Zone-selective Interlocking

Short-time and ground-fault protection can be interlocked to provide zone-selective interlocking.

Control wiring links several trip units in the distribution network and in the event of a fault, a trip unit will obey the set delay time only if receiving a signal from a downstream trip unit.

If the trip unit does not receive a signal, tripping will be instantaneous (with no intentional delay).

The fault is cleared instantaneously by the nearest upstream circuit breaker.
Thermal stresses (I²t) in the network are minimized without any effect on the correct time delay coordination of the installation.

Figure 16 shows circuit breakers 1 and 2 zone-selective interlocked.

A fault at A is seen by circuit breakers 1 and 2. Circuit breaker 2 trips instantaneously and also informs circuit breaker 1 to obey set delay times. Thus, circuit breaker 2 trips and clears the fault. Circuit breaker 1 does not trip.
A fault at B is seen by circuit breaker 1. Circuit breaker 1 trips instantaneously since it did not receive a signal from the downstream circuit breaker 2. Circuit breaker 1 trips and clears the fault. Circuit breaker 2 does not trip.

NOTE: Use I²t off with ZSI for proper coordination. Using I²t on with ZSI is not recommended as the delay in the upstream device receiving a restraint signal could result in the trip unit tripping in a time shorter than the published trip curve.

NOTE: Setting short-time delay (tsd) or ground-fault delay (tg) to the 0 setting will eliminate selectivity for that circuit breaker.