In uk.d-i-y, Toby wrote:

Well reasoned * well said = well done

blushthanks, Toby/blush Another possibility is that there's more
than the shower running off the single MCB! Of course, there *shouldn't*
be; just like the immersion in this place *shouldn't* have been put
onto the upstairs ring back at the CU when someone needed a couple of
new ways in the only-one-spare-way-left CU. If there was another load
sharing the shower feed - some recent hack-it-about alterations - then
that too would account for the behaviour reported. And the MCB would
be *right* to trip to protect the cable...

Stefek