Fault 5 — Broken 0 V Return Wire on the Contactor Coil

Broken 0 V Return Wire on the Contactor Coil

Symptom: the PLC is commanding contactor KM1 to pull in — the output is on, you can hear the program asking for it — but the contactor isn't engaging. No clunk. The motor downstream isn't running.

Reach for the multimeter. Voltage mode. It's tempting to probe the +24 V side of the coil and call it a day: A1 reads +24 V relative to the panel 0 V rail, so the supply is "obviously fine." But a coil needs a complete loop to energise. If the +V side is hot and the contactor still isn't pulling in, the return path is broken somewhere between A2 and the PSU's 0 V rail.

The trick is to probe BOTH sides of the load. Put the black probe on the PSU's 0 V terminal and red on A1: you'll read +24 V. Now move red to A2: on a healthy panel you'd read 0 V (A2 sits at ground). On a broken-return panel you'll read OL — A2 is electri- cally floating because its only path to ground (the wire to the terminal block) is open.

Continuity mode is the secondary check — once you've narrowed down the suspect segment, probe its two ends for a beep that should be there but isn't.