Often, there is hazardous equipment at a facility that potentially could injure an operator if they get too close. To mitigate the risk that this machinery poses, clients often ask us to design an emergency stop system (E-Stop). Emergency stop systems should not be wired to a traditional PLC because the PLC could potentially fail to stop the machinery in time to prevent an injury. There are two ways to ensure that E-Stop systems keep operators safe: they must be hard-wired to a relay or wired to a specially designed safety PLC. The most common of these two options is the simple hard-wired emergency stop relay. A hard-wired E-Stop system uses three parts:
- A safety control relay
- E-Stop push buttons or pull cords wired in an E-Stop chain.
- An E-Stop reset push button.
Safety Control Relay:
See figure 1 for an example of the E-Stop chain wiring shown on a power distribution drawing.
The safety control relay is the heart of the E-Stop system. The relay's coil is wired in series with the normally closed contacts of all the E-Stop push buttons or pull cords. The safety control relay contacts are wired in series with the motor starters or PLC outputs that feed the hazardous equipment. This way, if any E-Stop push button or pull cord is activated, the safety control relay loses power, which will cause the associated equipment to lose power as well.
E-Stop Push Buttons or Pull Cords:
See figure 2 for an example of wiring for an E-Stop Pull Cord. Notice how there are two wires needed for the E-Stop chain circuit.
The E-Stop push buttons or pull cords provide the user input for the E-Stop system. The buttons or pull cords are usually placed near the hazardous equipment so that an operator can easily activate them during an incident. After the E-Stop system is activated, the button or pull cord must be reset, so the system functions again. Once the system is reset, operators must press the E-Stop reset push button to re-activate the safety control relay and provide power to the equipment.
E-Stop systems create a safer work environment for your operators and avoid hazardous situations at your plant. If you are starting a new project or upgrading a facility, consider installing an E-Stop system to protect your employees.
Daniel Wunderink, Project Engineer