TL;DR

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.

What you might see

entire panel loses power randomly
all circuits drop at once then restore
main breaker trips without apparent cause
power loss correlates with high load events

Likely causes

Tap to mark as ruled out

Failing main breaker with degraded bi-metal element tripping at partial load
Upstream supply issue: loose service entrance lug or utility problem
Main breaker load-side lug intermittently loose under thermal cycling
Panel total load exceeding 80% of main breaker rating on continuous duty

Required tools

Data-logging voltmeter or power quality analyzer
Clamp ammeter (for both legs)
Torque screwdriver
Arc-flash PPE
LOTO kit

Safety first

Replacing a main breaker requires working directly adjacent to the energized line-side lugs. This task must be performed by a qualified electrician with arc-flash PPE and utility coordination if a service disconnect is not available.
Do not operate a panel with an intermittently tripping main breaker without investigation. Repeated trip cycles damage the breaker mechanism and increase the risk of a welded-contact failure.

Procedure

8 steps · ~60 min

1
Install a data-logging voltmeter at the panel output to capture the voltage when the next event occurs. This avoids guessing about timing and duration.
2
If the logger shows voltage drops at the panel but is steady at the meter, the issue is the service entrance connection or the main breaker.[1]
3
If the logger shows voltage drops at both the panel and the meter, the issue is upstream with the utility or transformer.
4
Open the main breaker and lock out the feeder. Remove the dead-front cover.
5
Check the main breaker load-side lugs for looseness and discoloration. Re-torque to spec on the breaker label.[1]
6
Measure the total connected load by clamping both legs of the main breaker output. If continuous load exceeds 80% of the main breaker rating, the main breaker is being stressed and should be upsized or loads redistributed.
7
If the main breaker trips at well below its rating, replace it with a new Square D QO main breaker of the same amperage. Note: replacement requires working near live line-side lugs. Only a qualified electrician should perform this step.
8
If the supply is the source, contact the utility or check the service entrance conductors and meter socket for loose lugs.

Sources

Square D QO and I-Line Load Center Installation Instructions, Square D (Schneider Electric)
Square D QO Load Center installation instructions, main breaker replacement and service entrance procedures (general)
View source

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