Schmersal AZM Guard Locking — Fault LED & Reset

Schmersal · intermediate · 20 min

A Schmersal AZM solenoid interlock signals its state with three LEDs — green power, yellow status, red fault. Read them, clear the cause, then reset by opening the guard or clearing the diagnostic bit.

⚠️ A guard-locking interlock keeps people out of a hazard until it's safe. Never defeat the lock or bypass the switch; resolve the real fault and reset by the documented method.

Read the three LEDs

A Schmersal AZM solenoid interlock (AZM201, AZM300, and similar) signals both its switching condition and malfunctions through three coloured LEDs:

• Green (Power) — supply voltage present.
• Yellow (Status) — operating condition (actuator detected / guard locked state).
• Red (Fault) — an error condition is active.

A red LED means the interlock has detected a fault; the yellow/green states tell you whether power and the guard/actuator are in the expected condition.

What a fault does to the safety outputs

The AZM continuously monitors the guard-locking function. If a fault results in the guard locking being unlocked when it shouldn't be, the solenoid interlock detects it and the safety outputs Y1/Y2 switch off — the machine is taken to the safe state. So a red fault often comes with the safety outputs dropping, which is the device protecting the operator.

Common fault causes

• Actuator misalignment — the coded actuator on the door is not entering the head squarely (door sag, hinge wear, or a knocked mount).
• Guard opened while locked / lock-monitoring mismatch between the commanded and actual lock state.
• Wiring / supply issue on the safety inputs or the guard-locking control.
• Series-diagnostic chain issue on devices wired in a diagnostic series.

Reset the fault

The AZM fault clears only when the cause is eliminated, and then by one of the documented reset paths:

1. Open the safety guard — with the cause removed, opening the guard resets the fault on many AZM variants.
2. Clear the diagnostic bit — on devices with serial diagnostics, the fault is reset when bit 7 of the request byte changes from 1 to 0 (or the equivalent reset bit is set/cleared in the call telegram from the PLC). The interlock's response and diagnostic data live in a PLC input byte, and the request/reset data is sent back through a PLC output byte, one per interlock in the series-wired chain.

Step-by-step

1. Read the LEDs — confirm green (power) is on and note that red (fault) is lit.
2. Inspect the actuator/door alignment — the most common cause; correct door sag/hinge/mount so the actuator seats squarely.
3. Check wiring and supply to the interlock and its safety outputs.
4. For serial-diagnostic installations, read the interlock's diagnostic input byte in the PLC to identify the specific fault.
5. Reset by opening the guard (cause removed) or toggling the reset/request bit per your diagnostic configuration.
6. Re-validate the guard-locking safety function before returning to production.

Never do this

Do not use a spare actuator to 'trick' the interlock or bypass the lock to keep running — that defeats the guard and is a serious safety violation. Fix the alignment or wiring instead.