What you'll learn
**Symptom:** the morning shift calls — the panel won't power up. You walk over: the PSU indicator LED is on, but the PLC base is completely dark. None of the input or output LEDs are lit; nothing on the panel responds.
Lab time: ~6 minutes.
Lesson briefing
Broken +24 V Supply Wire to the PLC
Symptom: the morning shift calls — the panel won't power up. You walk over: the PSU indicator LED is on, but the PLC base is completely dark. None of the input or output LEDs are lit; nothing on the panel responds.
The PSU is fine. Something between the PSU and the PLC has gone open — most likely a wire that's worked loose at a terminal screw, or a broken strand at a ferrule. There's no scorching, no smell, no tripped breaker — just silence.
Reach for the multimeter on the toolbar. Voltage mode is your friend here. Walk the +24 V rail from upstream to downstream: confirm the supply is healthy at the PSU output, then keep probing toward the PLC until the voltage disappears. The break sits between your last "good" reading and your first "OL". Continuity mode is the secondary check — once you suspect a segment, probe its two ends for a beep that should be there but isn't.
Hints
Start at the PSU output. Probe across the +V and 0V terminals in voltage mode — confirm the supply itself is alive (you should read +24 V).
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This lesson uses 5 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.
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