Lessons · Industrial control · fail-safe wiring
A broken wire must stop the machine
Wire stop and safety inputs normally closed so the input is TRUE when healthy; a cut wire looks like a press.
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
Safety standards require it. A normally open stop button with a broken wire can never stop anything.
How to think about it
If this wire broke, what would the input read? For any input whose loss must be safe, the healthy state carries current. In the program, a normally open contact then reads 'healthy'.
Worked example
[Start] [StopHealthy] (Motor)StopHealthy is TRUE while the NC button is unpressed and the wire intact.
Your turn
Name the guard input that must be healthy for the motor to run.
[Start] [] (Motor)
Run a rung and watch the scan
The trap
Inverting in the program to 'fix' a NC input. The wiring is the safety; the program must read it as designed.
Practise fail-safe wiring on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.