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Source: https://writers.stackexchange.com/a/22152
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

I'd say yes, but ... not if it loses clarity.

Warning labels have to be concise or people won't be able to read them, or won't bother to read them.

For example, a label that says "HIGH VOLTAGE" expresses the warning very briefly and concisely. Yes, it's not a complete, grammatically correct sentence. But you can write it in big letters so people can it from far away. It takes a fraction of a second to read it so they get the message.

Sure, you could write a more thorough explanation. "This machine has a variety of electrical and electronic components. If you were to touch bare wires, either with one hand on each of two separate wires, or a hand on the wires and your feet touching the ground, this might make it possible for electrical current to pass through your body, which can have adverse consequences on your health ..." etc. Would that be better because it uses complete sentences? Obviously not. Yes, that was a ridiculous extreme.

At the opposite extreme would be warnings that use incomplete sentences and invalid grammar in a way that makes them ambiguous. Like suppose someone posted a warning that read "DANGER RED BUTTON". Does that mean that it is dangerous to touch the red button? That you should push the red button when you believe that something dangerous is happening? That the button lights up or something when there is danger? It's hard to say because the warning has been abbreviated too far.

To take one of your examples:

"Ensure power is disconnected before servicing" versus "Ensure that the power is disconnected before servicing the machine"

I don't see how "that the" after "ensure" adds anything or clarifies the meaning. It's easily left out for brevity. Likewise, what does adding "the machine" help? Of course we mean "servicing this machine here that the label is on". As opposed to what? Servicing a customer's charge account? I suppose you could imagine someone thinking that you mean servicing the power supply. But the power supply is a machine too, so specifying "the machine" doesn't clarify the only remotely plausible alternative reading. Thus, I conclude this one is fine the way it is. The meaning is about as clear as it's going to get.

The one about cleaning with water is debatable. Someone might think it means that cleaning with water can generate an explosive atmosphere. That's not irrational: Perhaps chemicals can leak out of the machine, and if you put water on them, the chemicals react with the water. Or some such scenario. You'd have to know a lot about the machine and how it works and the environment that it's in to say, and if you knew all that, you wouldn't need the warning label.

Original score: 10