Description
2.5 Protection functions
3VA molded case circuit breakers with UL and IEC certification
Manual, 01/2019, L1V30435333-01
69
Short-time delayed short-circuit protection (S)
The ID letter for the short-time delayed short-circuit protection is "S" (stands for "Short-time
(protection)"). The S function of the trip unit can be used to implement time-selective short-
circuit tripping in low-voltage networks in which multiple molded case circuit breakers are
installed in series.
The short-time delayed short-circuit protection refers to the short-circuit protection of phases
L1 to L3 (called phases A to C in North America) and the neutral conductor. The protection
function responds if the current in at least one phase exceeds the set tripping current I
sd
for
the set delay period t
sd
.
The S release has a characteristic curve with current-dependent characteristic I
2
t, i.e. the
delay time is dependent on the energy content of the short-circuit current present. The
electronic trip units ETU 5-series and 8-series can be optionally switched over to a current-
independent tripping characteristic. In this case, a fixed delay time t
sd
applies.
Instantaneous short-circuit protection (I)
The ID letter for instantaneous short-circuit protection is "I" (stands for "Instantaneous").
This short-circuit protection function protects phases L1 to L3. The instantaneous short-
circuit protection function responds if the instantaneous value equal to the rms of the current
in at least one phase exceeds the instantaneous tripping current I
i
.
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