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3VT2 Molded Case Circuit Breakers up to 250 A
Overcurrent releases
3/16
Siemens LV 36 · 2008
3
■
Function
ETU LP, DP, MP and MPS- description of function
Proper functioning of releases does not depend on the form of
current in the main circuit. The function of the release is sup-
ported by a microprocessor, which processes a sampled
signal of the power circuit and recalculates it to obtain an rms
value. Therefore, the releases are suitable for protecting circuits
where the sinusoidal current is distorted by high harmonics
(e.g. circuits with controlled rectifiers, power factor compensa-
tors, pulse loading, and the like).
All the releases protect a circuit against short-circuiting and
overloading. The tripping characteristics are independent of the
ambient temperature. The release is fixed to the switching unit
by two bolts. The transparent cover over the adjustment controls
can be sealed.
Setting the tripping characteristic
The tripping characteristic of the overcurrent releases is defined
by standard EN 60947-2. For releases ETU DP, MP, MPS and
DPN, the characteristic is adjusted using latched switches on
the overcurrent release.
A visual demonstration on setting the tripping characteristic can
be found in the SIMARIS design program.
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. For MPS releases, the delay can be set
at 0, 100, 200 or 300 ms.
I
is a zone of high overcurrents and includes protection against
ultimate short-circuit currents. For MP releases, the time delay
can be set at 0 or 50 ms.
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 rated current of the
circuit breaker, with the characteristic shifting on the current
axis. By means of its internal circuitry, the release is set to one
type of characteristic.
• The dependent releases
ETU MP, MPS and DPN
are adjusted
using two switches,
I
r
and t
r
. The first (
I
r
) switch is used to ad-
just the circuit breaker’s rated current. The characteristic is
moved on the current axis.
By turning the other switch (t
r
), the time is adjusted 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
t
r
switch, it is possible to set a total of 8 characteristics. For
ETU MP and MPS releases there are 4 characteristics for mo-
tor protection and 4 characteristics for protecting lines avail-
able. Breaking times correspond with the release classes 10,
20, 30. By changing t
r
, it is possible to select the characteristic
according to the required motor starting (light, medium, heavy
or very heavy starting). For ETU DPN releases, there are avail-
able 8 characteristics for protecting lines or transformers. It is
not possible to turn the device back on right after the depen-
dent release has been actuated and circuit breaker tripped.
The release must be allowed to cool off, because it has a ther-
mal memory. The memory can be disabled by turning the
switch from the normal “T
t
” position to the “T
0
” position. The
dependent release remains active, and only its thermal mem-
ory 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
This release type is only in the ETU MPS overcurrent releases.
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 parameters
I
sd
and t
v
.
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
i
to
I
i
, will trip the circuit breaker with
delay t
v
, where t
v
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
v
.
3. Independent instantaneous release (short-circuit release)
I
• For releases
ETU DP, MP and DPN
, the independent instan-
taneous release is adjusted using one
I
rm
switch. The
I
rm
switch is used for setting up the short-circuit current that, upon
being reached or exceeded, causes instantaneous tripping of
the circuit breaker.