30.3 FAIL-SAFE DESIGN
All systems will fail eventually. A fail-safe design will minimize the damage to people and equipment. Consider the selection electrical connections. If wires are cut or connections fail, the equipment should still be safe. For example, if a normally closed stop button is used, and the connector is broken, it will cause the machine to stop as if the stop button has been pressed.
NO (Normally open) - When wiring switches or sensors that start actions, use normally open switches so that if there is a problem the process will not start.
NC (Normally Closed) - When wiring switches that stop processes use normally closed so that if they fail the process will stop. E-Stops must always be NC, and they must cut off the master power, not just be another input to the PLC.
Use redundancy in hardware.
Directly connect emergency stops to the PLC, or the main power supply.
Use well controlled startup procedures that check for problems.
Shutdown buttons must be easily accessible from all points around the machine.
