The primary purpose of circuit protection is to prevent damage to equipment and to minimize the extent and duration of power loss. The most suitable protective device for this application is the circuit breaker.
In the event that a fault occurs, it would be desirable that the fault would affect only the portion of the electrical system where the fault occurs. A circuit breaker coordination study is used to analyze the tripping times for the series of over-current devices being compared, normally from the source through the largest branch circuit breaker.
Single Line Diagram
Pictorial Diagram
IEC standard 60497-1 and 60497-2 defined two types of protection coordination
- Discrimination
total discrimination (total selectivity)
over-current discrimination where, in the presence of two over-current protective devices in series, the protective device on the load side effects the protection without causing the other protective device to operate
Full or Total Discrimination Curves
partial discrimination (partial selectivity)
over-current discrimination where, in the presence of two over-current protective devices in series, the protective device on the load side effects the protection up to a given level of over-current, without causing the other protective device to operate
Partial Discrimination
- Backup Protection
back-up protection
over-current co-ordination of two over-current protective devices in series where the protective device, generally but not necessarily on the supply side, effects the over-current protection with or without the assistance of the other protective device and prevents any excessive stress on the latter
Backup or Cascading Protection
Discrimination, also called selectivity, is the coordination of automatic protection devices in such a manner that a fault appearing at a given point in a network is cleared by the protection device installed immediately upstream of the fault, and by that device alone. This ensures that other parts of the network will remain in normal operation.
Backup protection is also called Cascading. The devices can be installed in different switchboards. In general, cascading refers to any combination of circuit breakers where a circuit breaker with a breaking capacity less than the prospective fault current at its point of installation can be used. Of course, the breaking capacity of the upstream circuit breaker must be greater than or equal to the prospective short-circuit current at its point of installation.
References:
1. IEC 60947-1 Low-voltage switchgear and controlgear  Part 1: General rules
2. IEC 60947-2 Low-voltage switchgear and controlgear  Part 2: Circuit-breakers
3. Schneider Electric The Electrical Installation Guide
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