Siemens SENTRON 3VA1 - Page 86

Description
2.6 Protection system
3VA molded case circuit breakers with IEC certificate
84 Manual, 03/2019, A5E03603177010-03
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 t
i
= 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 t
i
= 10 ms or t
zsi
= 50 ms.
Fault 3
Q22 signals short-circuit protection to Q11 with the result that only Q22 trips when delay time
t
zsi
= 50 ms expires. Without the ZSI function, the fault would not be cleared until t
sd
= 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 t
zsi
= 50 ms. Without the ZSI
function, Q11 would not trip until t
sd
= 300 ms had expired.
It is especially important to minimize breaking times when short circuits involving very high
short-circuit currents occur.