Zone-selective interlocking ZSI
The microprocessor-controlled, zone-selective interlocking function has been developed in order to control the total breaking time in low-voltage networks with multiple molded case circuit breakers connected in series.
Regardless of the number of series-connected molded case circuit breakers, all short circuits in the network can be cleared within a maximum time period of 50 ms.
The diagram below demonstrates the operating principle of zone-selective interlocking:
The ZSI function acts on the S and G ranges of the tripping characteristic. Currents within the I range (instantaneous short circuit) continue to cause instantaneous tripping.
If the short-circuit current is sufficiently large, the trip units of molded case circuit breakers Q41, Q33, Q22 and Q11 are activated. Since Q41 clears the fault within ti = 10 ms, none of the other molded case circuit breakers trips even though Q41 has no ZSI and cannot therefore transmit a blocking signal to Q33.
If the short-circuit current is sufficiently large, the trip units of molded case circuit breakers Q32, Q22 and Q11 are activated. As a result of the ZSI function, Q32 temporarily blocks Q22 which in turn temporarily blocks Q11. Depending on the magnitude of the short-circuit current, the fault is cleared either within ti = 10 ms or tzsi = 50 ms.
Q22 signals short-circuit protection to Q11 with the result that only Q22 trips when delay time tzsi = 50 ms expires. Without the ZSI function, the fault would not be cleared until tsd = 200 ms had expired.
The short circuit is detected only by Q11. Since Q11 does not receive a blocking signal from the downstream molded case circuit breakers, it trips after tzsi = 50 ms. Without the ZSI function, Q11 would not trip until tsd = 300 ms had expired.
It is especially important to minimize breaking times when short circuits involving very high short-circuit currents occur.