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3VT4 Molded Case Circuit Breakers up to 1000 A
Overcurrent releases
5/8
Siemens LV 36 · 2008
5
ETU DP and MP tripping times shortening with load
T - When tripping from the release’s “warm” state, the tripping
time of the characteristic is cut short during the standstill time t
u
by coefficient k.
Thermal standstill time of the characteristics
For all kinds of characteristics t
r
, the thermal standstill time for
ETU DP and MP releases is t
u
≥
30 min.
During this time, the short-circuit tripping time t
v
is cut short from
the cold-state characteristic by the coefficient k.
The real tripping time is t
s
= k . t
v
Example
The shortening constant can be read from the diagram. With
steady current 85% of
I
r
the real tripping time will be shortened
to:
t
s
= 0.74 . t
v
k [–] time shortening coefficient
I
r
[A] adjusted rated current of the overcurrent release
t
v
[s] tripping time of the release derived from the characteristic
t
s
[s] real tripping time of the release tripped from warm state
t
u
[s] standstill period for particular characteristics
Overcurrent releases are set by the manufacturer
I
r
= min
Restart = T
(t)
I
rm
= min, 0 ms
t
r
= TV, min