Verify the power path before replacing the PLC power supply.
A dark rack or repeated power fault does not always mean the supply itself has failed. Use a documented sequence to separate incoming-power, protection, backplane, loading and module faults.
Fault-isolation sequence
1. Confirm the symptom
Record PSU indicators, controller state, affected racks, alarms and whether the fault is intermittent or permanent.
2. Verify incoming power
Check the correct supply source, disconnect state, protective device status and documented input-voltage range.
3. Inspect connectors
Look for loose terminals, damaged plugs, heat discoloration, contamination or mechanical strain.
4. Review rack loading
Compare the installed module load with the correct manufacturer power-budget method for that platform.
5. Isolate downstream faults
If procedures permit, use the manufacturer troubleshooting method to determine whether a module or backplane fault is collapsing the supply.
6. Preserve evidence
Record measurements, indicators, event logs and the exact replacement catalog number before changing hardware.
Symptom matrix
| Observed symptom | Check first | Do not assume |
|---|---|---|
| All rack LEDs off | Incoming source, disconnect, fuse/breaker, PSU input | That the PSU is definitely failed. |
| PSU fault LED present | Manufacturer fault meaning and rack load | That replacing the same module will remove the cause. |
| Rack resets under load | Supply margin, loose connections, downstream faults | That software caused the reset. |
| Intermittent brownout | Input quality, connectors, thermal condition | That an intermittent part is easy to reproduce. |
| New PSU also faults | Backplane, module load, wiring, compatibility | That two supplies failed identically. |
Interactive checklist
Reusable worksheet
Download PLC power-supply checklist CSVMaintainer
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