JohdotaA safety relay is a relay made for monitoring emergency stops and other safety functions. Inside it are redundant relays that monitor each other, so a single fault does not prevent stopping.
Wiring
- Two channels. The two NC contacts of the emergency stop button are wired to the two input channels of the relay, for example S11–S12 and S21–S22. The relay also monitors for a short circuit between the channels.
- Reset. The reset button is wired to the reset input, for example S33–S34. A monitored reset is accepted only on a change of state of the button, so a stuck button does not reset the relay.
- Feedback. The NC auxiliary contacts of the controlled contactors are wired in series into the reset circuit. If a contactor has welded shut, its NC contact is open and the reset does not succeed.
- Safety outputs. The NO outputs, for example 13–14 and 23–24, control the contactors. They close only when both channels are healthy and the reset has been done.
When either channel opens, the outputs open at once. Terminal numbers vary between models, so always wire the relay according to the manufacturer's instructions.
Frequently asked questions
What is a safety relay?
A relay for monitoring emergency stops and other safety functions. Inside are redundant relays that monitor each other, so a single fault does not prevent stopping.
How does a safety relay work?
The safety outputs close only when both input channels are healthy and the reset has been done. When either channel opens, the outputs open at once.
How do you wire a safety relay?
The two NC contacts of the emergency stop go to the two channels, for example S11–S12 and S21–S22. The reset button goes to the reset input, with the NC auxiliary contacts of the contactors in series in the reset circuit. Terminal numbers vary, so follow the manufacturer's instructions.
What is the feedback loop on a safety relay?
The NC auxiliary contacts of the contactors, wired in series in the reset circuit. If a contactor has welded shut, its NC contact stays open and the reset fails.
What is a monitored manual reset?
A reset that the safety relay accepts only on a change of state of the reset button, so a stuck button does not reset the relay.