6EP1333-4BA00 Siemens — Unstable PLC power supply — cyclic restarts
At a glance
- Severity
- FAULT STOP
- Domain
- PLC
- Reference standard
- IEC 61131-2
- Product / context
- SITOP PSU8200 24V/5A — Recommended power supply for S7-1500
What it means
The PLC restarts cyclically (power cycling) or has intermittent interruptions. CPU LEDs turn off and back on in the boot sequence. The program is interrupted causing unpredictable machine stops. Actuators may behave erratically during restart phases.
Probable causes
- 24VDC power supply undersized or failing (voltage drops below threshold during peak loads).
- Loose or oxidized power terminals causing micro-interruptions.
- Disturbances on the upstream 230VAC mains (voltage spikes, mains micro-interruptions).
- Intermittent short circuit on the 24VDC line causing the power supply voltage to drop.
Reference values
| alimentazione 24VDC | {"nominale":24,"acceptable_range":"20.4-28.8 V","ripple":"< 5%","nota":"Also check the protective fuses on the modules."} |
|---|
Where to start
Measure with a multimeter the 24VDC voltage directly at the CPU power terminals. Is the value stable between 23.5V and 25.5V under load?
Full guided diagnosis in the app
The diagnostic tree for this fault has 7 guided steps with measurements, checks and resolutions. You'll find it in ElectraIQ, one of the five IndexOS apps: free, works offline.
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