Overload Relays In Motor Feeders; Motor Protection With Overload Relay - Siemens SIRIUS 3RU Manual

Configuration

6.2 Overload relays in motor feeders

6.2 Overload relays in motor feeders
6.2.1

Motor protection with overload relay

The individual overload relay families protect the following loads against the consequences
of an overload, a phase failure, and phase asymmetry.
Table 6- 1
For the protection of
Three-phase current loads
DC loads
Single-phase AC loads
Note
In the case of three-phase current loads, only 3-pole circuits (3 phases) are permitted.
4-pole circuits (3 phases + neutral conductor) are not permitted.
Note
Design of motor feeders
An overload relay alone cannot generally protect a load against overcurrents. The tripping
characteristic curve is too inert for a short-circuit and also the contactor actuated by the relay
in the event of tripping is not suitable for reliable breaking in the event of a short-circuit.
A protective device such as a 3RV2 motor starter protector or a fuse must be installed
upstream of the load to provide protection against short-circuits.
Appropriate contactors are required to protect loads. Chapter Overview of combinable 3RT
contactors (Page 61) provides an overview of the coordination between overload relays and
contactors, along with their power ratings.
60
Overload relays in motor feeders
SIRIUS 3RU thermal overload relays / SIRIUS 3RB electronic overload relays
3RU21
✓
✓
✓
Manual, 09/2016, A5E03656507420A/RS-AB/003
3RB20 / 3RB21
3RB30 / 3RB31
✓
-
-