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Why do alarms go off, not on?

fire-alarm.jpgQuestion

We were in a pub last week when fire alarm went off. I told my son the alarm had gone off, but he said: “No, it’s gone on, not off”. Why do we say it’s gone off?
Karen, Mortimer

Answer

** Definitive **
Name: Keith, Uxbridge
Qualification: Electrician
Answer: A fire alarm system is usually set, so you need something to trigger it. When it is triggered, it has offset, so you say it has gone off. It’s the trigger that has gone off. It’s not the word off, as in the opposite of on.

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