Siemens SIRIUS 3RU2 Manual page 23

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).
● The overload relays are especially suitable for use in fused switchgear assemblies. The
devices are also used in applications in line networks with operating voltages pf more
than 400 V. Compared with fuseless design, the fuses still have an extremely high short-
circuit breaking capacity in excess of 100 kA, even in these voltage ranges.
● Automatic RESET is easy to implement with the overload relays. Following an overload
trip, the load feeder need not be switched on again on-site.
● Attachable electrical or mechanical RESET modules 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.
● Applications with lengthy start times can be implemented thanks to the different trip
classes of the 3RB30 / 3RB31 solid-state overload relays.
● 3RB30/3RB31 solid-state overload relays reduce variation and simplify configuring and
inventory management thanks to their wide 1:4 setting range.
● 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 - SIRIUS 3RU2 thermal overload relays / SIRIUS 3RB3 solid-state overload relays
Manual, 09/2014, A5E03656507420A/RS-AA/002
Product description
3.3 Applications
23