On Sat, 30 Aug 2008 19:50:39 +0100, Bob Minchin wrote:

RCD devices only look for a difference in the current flowing in the two
connections. It has nothing to do with earth.

Yep. RCD's tripping with the "wrong" polarity strikes me as an big
indication that something isn't quite what it should be with the caravan
wiring.

You may however find that the test button does not work if L & N are
reversed as I think all this does is to connect a resistor between the
live and earth. This might only provide enough trip current if the live
is truly live.

I should hope it places a resistor between L and N and across the current
transformer. So that it actually tests what it is supposed to be doing
rather than relying on another path that might not exist.

L ------ CT -------+---
|
+-- R -- TB --+ LOAD
|
N ---+-- CT -----------

--
Cheers
Dave.