Rockwell Automation Low-Voltage Switchgear and Controlgear User Manual

Page 141

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Standard circuit breakers – above all in the range of higher rated currents – normally only offer
line protection and hence are not suitable for the overload protection of motors. For use in motor
circuits, additional suitable motor protective devices should be provided. It should thereby be
noted that the overload characteristic of the circuit breakers must be slower than those of the
motor protective devices, so that at overload the motor protective device and not the circuit
breaker trips (see IEC 60947-4-1 Annex B.4).

Fig. 4.2-6
Modern high current-limiting circuit breakers with motor protection characteristic

Circuit breakers with motor protection characteristic are used as so-called self-protected motor
starters (combination motor starter). At smaller rated currents the motor protection circuit
breakers often serve as manual motor starter and must then be verified under IEC 60947-4-1 as
motor starters. Modern motor protection circuit breakers have in comparison with the conven-
tional design of motor protection switches a high short-circuit switching capacity.

Circuit breakers for distribution systems and line protection

The requirements on circuit breakers for distribution systems and line protection are different
compared to circuit breakers for motor protection:

- The current range is often fixed

- The thermal triggers are less accurate

- Usually there is no temperature compensation

- The tripping level of the electromagnetic short-circuit release is usually lower and often

adjustable

- Switches of utilization category B have a (usually adjustable) time delay and a short-term

current carrying capacity (I

) and are thus suitable for time-selective cascading

In circuit breakers with motor protection characteristic, line protection is also automatically
assured as the lines are thermally less critical than motors. Depending on the respective
national standard, the lines may be rated according to the current-setting on the circuit breaker
or according to the upper end of the current scale. While with fuses of Type “gG” an overrating
of the fuse and hence of the cross-section of the protected line by one or two current steps is
necessary to prevent tripping during motor start-up, motor lines protected by circuit breakers
can have smaller cross-sections and hence be more fully utilized.

Circuit breakers as load switches