Normally open vs normally closed

“Normally open” and “normally closed” describe a contact at rest — nobody pressing the button, the coil de-energised, the float hanging. A normally open (NO) contact is open at rest and closes when something acts on it. A normally closed (NC) contact is closed at rest and opens when something acts on it.

The trap in the word “normal”

Normal means at rest, not “the way it usually sits while the machine runs”. A starter’s auxiliary contact is drawn open on the print because the coil is drawn de-energised — even though, with the motor running all day, that contact spends most of its life closed. Every ladder diagram is drawn with everything at rest, so the print shows the normal state, and you work out the running state from it.

How they look and what they do

On a ladder, a NO contact is two short parallel lines; a NC contact is the same with a diagonal line through it. A pushbutton is drawn the same way with a bar above. Stop buttons are NC, so pressing one breaks the rung; Start buttons are NO, so pressing one makes it. Series contacts behave as AND — every one must be closed. Parallel contacts behave as OR — any one closed path will do.

One worked, so the idea sticks

One rung, four states

The rung: a NO contact A in series with a NC contact B, feeding coil M.
A at rest, B at rest: A is open, so M is off.
A actuated, B at rest: A closes, B is still closed, so M is on.
A actuated, B actuated: B opens and breaks the rung, so M is off — the NC contact wins, which is exactly how a Stop button or an overload contact is meant to behave.

Work the state of every contact first, then read the rung left to right. The motor control worksheet has more.

Preguntas frecuentes

Is a Stop button normally open or normally closed?

Normally closed. It sits closed so the rung can carry current, and pressing it opens the rung. That is also why a broken wire to a Stop button stops the motor instead of leaving it unstoppable — it fails safe.

What is a contact that is “held closed”?

Some devices are drawn in their normal state but installed in a position that actuates them — a limit switch held down by a guard, say. Prints mark these as held open or held closed. Read the note; the symbol alone shows only the at-rest state.

Does NO/NC apply to relays and to pushbuttons the same way?

Yes. For a relay or starter, “at rest” means the coil is de-energised; for a pushbutton it means nobody is pressing it; for a float or pressure switch it means the process variable is below its setting. The rule is the same: at rest is normal.

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