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3VT4 Molded Case Circuit Breakers up to 1000 A
Overcurrent releases
5/7
Siemens LV 36 · 2008
5
1. Dependent release (thermal) L
• The dependent release ETU DP is adjusted using one
I
r
switch. The
I
r
switch is used to adjust the circuit breaker’s
rated current. The characteristic is moved on the current axis.
By means of its internal circuitry, the release is set to one type
of characteristic.
• The dependent release ETU MP is adjusted using two
switches,
I
r
and t
r
. The first (
I
r
) switch is used to adjust the cir-
cuit breaker’s rated current. The characteristics are moved on
the current axis. By turning the other switch (t
r
), the time after
which the circuit breaker will trip while passing through 7.2
I
r
.
The tripping characteristic thus moves on the time axis. Using
the tr switch, it is possible to set a total of 8 characteristics.
Four characteristics are available for motors protection.
Breaking times correspond with the release class 10 A, 10, 20,
30. By changing t
r
, it is possible to select the characteristics
according to the required motor starting (light, medium, heavy
or very heavy starting). For protecting transformers and lines,
4 characteristics can be set. It is not possible to turn the de-
vice back on right after the dependent release has been actu-
ated and circuit breaker tripped. The release must be allowed
to cool off, because it has a thermal memory.
The memory can be disabled by turning the “restart” switch
from the normal “T
t
” position to the “T
0
” position.
The dependent release remains active, and only its thermal
memory is inactivated. The thermal memory should be
switched off only in justified cases, and with the knowledge
that the temperature could rise in the protected device with re-
peated tripping.
2. Independent instantaneous release (short-circuit release) I
The independent instantaneous release in designs ETU DP and
ETU MP is adjusted using one switch,
I
i
. The
I
i
switch is used for
setting up the short-circuit current that, upon its being
reached or exceeded, causes instantaneous tripping of the
circuit breaker. Regulation of the short-circuit release takes in
settings for the characteristic appropriate for protecting lines
and motors. The wave form of the tripping characteristic is
adjusted using latched switches on the release’s front panel
according to the needs of the protected device. A visual
demonstration on setting the tripping characteristics can be
found in the SIMARIS design.
Tripping characteristics of ETU DP and MP releases 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 graph. 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 a half. If the load is less than 70% of
I
r
, the tripping
time does not become shorter.
NSO0_00375
I
r
[%]
1.0
T
0.8
0.6
0.4
0.2
0.0
50
60
70
80
90
100
k [-]