What you'll learn
**Symptom:** the line operator reports the conveyor contactor never pulled in this morning — the panel went dark on the load side. You walk over: the PSU's own indicator LED is on, but the contactor coil is silent and every downstream load is dead. None of the wiring looks scorched; nothing's tripped.
Lab time: ~7 minutes.
Lesson briefing
Rail Short at the Terminal Block
Symptom: the line operator reports the conveyor contactor never pulled in this morning — the panel went dark on the load side. You walk over: the PSU's own indicator LED is on, but the contactor coil is silent and every downstream load is dead. None of the wiring looks scorched; nothing's tripped.
An apprentice was on this panel yesterday — they may have stripped a feed wire a little aggressively at the terminal block. A stray strand across two adjacent terminal-block slots is enough to drop a rail.
Reach for the multimeter on the toolbar. Continuity mode is your friend here — you're hunting for a beep where there shouldn't be one. Voltage mode confirms the symptom; continuity mode finds the bridge.
Hints
Start at the load. Probe across the contactor coil (A1 vs A2) in voltage mode — the coil is wired to sit at +24 V at rest. What do you read?
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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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