Lessons · Industrial control · normally closed contact
Normally closed: passes power when FALSE
[/Name] passes power while its input is false; it opens when the input goes true.
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
Stops, faults, 'unless' conditions, and fail-safe wiring where a broken wire must stop the machine.
How to think about it
Should this block the rung when true? Whenever the rung should be blocked BY something being true, use a normally closed contact of that thing.
Worked example
[Run] [/Fault] (Motor)Runs while Run is true and Fault is false.
Your turn
Block when Estop is pressed.
[Start] [Estop] (Motor)
Run a rung and watch the scan
The trap
A physical NC stop button wired to an input is TRUE when nobody presses it. The program then uses a normally OPEN contact of it. Program contact and physical contact are different things.
Practise normally closed contact on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.