TL;DR

A burning smell in a panel almost always means a loose connection arcing under load. De-energize, locate the discolored wire or terminal, re-terminate, and torque to spec.

What you might see

acrid burning odor near the panel
discolored or charred wire insulation
warm panel cover or door
intermittent breaker trips on the affected circuit

Likely causes

Tap to mark as ruled out

Loose connection at a breaker terminal causing arcing under load current
Loose neutral bar connection causing current to arc at the neutral lug
Over-tightened connection that cut through wire insulation
Aluminum conductor not using anti-oxidant compound, causing resistance buildup

Required tools

Torque screwdriver or ratchet torque adapter
Wire strippers
Insulated screwdrivers
IR thermometer or thermal camera
Arc-flash PPE
LOTO kit

Safety first

The main breaker line-side lugs remain energized after the main breaker is opened. Only a qualified electrician should work inside the panel with the utility live.
If there is visible flame or smoke, do not open the panel. Disconnect power at the meter or utility disconnect, then evacuate the area if the smell persists.
Do not restore power until all loose connections are re-torqued and damaged components are replaced.

Procedure

9 steps · ~45 min

1
Open the main breaker to de-energize the panel. Lock out and tag out the upstream utility disconnect or feeder breaker.
Warning: Even with the main breaker open, the line-side lugs on the main breaker are still energized from the utility or feeder. Do not touch the line-side lugs.
2
Put on arc-flash PPE before opening the panel cover.
3
Remove the dead-front cover and visually inspect every breaker terminal, the neutral bar, and the ground bar for discoloration, charring, or melted insulation.
4
Locate the source of the smell: follow the discoloration or charred area. Mark it.
5
Check the wire at the suspect terminal. If insulation is melted back more than 0.5 inches, cut the wire to expose clean copper and re-strip.
6
Re-seat the wire in the breaker terminal or neutral bar lug and torque to the value on the panel label or breaker spec sheet.[1]
7
Photograph the damage before and after for the maintenance record.
8
Replace any breaker with melted or deformed body plastic. Replace the neutral bar lug if threads are damaged.
9
Restore power and re-check the area with a thermal camera or IR thermometer after 15 minutes under load.

Sources

Square D QO and I-Line Load Center Installation Instructions, Square D (Schneider Electric)
Square D QO Load Center installation instructions, terminal torque and wiring procedures (general)
View source

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