Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 140)

restarts after a voltage outage, for interlocking circuits, for EMERGENCY STOP functions and
for remote release.
Motor operators
Motor or remote operator units open the possibility to issue all commands to circuit breakers
remotely. The functions that are usually manually performed can thus be actuated from remote.
The load feeders can thus be switched-on and -off without direct intervention of an operator on
site. Resetting of a circuit breaker that has tripped is thus possible in remote-controlled distribu-
tion stations.
4.2.2.4 Application of circuit breakers
Depending on their design and accessories, circuit breakers can perform the following switch-
gear functions:
- Circuit
breaker
- Motor protection circuit breaker (manual motor starter)
- Load
switch
- Disconnector
- Main switch
- EMERGENCY STOP switch
The specific properties of the respective design should be taken into account to ensure that the
best suitable circuit breakers are selected for the respective applications. See also Section
2.2.1 .
4.2.2.4.1 Application as circuit breaker
Current-zero quenching circuit breakers
Current-zero quenching circuit breakers have little current limitation and break the short-circuit
close to the natural current zero crossover. Because of their high let-through values they are
mainly used for protecting lines and installations. In versions according to utilization category B
(with breaking delay) they are applied in selective power supply systems with time cascading.
Current-limiting circuit breakers
Current-limiting circuit breakers simplify the applications. They make complex supply-short-
circuit current calculations for every single location of switch installation unnecessary and make
short-circuit coordination nearly as simple as with conventional fuses.
Properties of circuit breakers to simplify planning work:
- High switching capacity renders supply calculations unnecessary:
If the switching capacity of the circuit breaker is higher than the short-circuit level at the site
of installation (with motor outputs typically 1 ... 20 kA) then there is no need to perform sup-
ply calculations.
- Low let-through values (cut-off current and I 2 ·t value):
Weld-free or only slightly welding starter combinations of circuit breakers and contactors are
often possible without overrating the contactors and are therefore economical. The manufac-
turers conduct coordination tests and issue coordination tables in accordance with for exam-
ple IEC 60947-4-1 coordination type “1” or “2”.
- The verification of the short-circuit withstand capacity in accordance with IEC 60439-1 (low-
voltage switchgear assemblies) is not required with cut-off currents
≤ 17 kA.
Circuit breakers for the protection of motors
These are at least equipped with to the motor current adjustable releases (bimetal or electronic)
with motor protection characteristic. Modern motor protection-circuit breakers are also featuring:
- Ambient temperature compensation (in case of bimetals)
- Safe single-phasing protection (for example special calibration, differential trip slider or
electronic fault detection). This is also the prerequisite for use with motors of the ignition
protection type “Increased Safety” (EEx e).
LVSAM-WP001A-EN-P - April 2009 4-32