Overload Relays In Motor Feeders - Siemens sirius System Manual

4.2.1 Overload relays in motor feeders

Fuseless motor feeders
Fused motor feeders
SIRIUS System Manual
GWA 4NEB 430 0999-02b
There are two categories of motor feeder:
Fuseless motor feeders consist of combinations of circuit breakers for
motor protection and contactors. These combinations are described in
Chapter 5, "3RA1 fuseless load feeders".
Fused motor feeders consist of combinations of contactors and overload
relays, often referred to as starter combinations, with upstream short-circuit
protection (e.g. fuses, circuit breakers for starter protection).
The advantages of fused motor feeders are as follows:
• It is easy to distinguish between tripping caused by an overload and trip-
ping caused by a short circuit. In the event of a short circuit, the fuses
limit the short-circuit current; in the event of an overload, the overload
relay switches off the contactor and thus the motor.
• At voltages > 400 V, fuses have a short-circuit breaking capacity of up to
100 kA. As a result, in 690 V systems, in particular, fused motor feeders
are often preferred.
• If automatic RESET is set, the overload relay resets itself automatically
and does not have to be switched on again locally.
• A remote reset can be implemented very easily by means of attachable
electrical and mechanical RESET modules for the 3RU11 and 3RB10 over-
load relays. The electrical remote RESET is already integrated in the
3RB12 multifunctional devices.
• Group fusing can be used with a circuit breaker to protect several motor
feeders against short circuit simultaneously, which has a positive effect
on the costs of a single feeder.
• If there are already outgoing feeders that are protected against short cir-
cuits in the cubicle, additional short-circuit protection is often unneces-
sary.
• Combinations of a circuit breaker for starter protection, a contactor, and
an overload relay also have the advantage that the feeder can be easily
isolated and that, in the event of a short circuit, it is disconnected in three
poles. 3RV13 circuit breakers for started combinations are available in the
SIRIUS modular system for building these combinations. These circuit
breakers do not have any overload releases.
3RU11, 3RB10, and 3RB12 overload relays
4-5