User Manual

Description
2.3 Protection system
3WA Air Circuit Breakers
Equipment Manual, 05/2021, 92310000002-03
51
2.3.5.3 Short time ST
With short time ST, the power distribution system is protected against power system faults
such as:
• Phase-phase short-circuit
• Phase-neutral conductor short-circuit
• Phase-ground short-circuit
The circuit breaker trips when the rms value of a phase current or the neutral current exceeds
the setting of the short-time-delayed tripping current for the duration of the set delay.
Characteristic
The characteristic is determined by the short-time-delayed tripping current I
sd, the adjustable
tripping time t
sd, the characteristic type, and a reference point Iref.
Depending on the setting for I
2
t, there are two characteristic curve forms:
• I
2
t = OFF (top left figure):
Tripping takes place independently of the current when the setting I
sd is exceeded after
the set delay t
sd.
• I
2
t = ON (top right figure):
After exceeding the setting I
sd, tripping occurs with an inverted time characteristic. When
the reference current value I
ref is exceeded, the current-dependent short-time delay ends
and the circuit breaker trips after the set delay time t
sd. This ensures the selectivity for
fuses even in the short-circuit current range.