TL;DR
Insulation degradation in an MCC is confirmed by a megohmmeter test. Test bus-to-ground and each bucket's wiring. Any reading below 10 megohms requires investigation before the equipment is returned to service.
What you might see
- ground fault indicator alarm active
- visual surface tracking or carbonized residue on bus insulation
- condensation or dust coating visible inside the MCC
- periodic nuisance trips correlated with humid weather
Likely causes
Moisture ingress from condensation or water entry through enclosure penetrations
Accumulated conductive dust (carbon, metallic, or chemical) bridging insulation surfaces
Age-related cracking or chalking of molded bus insulation
Prior over-voltage transient or fault arc scarring the bus insulation
Required tools
- Megohmmeter (1000V test voltage)
- True-RMS multimeter for zero-voltage verification
- Dry lint-free cloths
- Arc-flash PPE
- LOTO kit with grounds
Safety first
- A megger applies up to 1000V DC. Discharge all bus sections to ground with a grounding stick after each test before touching terminals.
- Confirm zero voltage on the bus with a meter before connecting the megger. Never megger a live bus.
Procedure
- 1
De-energize the MCC fully: open the incoming feeder breaker, lock out the feeder, and verify zero voltage at the main bus with a meter.
Warning: A full MCC de-energization typically requires utility coordination if the incoming feed is from a medium-voltage transformer. Follow your facility's switching permit procedure. - 2
Disconnect all motor leads from the starter buckets to prevent measuring motor winding insulation instead of MCC insulation.[1]
- 3
Set the megohmmeter to 1000V test voltage for a 480V MCC.
- 4
Test each bus phase to ground: connect the positive megger lead to the bus and the negative lead to the MCC chassis ground. Apply for 60 seconds and record the reading.[1]
- 5
Healthy bus insulation reads above 100 megohms. Between 10 and 100 megohms is degraded but operable. Below 10 megohms requires investigation and repair before re-energization.
- 6
For sections with low readings, inspect the bus insulation visually for cracks, tracking, or contamination. Clean contaminated surfaces with a dry, lint-free cloth. Do not use solvent-based cleaners on molded bus insulation.
- 7
Replace any bus section with cracked, tracking, or physically damaged insulation before re-energizing.
- 8
Re-test after cleaning and repairs. Record all readings in the MCC maintenance log.
Sources
Square D Model 6 Motor Control Center Instruction Bulletin, Square D (Schneider Electric)
Square D Model 6 MCC installation and maintenance instructions, insulation resistance test procedures and acceptance criteria (general)
View source
More guides for Square D (Schneider Electric) Model 6 MCC
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Nuisance trips with measured current well below the trip setting point to a trip unit fault or CT calibration drift. Verify with a secondary injection test and replace the trip unit if it fails.
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