Assuming that your shower is not faulty and having an abnormally high leakage current (not likely if its new) I need to clear up the exact wiring configuration you mention. Do you have an external RCD from your distribution board as your text implies, or is this the combined RCD in a modern "split load" distribution board (fuse box if you like).
If the RCD is separate from your fuse box then you have something really strange and quite dangerous going on (such as your oven is leaking to earth via its metal pipework (we are assuming its gas with electric fan, or an electric oven / gas hob style cooker) which is finding its way to earth via the water connection on the shower. While this earth leak is at a low level (all "element" based appliances leak to some degree) it sounds like your leakage sombined with that of the shower (which as it is new really shouldn't leak more than 1ma)
What is far more likely in reality is the following:
If you RCD is combined into a split load fuse box (lights not on RCD, sockets and other appliances on RCD) then this is just a combined leakage load exceeding the RCD rating.
I don't usually look at this forum, just tripped over it so drop me an email at email (at) ccsleeds . co . uk removing the characters etc. when you've posted a reply and I will help you out.
Pete K
January 2006
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