Electronic Overcurrent Trip Unit (Etu) - Siemens SENTRON 3VL System Manual

The magnetic trip unit
comprises a yoke mounting through which a current path runs, and a flap armature that is
kept at a distance from the yoke mounting by a tension spring. If a short-circuit current now
flows along the current path, the magnetic field thus generated causes the flap armature to
be moved towards the yoke mounting against the opposite force of the tension spring. The
release time is almost current-independent and instantaneous. The flap armature releases
the switching lock and thus opens the switching contacts before the short-circuit current can
reach its maximum; a current limiting effect is thus achieved. Immediately after release, the
flap armature is moved back to its starting position by the opposite force of the tension
spring.
3.3.4

Electronic overcurrent trip unit (ETU)

Electronic trip units (ETU)
In contrast to thermomagnetic trip units (TMTUs) where the overcurrent trip is unit caused by
a bimetal strip or magnetic release, electronic overcurrent trip units (ETUs) use electronics
with current transformers. The ETU captures the actual currents and compares them with the
default specifications.
All SENTRON 3VL circuit breakers with electronic overcurrent trips measure the actual
effective current (true RMS). This is the most accurate method of measuring.
ETUs are available from the VL160 circuit breaker up to and including the VL1600. The
SENTRON VL800, VL1250 und VL1600 circuit breakers are only available in the version
with electronic overcurrent trip unit.
The trip units can be replaced by the customer without special tools. Replacement is
described precisely in the operating instructions included with the ETUs. After installing the
electronic overcurrent trip unit in the relevant circuit breaker, the battery supply must be
tested with test function 3VL9000-8AP00.
Configuration
The electronic overcurrent tripping system consists of:
● 3 to 4 (3-pole or 4-pole) current transformers that also provide their own power supply.
This means an external auxiliary voltage is not required.
● Evaluation electronics with microprocessor
● Tripping solenoid
In all versions with electronic trip units for the SENTRON 3VL circuit breakers, the current
transformers are located in the same housing as the trip unit. At the output of the electronic
overcurrent tripping module, there is a tripping solenoid that releases the circuit breaker in
the event of an overload or short-circuit.
3VL molded-case circuit breakers
System Manual, 03/2009, 110 0110 - 02 DS 01
Product description
3.3 Configuration
21