219 / 267
3VT5 Molded Case Circuit Breakers up to 1600 A
Accessories and Components
Overcurrent releases
6/17
Siemens LV 36 · 2008
6
Tripping characteristic adjustment, trip unit ETU UP
The tripping characteristic of overcurrent releases is defined
by standard EN 60 947-2. The characteristic is 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-circuit currents without time delay.
I
2
t
- Characteristic setting in 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. Independent release (thermal) L
The dependent release ETU UP is adjusted using two switches,
Ir and tr. Using the first switch, Ir, the circuit breaker’s rated cur-
rent is adjusted. The characteristic is moved on the current axis.
Turning the second switch, tr, adjusts the time after which the cir-
cuit breaker will trip while passing through 7.2 Ir. The tripping
characteristic thus moves on the time axis. Using the tr 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 al-
lowed to cool off, because it has a thermal memory.
The memory can be disabled by turning the “restart” switch from
the normal “Tt” position to the “T0” position. The dependent re-
lease remains active, and only its thermal memory is inactivated.
Switching off the thermal memory should be used only in well-
justified cases, and with the knowledge that there could be rising
temperature in the protected device with repeated tripping.
2. Delayed independent release 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
i
, 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 more than 10 x 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
I
i
.
I
i
is short-
circuit current that, upon its being reached or exceeded, causes
the circuit breaker instantaneously to switch off. It is set up di-
rectly in kA on the release. The wave form of the tripping charac-
teristic 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 characteristic 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
⋅
=