Zone Selective Interlocking (ZSI)
Presentation
Use zone selective interlocking (ZSI) to reduce the electrodynamic stress on equipment when using selective coordination.
Principle of the ZSI Function
ZSI improves coordination by being selective about the position of the electrical fault. A signal wire links the installed circuit breaker trip units and manages the trip time delay for upstream circuit breaker Q1 according to the electrical fault position.
-
If an electrical fault occurs downstream of downstream circuit breaker Q2 (diagram 3), the trip units on circuit breakers Q1 and Q2 detect the electrical fault simultaneously. The trip unit on circuit breaker Q2 sends a restraint signal to the trip unit on circuit breaker Q1, which remains set on its time delay tsd. Circuit breaker Q2 trips and clears the electrical fault (instantaneously if circuit breaker Q2 is not delayed).
The other users downstream of circuit breaker Q1 still have power, the energy availability is optimized.
-
If an electrical fault occurs downstream of circuit breaker Q1 (diagram 4), the trip unit on circuit breaker Q1 does not receive a restraint signal from the trip unit on circuit breaker Q2. Time delay tsd is therefore inhibited. Circuit breaker Q1 trips and clears the electrical fault on the equipment instantaneously.
The electrodynamic stress created by the short-circuit current on the equipment is reduced to the minimum.
The ZSI function can be used to optimize the availability of energy (just like selective coordination) and reduce electrodynamic stress on the equipment. The ZSI function is applicable to both short-time and ground-fault protection.