Emergency stop circuits
Every machine that can hurt someone needs an emergency stop. But an E-stop is not just a red button wired to a PLC input: it must work even when the PLC, a wire or a contact fails.
The E-stop button
- Red mushroom head on a yellow background, easy to reach.
- Normally closed contacts with positive opening: pressing forces the contacts apart even if they are welded.
- Latching: it stays pressed until twisted or pulled to release. Releasing it must not restart the machine.
Why NC?
With an NC circuit, current flows when everything is OK. A pressed button, a broken wire, a loose terminal or a lost supply all give the same result: the circuit opens and the machine stops. With an NO button a broken wire would silently disable the E-stop.
Why not just the PLC?
A standard PLC can fail with its outputs stuck ON, and a program bug can ignore an input. So the E-stop must remove power from the hazards through hardware: a safety relay (or a certified safety PLC) that drives the contactors directly. The standard PLC gets a copy of the status for display and sequencing.
Safety relay basics
- Dual channel: the E-stop has two NC contacts wired to two separate inputs. The relay checks that both open together, detecting a short across one channel.
- Redundant, monitored outputs: two force-guided relays in series switch the contactor coils.
- Feedback loop (EDM): the NC auxiliary contacts of the contactors are wired back to the relay. If a contactor welded, the relay will not reset.
- Manual reset: after releasing the E-stop, the operator presses a reset button (monitored on the falling edge) before the machine can start again.
Stop categories (IEC 60204-1)
| Category | What happens |
|---|---|
| 0 | immediate removal of power (uncontrolled stop) |
| 1 | controlled stop (e.g. the drive brakes the motor), then power is removed; uses a time-delayed safety relay |
| 2 | controlled stop with power left on; not allowed for an E-stop |
The required safety level (PL per ISO 13849-1 or SIL per IEC 62061) comes from a risk assessment. Real safety circuits must be designed and validated by qualified people.
Other safety devices
Guard door interlock switches, two-hand control stations and safety light curtains are wired to safety relays in the same way.
The safety example in the PLC Sim Lab Playground combines an E-stop, a guard door, a light curtain and a tower light, so you can see how they interact.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
Open the Playground →Guided lessons →