The user must decide which devices will be disconnected from the controller when assigning inputs and outputs. These elements will not be wired to the controller (electromagnetic control logic). These elements often include devices such as hydraulic pumps and compressors that are rarely turned off after start. For safety reasons, components such as master start push buttons and emergency stops should be hardwired. If the controller fails or the system becomes unresponsive, the user can turn off the system by simply pressing the button.
Figure 10 shows an example of system components that are often left hardwired. The normally closed PLC Fault Contact 1 or watchdog timer contacts is wired in series to other emergency conditions. The contact is closed when the controller operates correctly but opens when there is a fault. This contact can be used by the system designer to shut down the PLC system in an emergency.
PLC fault contacts can be used to improve or implement a safety circuit. If a PLC operates correctly, the normally opened fault contact will close and the normally closed one will open when the PLC turns on. These contacts are connected in series to the hardwired circuit as shown in Figure 10. This means that, if the PLC stops working during normal operation, the normally opened contacts will also open. The hardwired circuit will be shut down at the point that the PLC is the controlling element. To control the power supply to the other control components, this circuit also uses an SCR safety control relay (SCR). To indicate an alarm condition, the normally closed fault contacts can be used.
Figure 10. Hardwired components in a PLC system.
Figure 10 shows how an emergency situation, such as a malfunctioning PLC, will cause power to be cut (L1) at the I/O module. Turning OFF the safety relay (SCR), will cause the SCR to open, preventing power from flowing to the system. The normally closed PLC fault contact (2PLC Fault Contact 2) will notify personnel of system failures due to a PLC malfunction. This type of alarm should be implemented by the designer in the main PLC rack as well as each remote I/O rack. Remote systems also have fault contacts integrated into their remote controllers. This allows subsystem problems to be notified quickly so that they can be addressed without putting people at risk.
