TL;DR
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.
What you might see
- thermal camera shows hot spot at a busbar joint or section
- IR thermometer reading above 60 degrees Celsius on the bus
- discoloration or oxidation at a busbar bolted joint
- downstream breaker tripping without overcurrent cause
Likely causes
Loose bolted joint at a busbar splice, tap, or stab reducing contact area and increasing resistance
Chronic overload on a section of the bus carrying current above its ampacity rating
Oxidized contact surface at a bolted joint, especially on aluminum bus
Missing or corroded Belleville washer reducing clamping force
Required tools
- Thermal camera
- Torque wrench (calibrated to bus bolt spec)
- No-oxide compound (for aluminum bus)
- Fine emery cloth or wire brush
- Arc-flash PPE rated for the MCC's incident energy
- LOTO kit with grounds
Safety first
- All thermal scanning inside an energized MCC requires arc-flash PPE and an energized electrical work permit. Do not reach into the bus section.
- Apply LOTO and verify de-energization before touching any busbar or bolted connection. MCCs contain multiple energy sources.
Procedure
- 1
With the MCC energized and under load, open only the front door panel for the section to be inspected. Do NOT enter the rear or pull out a bucket.
- 2
Perform a thermal scan of all visible busbar sections and joints using a thermal camera. Note the temperature of each joint relative to the bus average.
Warning: This is energized work. Wear arc-flash PPE rated for the MCC's incident energy. An energized electrical work permit is required. - 3
Flag any joint that is more than 15 degrees Celsius above the ambient bus temperature as a hot spot requiring attention.[1]
- 4
Coordinate a controlled de-energization: open the main incoming breaker, lock out both the incoming feeder and the main breaker, and apply grounds if required by your facility procedure.
Warning: MCCs contain live conductors above and below the main breaker even when the main is open. Follow your facility's arc-flash and energy isolation procedure exactly. - 5
Remove the dead-front covers of the affected section.
- 6
Inspect the hot-spot joint: check the bolt for looseness, inspect the contact faces for oxidation or contamination, and verify the Belleville washer is present and oriented correctly.[1]
- 7
Clean the contact faces with fine emery cloth or a wire brush and apply no-oxide compound if bus is aluminum. Re-torque the joint bolt to the value in the Square D Model 6 bus connection instructions.
- 8
Re-install dead-front covers, restore power, and re-scan with the thermal camera after 15 minutes at full load. Temperature should now match the rest of the bus.
Sources
Square D Model 6 Motor Control Center Instruction Bulletin, Square D (Schneider Electric)
Square D Model 6 Motor Control Center installation and maintenance instructions, busbar connection torque and inspection procedures (general)
View source
More guides for Square D (Schneider Electric) Model 6 MCC
How to respond to arc flash signs and pitting inside a Square D Model 6 MCC
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.
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 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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