Siemens SIRIUS 3RU - Overload Relays in Motor Feeders

Configuration
6.2 Overload relays in motor feeders
SIRIUS 3RU thermal overload relays / SIRIUS 3RB electronic overload relays
60 Manual, 09/2016, A5E03656507420A/RS-AB/003
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 Overload relays in motor feeders
For the protection of
3RU21
3RB20 / 3RB21
3RB30 / 3RB31
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 star
ter 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.