I'm an electrician and "normally open" / "normally closed" is well known to me. I recently started to also work on heating systems including the pneumatic system that controls some of the valves in the heating system.

In everything, other than the electrical world, normally closed means that no media/energy can pass in the resting state (of a valve).

Within the electrical world, normally closed means that current/energy can pass in the resting state (of a relay).

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I understand the reasoning/history behind the terminology. But this difference in terminology must without any doubt have caused so much confusion/annoyance through time, and it surely must have caused both expensive and tragic misunderstandings too.

It's unthinkable, that "Off" and "On" would mean the opposite within the two disciplines, and yet it seems to have been accepted that NO/NC means the opposite.

I wonder if IEC/ISO or any industry organatizations have ever tried to introduce a new set of terminology that would mean the same across all disciplines? Or if there are any practices/conventions used in some industries to avoid having such misunderstandings?

It is not impossible reform terminology

Some of the comments seem to suggest that established conventions/terminology is impossible to reform. I disagree. Wire coloring schemes were different all across Europe/the world, but in the end IEC chose to standardize it, and at least all of Europe are following those conventions now. Sweden changed from driving in the left side to driving in the right side of the road. Those transitions were messy, but only for some time.

Example from the Danish language

And a transition does not need to be messy. In the language of Danish, the default translation, and the most used translation from English to Danish is

  • normalt åben (normally open)
  • normalt lukket (normally closed)

But some engineers/electricians have chosen to use the following terminology:

  • normalt sluttet (normally connected)
  • normalt brudt (normally broken apart/disconnected)

They use this alternative terminology for the exact purpose of avoiding potential misunderstandings. This terminology leaves no room for misunderstandings. Its meaning is intuitive and it works for both fluids and electricity. This terminology never caused a misunderstanding, on the contrary, despite the fact that it is being used side by side with the NC/NO terminology.

  • $\begingroup$ I am an electrician and worked with hydraulics and found no issue. Perhaps one needs to pay attention the the different systems. The "old" water analogy for electricity does not always work well... $\endgroup$
    – Solar Mike
    Commented Nov 18, 2022 at 13:51
  • $\begingroup$ Obviously, it goes well most of the time. But that does not in any way mean that it has not caused both confusion and grave misunderstandings. To me, its quite obvious that it must have. $\endgroup$ Commented Nov 18, 2022 at 14:14
  • $\begingroup$ Well, I never had a problem fixing agricultural hydraulics on tractors from the circuit diagrams especially when IH aka Case had the nice mixing of electro-hydraulic... $\endgroup$
    – Solar Mike
    Commented Nov 18, 2022 at 14:53
  • 1
    $\begingroup$ If you think that's bad, wait until how you hear that electrons have negative charge and "saturation mode" means different things for different kinds of transistors. I like to think of a frankenstein switch: mechanically closed = conducting, mechanically open = not conducting. Or like a draw bridge where you are the car, not the ship. $\endgroup$
    – DKNguyen
    Commented Nov 18, 2022 at 15:27

1 Answer 1


It may help to think of the switch in terms of contact gap. A "closed" contact has no gap and conducts. An "open" contact has a gap and doesn't conduct.

In the case of valves it makes more sense to think of them in terms of a gate or door - open or closed to the passage of fluid.

It's unthinkable, that "Off" and "On" would mean the opposite within the two disciplines, and yet it seems to have been accepted that NO/NC means the opposite.

It's a fact of life! Start making it thinkable. It's too late to change it.

  • $\begingroup$ So why not just call a diode a backflow preventer... or one way valve. $\endgroup$
    – Solar Mike
    Commented Nov 18, 2022 at 18:13
  • $\begingroup$ @Transistor: I do understand the logic behind the naming, and the reason why they are named as they are. That does not mean that it wont cause confusion, misunderstandings and accidents. $\endgroup$ Commented Nov 18, 2022 at 20:09
  • $\begingroup$ @SolarMike, see what I've written here: lednique.com/what-is-an-led. $\endgroup$
    – Transistor
    Commented Nov 18, 2022 at 21:20

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