On Mon, 28 Sep 2009 12:19:07 +0100, David J wrote:

Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

What do you exactly mean by "main circuit switch" and "lighting
circuit protection"?

The arc in a expiring tungsten light bulb is very low impedance so
there is quite a surge in current, this can easyly trip an MCB.

RCD trips are also possible depending on how high the existing
leakage is. These days of everything having SMPSU's and filter
components between all three mains wires the standing leakage through
an RCD serveing a couple of rings etc can be quite high and thus the
RCD only needing a little push to trip it.

--
Cheers
Dave.