Functions
5.2 Ground-fault protection in the case of 3RB21 / 3RB31
SIRIUS 3RU thermal overload relays / SIRIUS 3RB electronic overload relays
54 Manual, 09/2016, A5E03656507420A/RS-AB/003
Ground-fault protection in the case of 3RB21 / 3RB31
Introduction
In addition to the current-dependent protection of loads against impermissibly high
temperature rise resulting from overload, 3RB21 / 3RB31 electronic overload relays offer
ground-fault protection.
A ground fault can arise as a result of insulation damage or ingress of moisture or
condensate. This results in displacement of the neutral point voltage of the power network,
and in an unbalance. The neutral point voltage is simulated virtually in the device and when
a neutral point displacement voltage arises, the overload relay switches the contactor off
instantaneously. This prevents thermal follow-on damage and a resulting, and significantly
more critical, double ground fault.
Note
Contactor assembly for star-delta (wye-delta) start
Internal ground
-fault detection is not possible with contactor assemblies star-delta (wye-
Table 5- 3 Ground-fault detection
Type of ground-fault detection
Internal ground-fault detection For motors with three-wire connection for the
detection of fault currents ≥ 75 % of the current
can be found in the Chapter ...
about ground-fault protection of the electronic
overload relays in conjunction with star-delta
Contactor assembly for star-delta (wye-delta) start
(Page 63)
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