An in-running nip under a roller is guarded by a hinged guard with its associated interlock switch. The LH picture shows the guard closed and the RH one shows it open.
Some of the material curled and jammed the machine so that it stopped mid-cycle. The operator lifted the guard and did nothing wrong; for example, he did not override the interlock.
After freeing the jam, the machine restarted by itself, which theoretically should not happen with an interlocked guard. "How could this happen?", asked the company directors.
When I examined the electrical circuit diagram, I found the classic mistake of the high-reliability safety circuit being used as a feed to the PLC, rather than being downstream of the PLC.
The PLC was not a safety-rated PLC and what appears to have happened is that the program was paused mid-cycle by the jam-up and therefore could not control the motor.
The lesson is to either use safety-rated PLCs or to use the safety circuit downstream of the PLC.
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