396LE005550A
Function of the Energy trip unit
3.2 Protection function
3.2.6
Zone Selective Interlocking
function (ZSI)
The Zone Selective Interlocking function is designed to reduce electrodynamic
stress in the electrical power distribution system (conductors and busbars) when
time selectivity is activated.
It applies to the upstream section of the electrical power distribution system
composed mainly of open circuit breakers and moulded case type circuit breakers.
It involves connecting the upstream and downstream circuit breakers with a special
cable. This connection may or may not inhibit the tsd and/or tg time delay of the
circuit breakers depending on the location of the short-circuit fault.
When the Zone Selective Interlocking function is activated on a circuit breaker,
it inhibits its time delay settings and has an almost instantaneous trip time.
When it is not activated, the circuit breaker operates according to the trip time delay
settings.
The Zone Selective Interlocking function is supplementary to time selectivity
(tsd and tg time delay). Under no circumstances can it replace it.
It is applicable to short time delay protection and ground protection.
Here are two examples to explain the functioning.
Q1
ZSI2
ZSI1
Q2
Q3 Q4
ZSI2
ZSI1
ZSI2
ZSI1
a)
b)
ZSI2
ZSI1
Zone Selective Interlocking: Example
First, circuit breakers Q1, Q2, Q3, Q4 are set to their respective thresholds enabling
time selectivity to be activated.
Fault example a):
- In the event of a fault downstream of circuit breaker Q3, circuit breakers Q1,
Q2 and Q3 detect the fault at the same time. Thanks to the connection cable
between the circuit breakers, circuit breaker Q3 informs circuit breaker Q2 that
it has detected the fault. Circuit breaker Q2 then informs circuit breaker Q1 that
it has also detected the fault. Circuit breakers Q1 and Q2 then maintain their
respective time delays so that circuit breaker Q3 can eliminate the fault instantly.
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