Fault 8 — Relay Contact Card Swap (Alarm Stuck ON)

Relay Contact Card Swap — Alarm Stuck ON

Symptom: the night-shift operator phoned: the panel's alarm beacon and X0 input have been ON since power-up, and there's no actual alarm condition — the safety chain is healthy, the door switches are closed, nothing has tripped. You walk to the panel. PSU LED on. Relay KA1 coil de-energised (correct — there's no alarm condition to drive it). PLC input X0 LED lit when it should be dark.

The schematic on the door says X0 is sourced from KA1's NO contact: alarm signal goes high (+24 V at X0) ONLY when the relay coil energises. At rest, with the coil dropped, the NO contact is OPEN and X0 should sit floating — the PLC reads it as OFF. So why is the alarm ON when it should be OFF?

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.