080727-2057 EST
An incident of maybe 20 to 30 years ago.
At an auto plant a very competent electrican was troubleshooting a lift and transfer function. In his judgement it was probably necessary to troubleshoot with power on. All over this plant are signs with a bucket labeled CHOPSTICK. In this is a wood dowel rod. This is to be used to probe a machine and keep your body out of the machine.
He did not use a chopstick but used his hand to operate a limit switch. This switch was the problem, and he lost his hand. This cost the companies or their insurance companies 1 or 2 million dollars.
If you really have to work hot on a machine use a CHOPSTICK.
Also never trust a PLC and the soild state or mechanical outputs. These could randomly fail although the probability is low. Stay out of a machine when the power is on.
Many light curtains have an auto test at power up. However, when the machine cycle allows entry during a portion of a cycle I have always suggested to the circuit designer to incorporate a logic circuit to verify that the light curtain changed state at this time. In effect this adds a verification test on the light curtain each machine cycle. If it does not verify, than a light currtain fault is generated.
.
