David explained :
Must remember to test the RCD when hooking up to a caravan site mains.
Usually it is such a relief to find a blue plug that the lead just gets
banged in then the kettle plugged in.

I also do a rough polarity check, though I must admit I have never
found one with an obvious wrong polarity. Make sure that if the EHU
cable is on a reel, that it MUST be run completely off the reel to
prevent it over heating.