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Comment by bryanlarsen | Hacker News Reader

Does it fix my favorite pet peeve, the overuse of the wrong meaning of "fail closed"?

"Fail open" usually refers to a fuse that opens and kills power, meaning the system is inert and safe on failure.

"Fail closed" is the opposite -- system has power and is live.

Computer security people have appropriated the term but use it for the completely opposite meaning. When your work straddles electrical engineering and computer security the best way to avoid confusion is just to never use the term.

I can tell my Claude to never use the term, but of course now I'm seeing it everywhere in comments from other people and it drives me batty.

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> fail closed

I understand fail closed to mean, be secure when in failure. And fail open to be continue to operate during a failure. A door that fails closed would not let anyone in; one that fails open lets everyone in.

But I can see how these are not the mutually exclusive definition the labels imply, especially if you apply the concept to entities that aren't doors or otherwise have explicit open/closed states. It's probably best to just be specific in those cases.

Similarly, open loop vs closed loop seems to trip people up enough that I no longer use it. But the confusion is understandable since "closed loop" being "has a feedback loop" sounds backwards. Which, is the same way it's being used in your fuse example; a "closed" fuse closes the circuit making it live. But it's still backwards from the colloquial usage, even if it's correct in that context.

Is there colloquial usage? It's common among computer programmers which is where Claude picked it up, but it's still an engineering term there.

It's comparable to "literally", which has picked up two opposite meanings, one of which appears to be obviously wrong. But because of the opposite meanings, using it the "correct" way is still wrong -- the only way to win is to not play, to stop using the terms "literally" and "fail-closed".

That doesn't make sense at all. Fail open means the method of it's use is still in use.

Say you have a door that has powered locks. You want it to fail "open" so that when the power goes out, it's still useable, and people can get out. That's the source of the term.

Assuming the guy is for real (the closest relation I have to EE is accidentally electrocuting myself at times), I'm pretty sure they're referring to circuits breaking open or remaining closed, hence the opposite meaning.

Took me a minute as well, cause indeed with a computer background, the meaning is completely the opposite. Just like in other security contexts (door locks).

The ambiguity is a good reason to avoid the expression entirely.

MIL-P-1629 from 1949 formally defines fail-open mechanical switches that release pressure on failure.

The concept goes back to a pressure cooker invented in 1679 by Papin.

contextual, as are air brakes 'failing closed'. However, I wonder how fast Papin made his spagbol with his pressure cooker

Nah, "fail open/closed" means that in failure mode something is open. It's "good" when something is a circuit and what failed is a fuse, but it's "bad" when it's your API security. If it's a valve, it probably can be good or bad depending on the use case.

It doesn't mean "fail open" is always the desired/safe outcome. It goes back to 1872 air brakes on a train. The goal is to "fail in safe mode", sometimes it's open, sometimes it's closed.

From the top of my head, where "fail open" is the desired outcome:

- emergency doors

- industrial cooling

- pressure valves

- probably something in HVAC

Note that none of these are "computer security people".

> "fail open/closed" means that in failure mode something is open.

That sentence doesn’t logically parse. Failing open or closed is a concept with two outcomes, it doesn’t mean one or those two outcomes.

I added "/closed" later and forgot to update the rest of the sentence. Too late to edit.

I noticed Claude Opus 5 did this about 30 minutes after reading your comment. In a discussion of price feeds that have gone silent, Opus said - program should fail closed. I don’t want my circuits operating without data!

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