TL;DR

Flickering lights or indicators fed from a QO panel usually trace to a loose neutral connection. Measure neutral-to-ground voltage, locate the loose lug, and re-torque.

What you might see

lights on affected circuits flicker randomly
panel voltage indicator fluctuates
sensitive equipment reboots intermittently
surge protectors click on and off

Likely causes

Tap to mark as ruled out

Loose neutral bar connection allowing neutral current to arc intermittently
Poor supply voltage regulation from a sagging utility or upstream transformer
Loose line-side connection at the main breaker
Large switching loads on the same service causing momentary dips

Required tools

True-RMS multimeter (CAT III or IV rated)
Torque screwdriver
Insulated screwdrivers
Arc-flash PPE
LOTO kit

Safety first

Voltage measurements inside an energized panel require arc-flash PPE and an energized electrical work permit.
Never loosen or re-torque neutral lugs with the main breaker closed. Open and lock out first.

Procedure

8 steps · ~30 min

1
Put on arc-flash PPE and open the panel with an energized work permit in effect.
2
Measure voltage at the panel: L1-N, L2-N, and L1-L2 with a true-RMS multimeter. Record the values and observe for fluctuation over 30 seconds.
Warning: These measurements are on energized conductors. Use category III or IV rated meter probes.
3
Measure neutral-to-ground (N-G) voltage at the neutral bar. More than 0.5V N-G indicates a loaded neutral with a resistance issue.
4
If N-G voltage is elevated, open the main breaker, lock out the feeder, and remove the dead-front cover.
5
Inspect every neutral bar lug for looseness. Tug each neutral wire gently. Any movement indicates an undertorqued lug.[1]
6
Re-torque all neutral bar lugs to the value on the panel label.
7
If voltage fluctuates on both legs equally, the issue is upstream. Check the utility meter connection and the service entrance conductors for looseness at the meter main.
8
Restore power and re-measure N-G voltage. Should be at or below 0.1V under normal load.

Sources

Square D QO and I-Line Load Center Installation Instructions, Square D (Schneider Electric)
Square D QO Load Center installation instructions, neutral bar torque specifications and voltage measurement guidance (general)
View source

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