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
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
- 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
More guides for Square D (Schneider Electric) QO / I-Line
How to find and fix a burning smell caused by arcing in a Square D QO panel
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.
How to diagnose buzzing or humming noise in a Square D QO panel
A hum or buzz from the panel is usually a loose bus bar connection or a breaker under high harmonic load. Locate the noisy component with a listening scope and re-torque the bus connections.
How to address corrosion and moisture ingress in a Square D QO load center
Corrosion inside a panel means moisture is getting in. Find and seal the entry point, clean the corroded components, replace anything with green or white deposits on the terminals, and upgrade the enclosure rating if needed.
How to diagnose intermittent power loss from a Square D QO main breaker
Intermittent total power loss points to the main breaker or the upstream supply. Check supply voltage first, then swap the main breaker if voltage is steady upstream but the panel drops intermittently.
Stop fixing the same fault twice.
Dovient turns guides like this into your team's shared playbook, with AI that catches recurring issues before they break the line.