The user should decide which devices will not be wired to the controller during the assessment of inputs and outputs. These elements will remain part of the control logic of electromechanical. These elements usually include devices that are not switched off frequently after start, such as hydraulic pumps and compressors. Components like master start push and emergency stops buttons should also remain hardwired, principally for safety purposes. This way, if the controller is faulty and an emergency occurs, the user can shut down the system without PLC intervention.
The normally open PLC Fault Contact is wired in series with other emergency conditions. This contact stays closed when the controller is correctly operating, but opens when a fault occurs. The system designer can also use this contact if an emergency occurs to disable the PLC system operation.
PLC fault contacts are the contacts of safety that are available to the user when enhancing or implementing a safety circuit. When a PLC is operating correctly, the open fault contact closes and the normally closed on opens when the PLC is first turned on. These contacts are connected with the hardwired circuit in series, so that if the PLC fails during standard operation, the normally open contacts will open. This will shut down the hardwired circuit at the point where the PLC becomes the controlling element. This circuit also uses a SCR (safety control relay) to control power to the rest of the control components. The normally closed fault contacts are used to indicate an alarm condition.
An emergency situation including a PLC malfunction will remove power to the module I/O. the turning OFF of the SCR (safety control relay) will open the SCR contact, stopping the power flow to the system. Furthermore, the normally closed PLC fault contact will alert personnel of a system failure due to a PLC malfunction. The designer should implement this alarm type in the mail PLC rack as well as in each remote I/O rack location.