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Topic: Why did fuse blow? (No, not a joke!) (Read 4365 times) previous topic - next topic


Nov 29, 2012, 06:24 pm Last Edit: Nov 29, 2012, 06:29 pm by tkbyd Reason: 1
I live in the UK.

Here, anything powered by "the mains"... i.e. "household voltage"...  "plug it in to a socket" (thank you, Mr Hubbell... they had to be INVENTED... plugs, sockets! Ever think about that?)... things have a fuse IN THE PLUG. There are also fuses on the whole circuit, of course... but the fuse-in-the-plug IS a good idea, really.

Anyway... just now, I unplugged a "fan heater"... a combination of fan + electric heating element, for space heating. I didn't turn off before unplugging. For some reason, this caused the 13 amp fuse in the plug to blow. (Over 13A flowed at 230v AC, running a motor and a piece of hot-at-the-time resistance wire.) WHY? There WAS a "flash" as the pins came out of the socket. But what would have caused a current surge as the circuit was disconnected? I don't THINK that as I unplugged, the resistance wire broke, shorted out to fan's case, etc. Collapsing field in motor? But why no fatal surge when I "disconnect" more elegantly... with the switch?

I'm sure someone out there can tell me?

(I'd check the matter of whether the wire broke and shorted, except that as I opened the plug to replace the fuse, I dropped one of the plug's pins... IN my house... not "out in the garden" or something... and CAN'T FIND IT. (Just not my day in general. Using fan heater because central heating has been out for 10 days, waiting for a part, and the part came this morning... didn't fix problem. Two instances of the part, actually, as I'd got fed up waiting for first supplier.))


(Oh yes... and when the fuse blew, the circuit's breaker went, too... taking down a computer with several open projects on it. THANK HEAVENS: I HAD saved one essay which would have been a heartbreak to lose.) I'll let you know when I start having fun.


I'm very tempted to suggest frayed wire that shorted as you pulled the plug
especially as it took the circuit breaker out as well!
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Nov 29, 2012, 07:37 pm Last Edit: Nov 29, 2012, 07:54 pm by tkbyd Reason: 1
Good idea... thank you... but I am fairly careful about frayed wires, wire wiskers, loose plugs.

(Writing that inspired me to go Try Again to find the wretched pin. Found it. Fan working fine. Wiggled wire, both ends... no problem.)

Something to do with "switch bounce"... circuit between socket and plug breaking, then re-making in a moment, during the unplugging process? How would that cause current surge? Yes, I THINK it "could" be collapsing fields around the motor coils. Does anyone KNOW that could be it? Any other ideas?

Just curious... don't "need" the answers. Sad, but not that sad.

Saw the spectacular results of a short in a vacuum cleaner in the US, no fuse-in-plug... a series of sinusoidal deep scorch marks on carpet. (I was firefighter at time). The (spiralling) wire inside the 50' cable had flashed VERY hot, along the whole length of the cable, giving rise to the scorch marks described.

Any other ideas?


Some call back emf and it is a physics principle about coils (inductors) in general. Removing the plug without turning off your fan-heater creates an induced voltage proportional to the rate at which the original current drops. If you remove the plug fast enough, you can create a voltage across the plug bigger than the 230V then current may go beyond the fuse's rated value and then "pop".


Look at the first equation on the wiki page.
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But why, then, does fuse NOT blow when the thing is merely switched off? Is the switch more than a fancy and convenient way of "unplugging"?

But I'm encouraged that you, too, seem to be headed towards the "collapsing fields" theory.

Maybe a brief "re-make" of the contact, at just the wrong moment in the "fields collapsing" process creates a "clash" between what the grid is supplying and the voltages inside the very-recently-disconnected motor? (A "re-make" which the switch is properly designed to avoid?) If that makes sense, can anyone express it more accurately?


Good question. Maybe there is a snubber circuit to prevent the switch from arcing?
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The circuit breaker in my light circuit trips when an incandescent  lightbulb blows, why does that happen?


Imagine the filament as a short length of special high resistance, high melting point wire... which it is, but with fancy coiling.

The bulb fails when finally enough of the wire has sublimed away, or when a mechanical shock breaks it... it is brittle, etc.

If one of the ends of the wire, as it falls from its normal position hits the wrong part of the support structure, you get a path for the voltage through a shorter-than-intended piece of the "special" wire. Short piece. Less resistance. Higher current... high enough to trip breaker.

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