X filter goes bang

[maniacminer]

Not from a Beeb or anything remotely Acorn - but a good old X cap failure - this time an old (2000s) Black and Decker hot air gun - the wire coming into the handle had perished internally and created an arc between live and neutral (there is no earth/ground as it is double insulated) In this design, the suppression capacitor is held within the handle. I smelt something odd (I wasn't using it) and then around a minute or so later there was a really strange squeaking/buzzing sound followed by a loud bang and a scream (grown men shouldn't be able to have access to that register) Despite all these fireworks, the plug fuse hadn't blown, the wire had burnt itself out and the capacitor sacrificed itself trying to absorb the high frequency transients.

[sP1d3r]

I'd have thought that a short like that would trip your fuse box?

[1024MAK]

Fuses take time to blow, and can survive short term overloads that are significantly greater than the current rating of the fuse (typically one and a half times).

MCBs (miniature circuit breakers) normally have a magnetic trip and a thermal trip. The thermal trip will take time to operate, and will tolerate an overload significantly greater than the current rating for short durations. The magnetic trip is much faster, but is designed more to deal with short circuits. An arc may not have caused enough current to flow to trip either MCB mechanism if this was on a ring circuit. Ring circuits are fused at 30A, or have an 30A or an 32A MCB.

Either way, 30A at 240V is 7.2kW.

A plug top 3A fuse would allow around 1kW before blowing. A plug top 5A fuse would allow around 1.8kW before blowing. Either way, I don't think there would be much of a hot air gun left if that much power had been converted to heat...

Mark

[sP1d3r]

Coo, sounds heavy!
My mains fuse box was tripped by a Rodime drive sometime ago, nothing apparent damage wise.

[maniacminer]

There was a 13A fuse in the plug (it's a 2kW model) I cut the wire off, cleaned the blast area, replaced the capacitor and crimped the connections - all good and it's working again. The cable is around 15cm shorter than it was. I had to put a new fuse in because I broke open the ceramic to see what state the fuse wire was in - there was sand inside, wow, back in the days when a Chinese fuse was ok, the wire still solid and securely fastened to each cap end. The arc wasn't strong enough to trip the over current breaker and there was no short to ground to trip the residual current breaker. I think this is one good reason for arc trip breakers to be installed, it only takes a pinched/frayed cable and the cable lying on something flammable and you've got a big fire on your hands