Lessons · Industrial control · wiring an emergency stop
Wired to fail safe
An emergency stop is wired normally closed, so the input is on while everything is fine. A pressed button and a broken wire both read as off, and both stop the machine.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
A stop that depends on a signal arriving fails silently when the wire breaks. A stop that depends on a signal being present fails safe: any fault looks like a stop.
How to think about it
In the program, read the healthy state as a normally open contact of the NC-wired input: [EstopOK]. Never invert it with a slash; the wiring already did the inversion.
Worked example
[EstopOK] [Start] (Motor)EstopOK is on while the button is unpressed and the wire intact. Press it, or cut it, and Motor stops.
Your turn
Require the guard circuit healthy as well.
[EstopOK] [] [Start] (Motor)
Run a rung and watch the scan
The trap
Writing [/Estop] in the program with a normally open button means a broken wire reads as 'not pressed' and the machine keeps running. The safety is in the wiring, not the slash.