RCD switch failed to trip on a fault?

The RCD could be a faulty unit. Just replace with a new unit.
No need to go near "electricity boards fuse". It is sealed and should not be tampered with by anyone other than the electricity supplier. Removal of fuse from main switch fuse unit (which is part of the consumer unit) should be sufficient isolation for safe replacement of RCD.

J
May 2009

RCD's can either be fitted into the consumer unit (£35 or thereabouts) or separately from the consumer unit (£55 or thereabouts). Fitting needs to be done by someone who knows what they are doing as the electricity boards fuse has to be removed for safe installation.

John
May 2009