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Skipweasel[_2_]

In article ,
says...
I've got some 75 watt halogen mains spots (a bit like a small PAR38) on a
dimmer, and when one failed it not only blew the dimmer to bits but
tripped the 10 amp MCB.

Ah, that'll be shorting along the plasma from the fail-point. When
thefilament fails it creates a happy little ball of plasma which has a
very low resistance. If you're unlucky this can reach from feed-wire to
feed-wire and, in effect, present a dead short.

Bulbs have fuses in them lately, but they still don't seem to actually
work, hence the blown bits.

--
Skipweasel - never knowingly understood.