What you'll learn
**Symptom:** the line operator reports the conveyor's safety-OK indicator never came on this morning. You walk to the panel — the PSU LED is on, the relay coil is de-energised (as it should be at rest, since the safety chain holds it dropped out), and the PLC input X0 LED is dark. The schematic on the inside of the door says X0 should be sourced from the relay's NC contact and read **high (+24 V) at rest**.
Lab time: ~8 minutes.
Lesson briefing
Relay Contact Card Swap
Symptom: the line operator reports the conveyor's safety-OK indicator never came on this morning. You walk to the panel — the PSU LED is on, the relay coil is de-energised (as it should be at rest, since the safety chain holds it dropped out), and the PLC input X0 LED is dark. The schematic on the inside of the door says X0 should be sourced from the relay's NC contact and read high (+24 V) at rest.
On a relay marked per IEC contact numbering, terminal 11 is the contact common, 12 is the NC contact, and 14 is the NO contact. Two physically distinct screws on the side of the relay body — easy to land a wire on the wrong one if the contact card was specified wrong on the BOM.
Reach for the multimeter on the toolbar and probe the panel.
Hints
Start at the PLC input. Probe X0 against the panel 0 V — what do you read, and what should you read at rest if the input is sourced from the relay's NC contact?
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This lesson uses 6 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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