User Manual

SIRIUS 3RU2/3RB3 overload relays

5.4 Functions

SIRIUS Innovations

System Manual, 01/2011, A8E56203870002-03

499

5.4.2 Ground-fault protection (3RB31 only)

Introduction

In addition to current-dependent protection for loads against impermissibly high temperature

rise caused by overload, phase asymmetry, and phase failure, 3RB31 solid-state overload

relays feature internal ground-fault detection (not possible in conjunction with contactor

assemblies for star-delta (wye-delta) start). This provides protection of loads against high-

impedance faults to ground caused by damaged insulation, moisture, condensation, etc.

Ground-fault protection

In the event of a ground fault, the device trips immediately, disconnecting the contactor (and

thus the load) by means of the auxiliary contacts. To also protect the load and the system

against ground faults caused by damaged insulation, moisture, condensation, etc., the

overload relays support ground-fault monitoring by means of internal ground-fault detection.

Note

Contactor assembly for star-delta (wye-delta) start

Internal ground-fault detection is not possible with contactor assemblies star-delta (wye-

delta) start.

Table 5- 6 Ground-fault detection

Type of ground-fault detection Application

Internal ground-fault detection For motors with three-wire connection for the detection of fault

currents ≥ 75 % of the current setting I

in operation