Intermittent CPU Stop Traced to a Weak Rack Power Supply

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Fault Log · Field Report

Siemens S7 PLC — Intermittent CPU Stop Traced to a Weak Rack Power Supply

Plant: Cement  |  System: Siemens S7 PLC, rack power supply module  |  Downtime: Multiple short unplanned stops before root cause found

Symptom

An S7 PLC controlling a process area started going into CPU STOP intermittently — no consistent time of day, no consistent load condition. Each time, the CPU would need to be manually switched back to RUN before the process could resume. The diagnostic buffer wasn't pointing to a clean, repeatable cause, which made it feel more like a hardware/power issue than a program fault.

Investigation

Started with the software side since that's the fastest to rule out:

  • Diagnostic buffer — reviewed the CPU's diagnostic buffer entries around each stop. Entries were inconsistent between occurrences, not pointing to a single repeatable program fault or module error.
  • Program logic — checked for watchdog timeouts or scan time issues that could trigger a stop under high load. Scan times were within normal range even on the cycles just before a stop.
  • I/O modules — checked individual I/O module status LEDs and diagnostics. No module was flagging a persistent fault.

With the program and I/O side clean, and no repeatable software cause showing up, the CPU itself became the prime suspect — random STOPs with no clear trigger is a classic sign of a failing CPU module. Replaced the CPU as the next logical step. The fault continued afterward, on the new CPU as well, which ruled the CPU out completely and meant the actual cause was somewhere else feeding into it.

Attention then shifted to the rack's power supply module. Under a clamp meter check during normal running, supply output looked acceptable — but checking it only under steady-state conditions wasn't enough, since the stops weren't happening at consistent load points. Monitored the supply output more closely across load transients — motor starts and other loads switching on the same rack — and found the voltage dipping briefly under those transient conditions, enough to momentarily drop below the CPU's minimum operating threshold before recovering.

Root Cause

The rack power supply module was aging and no longer holding output steady under transient load conditions. A brief voltage dip during a load transient was enough to trip the CPU into STOP, even though the same supply looked fine under steady, no-transient conditions — which is exactly why a single steady-state voltage check hadn't caught it earlier.

Fix

Replaced the rack power supply module. Monitored the rack through several of the same load transients that had previously caused a stop — supply held steady, and the CPU stayed in RUN through all of them.

Lesson

A power supply that reads fine under steady-state conditions can still be the cause of an intermittent fault if it's failing under transient load. If a PLC is dropping to STOP with no repeatable software cause, don't just check supply voltage once and move on — watch it through the actual load transients happening on that rack, since that's where a weakening supply will show itself first.

It's also worth remembering that random CPU STOPs will naturally point suspicion at the CPU itself, and replacing it can feel like the obvious first move. Here it didn't solve anything — a good reminder that the module reporting the fault isn't always the module causing it. When a straightforward part swap doesn't fix an intermittent fault, step back and look at what's feeding into that module rather than replacing it again.

Had a similar intermittent CPU stop traced back to the power supply? Drop a comment — worth comparing how others catch transient-only power issues.

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