TL;DR
Evidence of prior arcing inside an MCC means the equipment is unsafe until fully inspected and repaired. De-energize, assess the damage, replace any component with arc pitting or carbon tracking, and perform a root-cause review before re-energizing.
What you might see
- carbon deposits or burn marks on bus insulation or component housing
- pitting visible on contact surfaces or bus bars
- acrid smell of burned plastic or copper when door is opened
- protection relay logged a ground fault or arc flash event
Likely causes
Loose connection failing under fault current and creating an arc
Insulation failure from contamination or aging allowing phase-to-phase or phase-to-ground arcing
Incorrectly racked bucket with stab misalignment causing an arc on insertion
Inadequate or absent arc flash mitigation allowing a fault to escalate
Required tools
- Megohmmeter (1000V)
- Thermal camera (for post-repair verification)
- Vacuum cleaner with fine-dust filter
- Replacement bus sections and insulators as required
- Arc-flash PPE (full arc-rated suit, face shield, gloves)
- LOTO kit with grounds
Safety first
- After any arc flash event inside an MCC, the equipment must not be re-energized until a qualified engineer has signed off on the repair. Structural damage from the arc may not be visible without disassembly.
- Carbon dust from an arc is electrically conductive. Do not use compressed air inside the MCC enclosure. Use a vacuum instead.
Procedure
- 1
Treat the MCC as unsafe until fully inspected. Do not re-energize after evidence of arcing without completing this procedure.
Warning: Arc pitting indicates a high-energy electrical fault occurred inside the enclosure. There may be structural damage to the bus or insulation that is not visible from the front. Full de-energization and inspection is mandatory. - 2
De-energize the MCC: open the incoming feeder, apply locks and tags to all energy sources, and verify zero voltage on the bus.
- 3
Open the affected section. Photograph all arc damage before cleaning or removing anything.
- 4
Inspect every bus bar and insulator in the affected section for pitting, melting, carbon tracking, or structural damage. Check adjacent sections as well, arc blasts can propagate.[1]
- 5
Replace every component showing pitting on contact surfaces, melted insulation, deformed bus bars, or cracked insulators.
- 6
Clean carbon deposits from the interior using a dry lint-free cloth or a vacuum. Do not use compressed air (it redistributes conductive carbon dust).
- 7
Perform a megohmmeter insulation resistance test on all bus sections after cleanup (see insulation degradation procedure for this machine).
- 8
Conduct a root-cause review: identify what failed and why, and correct the underlying condition before re-energizing.
Sources
Square D Model 6 Motor Control Center Instruction Bulletin, Square D (Schneider Electric)
Square D Model 6 MCC installation and maintenance instructions, post-fault inspection and bus replacement procedures (general)
View source
More guides for Square D (Schneider Electric) Model 6 MCC
How to diagnose a circuit breaker that fails to close in a Square D Model 6 MCC
A breaker that won't close usually has a discharged spring or a coil fault. Check the spring charge indicator, test the closing coil voltage, and replace the coil or the breaker if the mechanism is damaged.
How to find and fix hot spots on busbars in a Square D Model 6 MCC
Hot spots on MCC busbars are caused by loose bolted connections or sustained overload. Scan with a thermal camera, de-energize, re-torque, and re-scan under load to verify the repair.
How to assess insulation degradation in a Square D Model 6 MCC
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.
How to fix nuisance tripping from a faulty trip unit in a Square D Model 6 MCC
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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