On Sep 28, 11:19 pm, David J wrote:
Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

There is no way that the broken filament can arc internally to the
bulb's bayonet/earth is there?

Was the bulb pointing upwards or downwards? If it's pointing upwards,
bits of broken filament could short out the two solid wires going to
the base, and that could melt a wire outside the bulb and it might
touch the earthed base. I'm sure I've had bulbs blow a fuse even when
hanging downwards.

When a filament is simply broken, I have a party trick where I plug
the bulb in and rotate it so the broken ends touch, and magically the
bulb works again! It works for quite a few hours, good for an
emergency when there are no new bulbs.