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3VT4 Molded Case Circuit Breakers up to 1000 A
Overcurrent releases
5/6
Siemens LV 36 · 2008
5
■
Technical specifications
The electronic overcurrent release consists of a separate and
interchangeable unit, which is supplied with the 3VT4 710-3AA..-
0AA0 switching unit. By exchanging the overcurrent release, the
range of the circuit breaker’s rated current can be easily
changed.
Releases for the 3VT4 710-3AA30-0AA0 switching unit are pro-
duced in four current ranges
I
n
= 315, 630, 800 and 1000 A.
The releases, including their adjustment, cover rated currents
ranging from 125 to 1000 A.
Depending on the needs for adjusting the release’s tripping
characteristics to the protected device and to the variability of
the characteristics with regard to selectivity, the following re-
lease devices are available:
ETU DP
They have one type of characteristics with adjustable
I
r
and
I
rm
.
ETU MP
They have more kinds of characteristics with adjustable
I
r
, t
r
and
I
rm
.
ETU UP
They have universal characteristics, with the greatest variability
in adjustment:
I
r
, t
r
,
I
sd
, t
sd
and
I
i
.
ETU DP, MP and UP
Proper functioning of releases does not depend on the form of
current in the main circuit. The function of the release is
supported by a microprocessor, which processes a sampled
signal of the power circuit and recalculates it to obtain an rms
value. Therefore, digital releases are suitable for protecting cir-
cuits 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. Setting of selective cascading of circuit breakers is
especially enabled by the ETU UP release. Tripping characteris-
tics of the releases are independent 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.
Adjustment of the tripping characteristics for ETU DP and MP
releases
The tripping characteristics of the overcurrent releases are de-
fined by standard EN 60 947-2. The characteristics are adjusted
in two zones using latched switches on the overcurrent release
unit:
L is a zone of low overcurrents and includes the area of thermal
protection.
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.
NSO0_00373
t
I
L
I
I
r
I
i
t
r
NSO0_00374
t
I
L
I
I
r
I
i