Quote Originally Posted by Emperor Fabulous View Post
What do they actually check ? - and how do they work (preferably in detail, if anyone knows) ?
They basically check that all three pins are wired correctly ... i.e Earth con present and Live is Live and Neutral is Neutral etc. The system normally has three lights with any fault causing one or more lights not to light up. It's a handy gadget to have when you suspect there's a fault on a mains ring.

All our stuff is Pat tested for hotel work, never had a pub ask for a certificate but the PAT testers put stickers on everything anyway.