Multiple Units Fail After the Same Power Event: Decision Tree
Why this matters
When one device fails, you diagnose the device. When several unrelated devices fail at once, you have to diagnose the building's electrical system first, or you will replace part after part only to have the next one fail the same way next week. This is also the scenario where a customer's frustration compounds fastest, because every callback looks like you missed something, when the real miss was not checking upstream. Get the sequencing right and you save both the customer and yourself repeat trips.
Start here: check for an active hazard before touching anything
A power event severe enough to damage multiple pieces of equipment can also mean damaged wiring, a compromised panel, or a grounding fault that is still live. Before opening any equipment, verify there is no ongoing hazard: look for scorch marks, smell for burning, and confirm the main panel and any protective devices are intact and not showing damage themselves. Do not proceed with component-level diagnosis if you suspect the panel or service entrance itself was damaged; that gets a qualified inspection first.
Step 1: map what actually failed, not what the customer thinks failed
Get a full inventory before you form a theory. Ask what stopped working, and independently check anything else that shares the same panel, sub-panel, or circuit, since the customer may not think to mention a device that "just seems a little off" rather than fully dead.
- List every affected device and which circuit or panel it's on.
- Note whether the failures are identical in symptom or different.
- Note the physical distance between them (same room, same panel, different parts of the building).
Step 2: read the pattern the map gives you
| Pattern observed | Most likely explanation | Where to look next |
|---|---|---|
| Everything on one circuit failed, other circuits fine | Localized fault on that circuit (bad connection, overloaded circuit, damaged wiring) | That circuit's wiring and breaker, not the panel as a whole |
| Everything on one leg of a split-phase or one phase of three-phase service failed | Lost or high-resistance neutral, or single-phase loss upstream | Service entrance, neutral connection, utility-side supply |
| Failures scattered across multiple circuits and panels | Building-wide event: surge from utility side, lightning-related transient, or a major grid disturbance | Main service entrance protection, grounding system, utility history for the area |
| Only the most sensitive electronics failed, motors and heaters fine | Voltage transient (spike), not a sustained overvoltage | Surge protection at the electronics, not a wiring problem |
| Only motors and heavy loads failed, electronics fine | Sustained overvoltage or lost neutral causing voltage imbalance | Neutral and voltage balance across legs, not a simple surge |
A lost or high-resistance neutral deserves special attention: it can push one leg of a split-phase system well above normal voltage while the other drops, which reads exactly like "half my house is fried and the other half is fine." This is a utility-side or service-entrance problem, not something you fix by replacing the failed devices alone.
Step 3: check voltage and balance before replacing anything
With a meter, verify voltage at the panel is correct and balanced across legs or phases before you condemn individual components. If voltage is currently normal, that does not rule out a transient event that has since passed, but it does let you move forward with confidence that you are not about to re-energize new replacement parts into an ongoing overvoltage condition.
Step 4: decide who needs to be involved
- If the pattern points to a building-wide event or a neutral/service problem, this is beyond swapping parts. Loop in a licensed electrician for the service entrance and grounding, and hold off on replacing every downstream device until that root cause is confirmed fixed.
- If the pattern is confined to one circuit or one device, proceed with normal component-level diagnosis on that scope.
- If you are not the trade responsible for the affected equipment (for example, an HVAC tech finding fried kitchen appliances on the same event), document what you found and recommend the right trade rather than guessing at equipment outside your scope.
Step 5: fix the root cause before or alongside the repairs
Replacing every damaged device without addressing a confirmed upstream cause (unprotected service entrance, damaged neutral, no surge protection) means the next event takes out the new parts too. Present the upstream fix and the individual repairs as one plan, not sequential surprises.
Recap
- Confirm no ongoing hazard before you start.
- Build a full map of what failed and on which circuits.
- Match the pattern to a likely upstream cause using the table.
- Verify voltage and balance before replacing parts.
- Escalate to the right trade for the root cause, then fix devices.
References
- NFPA 70 (National Electrical Code) for service entrance and grounding requirements
- IEEE guidance on voltage sag, swell, and transient classification
- See related: The Protective Devices That Should Have Prevented Power-Event Damage; What a Brownout Does Differently Than a Full Outage