the source of power. In any case, fuses, circuit breakers, and switches should not be installed in battery compartments because of the risk of corrosion coupled with the potential presence of explosive gasses. The question then arises, how close is close enough?
Thus, the concerned feeder belongs to the protection area of the relay 6 and relay 7, providing an inherent backup protection for the feeder. Should relay 6 or its circuit breaker fail to operate, relay 7 will be allowed to operate.
The NEC ® 7569 defines two levels:
- Selective Coordination for all currents
- Coordination to second
It's important to know the difference.
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The protection of ring and meshed networks can also be carried out using directional definite time underimpedance or distance relays. These relays are frequently used for the protection of transmission and sub-transmission networks, meshed or ring-operated distribution networks or weak radial networks.
The selectivity diagram of Figure 6 shows that should a fault arise, for example, in the far end of the feeder (outgoing feeder 6) protected by relay 6, the fault current magnitude will be on the level indicated by [ 8 ]. This fault causes both the relay 6 and relay 7 to start (outgoing feeder 6).
Studying and planning of time-selective protection schemes is most conveniently carried out using selectivity diagrams.
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According to the standards, the relay should start once the energizing current exceeds times the set start current when the normal, very or extremely inverse time characteristic is used. When the long-time inverse characteristic is used the relay should start when the energizing current exceeds times the set start current.