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3VT4 Molded Case Circuit Breakers up to 1000 A
Overcurrent releases
5/9
Siemens LV 36 · 2008
5
Adjustment of tripping characteristics, Trip unit ETU UP
The tripping characteristics of overcurrent releases are defined
by standard EN 60 947-2. The characteristics are adjusted in
three zones using latched switches on the overcurrent release
unit:
L - is a zone of low overcurrents and includes the area of thermal
protection.
S - is a zone of medium overcurrents and includes long-distance
short-circuit protection for lines. Intentional delay in tripping of
these low short-circuit currents can be used to achieve selectiv-
ity of protective devices. This type of delay can be set only in
self-contained releases (full version).
I - is a zone of high overcurrents and includes protection against
ultimate short-circuiting without time delay.
I
2
t - Characteristic setting in the ON position represents a
constant value of energy passed through. If fuses are used as
protective elements for outgoing branch feeders, it is possible to
adjust the selective part of the characteristics to better suit the
shape of the fuse characteristics.
1. Dependent release (thermal) L
The dependent release ETU UP is adjusted using two switches,
I
r
and t
r
. Using the first switch,
I
r
, the circuit breaker’s rated
current is adjusted. The characteristics are moved on the current
axis. Turning the second switch, t
r
, adjusts the time after which
the circuit breaker will trip while passing through 7.2
I
r
. The
tripping characteristics thus move on the time axis. Using the
t
r
switch, a total of 8 characteristics can be set. Breaking times
correspond with the release class 10 A, 10, 20, 30. It is not
possible to turn the device back on right after the dependent
release has been actuated 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 inacti-
vated. The thermal memory should be switched off only in justi-
fied cases, and with the knowledge that there could be rising
temperature in the protected device with repeated
tripping.
2. Delayed independent releases S
The delayed independent release has the function of a delayed
short-circuit release. It is used to set up a selective cascade of
circuit breakers. It is set up using specifications
I
sd
and t
sd
.
I
sd
is an n-multiple of current
I
r
(
I
sd
= n ×
I
r
). It is a short-
circuit current that, within the span of
I
sd
to
I
rm
, will trip the
circuit breaker with delay t
sd
, where t
sd
is a delay set up for
switching off the release.
The delayed independent release actuates the circuit breaker
if the current in the circuit reaches at least the preset n-multiple
and lasts at least the preset delay time t
sd
. The independent
release can be disabled by setting the parameter n
(
I
sd
= n ×
I
r
) into the position. Parameter t
sd
can be set to values
with respect to the energy that passed through l
2
t (switch
position l
2
t on). The preset time values are then applicable for
currents higher than 10x current
I
r
. Tripping times of k-multiples
of
I
r
for k < 10 are defined as follows:
3. Independent instantaneous release I
The independent instantaneous release has the function of a
short-circuit release. It is set up only on parameter Irm. Irm is a
short-circuit current that, upon its being reached or exceeded,
causes the circuit breaker to switch off instantaneously. It is set
up directly in kA on the release. The wave form of the tripping
characteristic is adjusted using latched switches on the re-
lease’s front panel according to the needs of the protected
device. A visual demonstration on setting the tripping character-
istic can be found in the SIMARIS design.
NSO0_00456
t
I
L
S
I
I
r
I
sd
I
i
t
sd
t
r
I
2
t
I
2
t
OFF
ON
t
t
v
10
k
⎝ ⎠
⎛ ⎞
2
=