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3VT3 Molded Case Circuit Breakers up to 630 A
Overcurrent releases
4/17
Siemens LV 36 · 2008
4
ETU LP, DP, MP and MPS - description of function
Proper functioning of releases does not depend on the type of
current in the main circuit. The function of the release is
supported by a microprocessor, which processes a sampled
signal of the main 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. Tripping characteristic of the releases is indepen-
dent of the ambient temperature. The release is affixed to the
switching unit by two bolts. The translucent cover over the
adjustment controls can be sealed.
Setting the tripping characteristics
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 unit.
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 ETU 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 ETU MP releases, the time de-
lay 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 circuit breaker’s
rated current, 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
adjust 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 amd MPS releases there are available 4 characteris-
tics for motor protection and 4 characteristics for protecting
lines. Breaking times correspond with the release classes 10
A, 10, 20, 30. By changing t
r
, it is possible to select the char-
acteristic according to the required motor starting (light, me-
dium, heavy or very heavy starting). For ETU DPN releases,
there are available 8 characteristics for protecting lines or
transformers. 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, be-
cause it has a thermal 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 memory is inactivated. Switching off the thermal mem-
ory should be used only in well-justified cases, and with the
knowledge that there could be rising temperature in the pro-
tected 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
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 Irm, 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 pre-
set n-multiple and lasts at least the preset delay time t
sd
.
3. Independent instantaneous release (short-circuit release) I
• For releases ETU DP, MP and MPS, the independent instanta-
neous release is adjusted using one
I
i
switch. 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.