Deadman Switch in PLC

A motor will be controlled by two switches. The Go switch will start the motor and the Stop switch will stop it. If the Stop switch was used to stop the motor, the Go switch must be thrown twice to start the motor. When the motor is active a light should be turned on. The Stop switch will be wired as normally closed.



A deadman switch, also known as a deadman’s switch or emergency stop switch, is a safety mechanism used to immediately stop a process or machine in the event of an emergency or hazardous situation.

Deadman Switch

In the Programmable Logic Controller (PLC), a deadman switch can be implemented as an input device to trigger an emergency stop function.

Here’s how it typically works:

  1. Input Device: The deadman switch is a physical input device designed for quick and easy activation. It can take the form of a push button, toggle switch, or other types of safety switches. The switch is usually designed to be manually held or operated by the operator, requiring continuous pressure or engagement.

  2. PLC Input Connection: The deadman switch is connected to a digital input channel of the PLC. The connection is made through wiring that runs from the switch to the input module of the PLC.

  3. PLC Programming: In the PLC program, a specific input address or bit is assigned to the deadman switch input channel. The program continuously monitors the state of the input channel.

  4. Emergency Stop Function: When the deadman switch is engaged or activated, it sends a signal to the PLC input channel, indicating the need for an emergency stop. The PLC program is designed to respond to this input signal by triggering the emergency stop function.

  5. Emergency Stop Action: The emergency stop action can involve shutting down motors, de-energizing actuators, closing valves, or stopping any hazardous operations. The specific actions and sequence of the emergency stop are programmed in the PLC to ensure a safe and controlled stoppage of the process or machine.

  6. Safety Monitoring: The PLC continuously monitors the state of the deadman switch input. If the switch is released or disengaged, indicating a loss of control or operator presence, the PLC program should trigger the emergency stop function to ensure safety.

The implementation of a deadman switch in a PLC system helps enhance safety by providing a means to quickly and effectively stop potentially dangerous operations. It allows operators to take immediate action in emergency situations, reducing the risk of accidents or injuries.

It’s important to ensure that the deadman switch is properly installed, wired, and maintained. Regular testing and inspection of the deadman switch and its associated circuitry are necessary to ensure its reliability and effectiveness as a safety mechanism.