Applicable Systems
MNS-MCC
Symptoms
The earth-fault relay trips and the main incomer breaker opens. The fault log displays G/F and the magnitude is recorded as >30% of rated current. The green Healthy LED extinguishes.
Possible Causes
- Insulation breakdown in motor windings or cable insulation
- Moisture ingress into cable glands or terminal boxes
- Damage to cable sheath causing phase-to-ground contact
- Incorrect grounding of MCC enclosure creating stray currents
Troubleshooting Steps
- Step 1: Isolate the affected motor feeder and megger the motor windings to ground
Insulation resistance reads >10 MOhm at 500V DC - Step 2: Inspect all cable entry glands and terminal box seals for water ingress
Glands are dry, seals intact, and drain holes are unobstructed - Step 3: Perform a step-voltage insulation test on the power cables from breaker to motor
No breakdown occurs up to 2kV DC and leakage current is stable - Step 4: Verify the MCC ground bus is bonded to the building ground electrode with a low-impedance path
Ground impedance is <1 Ohm per IEEE 142
How to Reset
Reset the earth-fault relay by pressing the reset pushbutton after isolating and rectifying the ground fault. Reclose the incomer only after confirming zero residual ground current on the relay display.
Common Replacement Parts
- 1SFA897502R1000: ABB REJ603 earth-fault relay (Compatible with MNS 2.0 and MNS 3.0 platforms)
Frequently Asked Questions
- Q: How sensitive should the earth-fault setting be?
For motor feeders, a typical setting is 30% of FLA with a 0.1-0.3s delay to allow for motor starting transients.
- Q: What if the fault clears when I disconnect the motor?
This indicates the fault is likely in the motor windings or the cable segment between the breaker and the motor. Megger each component separately.
Source: ABB MNS-MCC Technical Manual, Edition 04/2022
Category: Electrical