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However, back in a realistic world.
Is it ever moved, and what sort of people use it ?-
If it gets a a lot of rough use by unskilled hands or is remotely portable (!) then users need to know who to report odd behaviour to, and a retest period that again depends on the above, it could be anything from every time it is used, to 5 years or more.
What to look for
Reliable earthing to all outlets.
If it has not been seen before, polarity and phase sequence to all outlets.
Open and sniff for burning, give anything that could work loose a wiggle.
Look inside for anything more complex than switching, surge arestors, isolation transformers etc that may make testing tricky.
With the power off, insulatation L+L+L+N in parallel to E.
Is RCD protection required, is it upstream.
May be worth reposting in the electrics/wiring part of the forum, or better reporting yourself and asking the mods to move it - there are more electrical folks there, and you may get more focussed advice from folk who have seen that type before..