Guide to Low Voltage Electrical System Design and Selectivity

What is selectivity?

The electrical design industry has historically required electrical system circuit breaker selections and settings be validated with a short circuit and coordination study performed by a licensed engineer. These studies assure that circuit breakers are capable of interrupting the available current and would operate “selectively”.

Traditionally, “selectivity” in a low voltage electrical system meant that the long time and short time portions of time-current curves (TCCs) would be selective, i.e. the circuit breaker closest to the fault would trip first, maximizing the amount of the electrical distribution system left in service

In most cases, the circuit breaker instantaneous overcurrent (IOC) TCCs would not be selective “on paper,” as they typically overlap. 

Traditionally, selectivity between molded case circuit breakers (MCCBs) and insulated case circuit breakers (ICCBs) was considered effective, even if there was an overlap of TCCs in the instantaneous region.

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Guide to Low Voltage Electrical System Design and Selectivity