You squeeze an arm which opens the jaws and permits it to be clamped
(hence the name) around a cable. The display shows the total current
being consumed. The used to be analogue but most these days are
digital. You just clamp it around one cable only, either live or
neutral will do - but not both as the current in one would cancel out
the other.

..addendum

You can get ones that go around the sheathing of 2 or 3 core cable +/or
flex. We use them all the time [sorry for the rather long link]




http://rswww.com/cgi-bin/bv/rswww/se... cacheID=ukie




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