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.


Downward compatibility

The ZSI function of 3VA molded case circuit breakers is compatible with the ZSI functions of Siemens 3VL molded case circuit breakers and 3WL air circuit breakers.

Operating principle

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.

Fault 1

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.

Fault 2

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.

Fault 3

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.

Fault 4

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.

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Communication cable


"Virtual" tripping time of I protection


Output, transmits the blocking signal


Delay time setting of S protection


Input, receives the blocking signal


Delay time of all molded case circuit breakers which detect the short circuit but do not receive a blocking signal when ZSI is activated