Why does a Fan Isolator have to be a 3 pole Switch? E.g. Gu1413

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Fans are 3 pole because in bathrooms, there are usually three lines requiring disconnection to completely isolate the fan. Which are: - permanent live (enables run-on under timer control, also for PIR or humidistat controls, where fitted) - switched live (from light circuit, brings fan on with light) - neutral. BS EN 60669-1 refers.