Discrimination (selectivity) is achieved by automatic protective devices if a fault condition, occurring at any point in the installation, is cleared by the protective device located immediately upstream of the fault, while all other protective devices remain unaffected.
Discrimination is based on the principles of current levels, or time-delays, or a combination of both.
Discrimination may be total or partial:
Total discrimination: for all fault values from overload to bolted short-circuits , distribution is totally selective if only the circuit breaker immediately upstream located above the fault opens.
Partial discrimination: selectivity is partial if the above condition is not fulfilled up to the bolted short-circuit current but only up to a lower value. This value is referred to as the selectivity limit.
Standard IEC 60947-2 on industrial circuit breakers deals with coordination between a circuit-breaker and another short-circuit protection device combined in the same circuit. This protection device may be a fuse or another circuit breaker.
Selectivity by curves
elec calc™ allows users to check the selectivity of the different protections through a specific window showing the time/current curves of the selected protection devices.
In addition, the curves showing transient inrush currents associated with motor start-up or transformer magnetisation and the thermal withstand of cables can be displayed on the same window to check the adequation of the protection system.
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