Siemens SIRIUS 3RT Series System Manual page 489

The advantages of load feeders with overload relays
Installing load feeders with overload relays (fuses + contactor + overload relay or MSP for
starter combinations/circuit breaker (acc. to UL) + contactor + overload relay) has the
following advantages over configurations without overload relays (motor starter protector +
contactor):
● Overload release and short-circuit release are signaled separately. In the event of a short
circuit the fuses or the MSP for starter combinations/circuit breaker (acc. to UL) limit the
short-circuit current and in the event of an overload the overload relay disconnects the
contactor (and thus the load).
● At voltages higher than 400 V, fuses have a short-circuit breaking capacity of more than
100 kA. Therefore, the fused option comprising fuses + contactor + overload relay is often
used in 690 V supplies.
● An automatic RESET can be performed easily in conjunction with an overload relay. As a
result, the feeder does not have to be powered back up locally following an overload
release.
● Attachable electrical and mechanical RESET modules which are compatible for use with
all sizes enable 3RU21 thermal overload relays to be reset remotely. Mechanical RESET
modules which are compatible for use with all sizes can also be attached to
3RB30/3RB31 solid-state overload relays. An electrical remote RESET is an integral
component of the 3RB31.
● Lengthy start times can be achieved with 3RB30/3RB31 solid-state overload relays.
●
● MSP for starter combinations/circuit breaker (acc. to UL) + contactor + overload relay
combinations have the advantage that the load feeder can be isolated easily and all three
poles can be disconnected in the event of a short circuit.
SIRIUS Innovations
System Manual, 01/2011, A8E56203870002-03
3RB30/3RB31 solid-state overload relays support wide setting ranges of 1:4.
SIRIUS 3RU2/3RB3 overload relays
5.2 Product description
489