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3VT4 Molded Case Circuit Breakers up to 1000 A
Overcurrent releases
5/10
Siemens LV 36 · 2008
5
Tripping characteristics for ETU UP release with load
The tripping characteristics from the cold state indicate the
tripping times during which it is assumed that, up to the moment
when an overcurrent develops, no current is flowing through the
circuit breaker. The tripping characteristics tripped from warm
state indicate the tripping times during which it is assumed that,
before the moment when an overcurrent develops, current is
flowing through the circuit breaker. Characteristics of electronic
releases are independent of the ambient temperature and are
plotted in a cold state. Digital releases enable simulation of a
release in warm state. The tripping times become shorter in a
steady state, as shown in the following diagram. The steady
state is a period during which the characteristics do not change.
If the circuit breaker is loaded with a reduced current for at least
30 minutes, the tripping times will be cut by half. If the load is
less than 70% of
I
r
, the tripping time does not become shorter.
T - When tripping from the release’s “warm” state, the tripping
time of the characteristics are 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 period for
ETU UP releases is t
u
≥
30 min. During this time, the short-circuit
tripping time t
v
is cut short from the cold-state characteristics by
the coefficient k.
The real tripping time is ts = 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 release
t
v
[s] tripping time of the release derived from the characteristics
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
t
r
= min
t
v
= min,
I
2
t - ON
I
sd
= min
Manufacturer
I
r
= min
Restart = T
(t)
I
rm
= min, 0 ms
t
r
= TV, t
(t)
, min
I
sd
= 0 ms, min
I
= 0.5
I
r
NSO0_00375
I
r
[%]
1.0
T
0.8
0.6
0.4
0.2
0.0
50
60
70
80
90
100
k [-]