Schneider Electric 3.0A User Manual

© 1999–2012 Schneider Electric All Rights Reserved

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 (I

2

t) in the network are 

minimized without any effect on the correct 
time delay coordination of the installation.

NOTE: Use I

2

t off with ZSI for proper coordination. Using I

2

t 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.

Figure 6 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.

06133

376