PLCs and Safety Circuits

[pfalcon]

Decided this needed its own thread. Regards the use of traditional (non-safety) PLC I/O and safety circuits.

I am not so sure about that.

It is common to put a dry contact controlled by the PLC into the estop string so that if the PLC fails, the contact will open, and the machine will go to a known safe condition.

It is also very common to bring some kind of feedback from the estops and MCRs to the PLc so one can determine just what tripped the machine.

What you do not want to be doing is to use the PLC to make any kind of decision that the safety circuits cannot override.

Agreed. An output from a PLC (dry contact) that trips the Estop string is acceptable and common in practice. (PLC Disable okay)

Agreed. An input to the PLC from safety devices where the input is used to EXCLUSIVELY drive indicator lights or screen messages is acceptable and common in practice. (PLC Indicator okay)

Clarification. An output from the PLC that enables the Estop string is NOT acceptable in that a traditional PLC output may fail ON therefore restoring machine function unexpectedly. (PLC Enable very bad)

Clarification. Safety device inputs such as from MCRs, CREs, Estops are not to be used for PLC program control because they will mask defective wiring. This is typically violated by kicking the machine out of automatic mode. Sometimes it is used to trip the PLC Disable (see above) output. Once a month I get to visit a new machine with the safety department. They usually insist on doing their inspection first. This includes testing all the Estops. Typically they pass. Then I force the PLC inputs for safety devices into the ON state. Typically some of the Estops then fail proving the hardwiring was done incorrectly.