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Low-melt fuses for regular plugs

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Our dishwasher stopped working recently - apparently he wasn't happy with the pay and conditions. No - I digress.

The dishwasher stopped working for the simple reason that the connection inside its plug (on the neutral side, as it happens) wasn't great - the grub screw holding the wire was loose.

By the time the problem came to light, the plug and socket were both scorched - clearly, there had been arcing inside the plug across the poor connection, and perhaps conduction via a layer of carbon that formed as the wire charred.

This is a not-uncommon occurrence, and must surely lead to fires now and again.

Why, then, cannot regular domestic fuses (the ones that go in the plug) be heat-sensitive as well as sensitive to overcurrent? All that's needed is to make the wire inside them (or a part of it) from an alloy that melts at, say, 70°C. This way, if the plug got seriously hot from a poor connection (and assuming the heat spreads enough to heat the fuse), the fuse would fail before things got more interesting.

I know there are thermal fuses available, but not (as far as I know) as replacements for regular plug fuses.

MaxwellBuchanan, Oct 17 2014


       'Cos Big Dishwasher would rather sell you the new model than a new fuse.
the porpoise, Oct 17 2014

       The fault was in the plug, not the dishwasher.
MaxwellBuchanan, Oct 17 2014

       Interesting. Most plugs here (USA) don't have fuses in them. Christmas lights are the only exception I can think of. Do all of your plugs have fuses or just some of them? Is this because the circuit has a large main breaker that could supply too much current for most power cords?   

       It seems like adding a thermal fuse would be a good idea, but I doubt a retrofit would work. Since the heat would primarily be conducted through the metal contacts, you'd either need a thermal fuse on each wire, or have the fuse in very close proximity too all the wires so it would melt if any of the three wires got hot.   

       Alternatively it might be good to include the thermal fuses in the outlets which would work here (USA) as well as other locations.
scad mientist, Oct 17 2014

       // Do all of your plugs have fuses or just some of them?// All. Admittedly, they are probably less important now that RCDs are standard, since a short to earth will trip the breaker. I think any domestic mains circuit (even in the US) can supply more than a single device or light power cord can handle, in the event of a short.   

       //you'd either need a thermal fuse on each wire, or have the fuse in very close proximity too all the wires// Perhaps. But when a plug overheats (it used to happen from time to time in my parents' house - lots of badly-wired plugs and 3kW electric heaters), the whole thing gets hot - at least 45-50°C on the outside, and we're talking a substantial piece of plastic. So I'm pretty sure the whole of the inside gets to 60 or 70°C in most cases.
MaxwellBuchanan, Oct 17 2014

       scad: If you haven't already, I'd recommend a good look at the UK plug/outlet combination. It's beautiful piece of engineering, as far as I know, it's the only polarized, grounded, fused and shuttered (little doors to prevent accidental contact) system. Every plug has an indevidual fuse, which is great, a lamp does not need 13 Amps. What Max here is trying to do is improve on this design. A noble and ambitious and probably futile endevor*.   

       Max: How about going a different route? Make the wire inside the fuse out of something very easily oxidized, Magnesium or something. Surround the wire with inert gas, like in many high power lamp bulbs. Encase the fuse not in the usual glass, but in plastic. Use special plastic that degrades very rapidly when exposed to the UV light from electrical arcing. A few mins of arcing, the plastic degrades enough to let the gas out, the fuse blows at well below the rated current.   


       *The noble, ambitious and futile endevor is a British speciality.
bs0u0155, Oct 17 2014

       //Use special plastic that degrades very rapidly when exposed to the UV light from electrical arcing.// Yes, sort of maybe. But then if you left a spare fuse in daylight - it would be sort of like a vampire fuse.   

       Alloys can be made to melt at very precise temperatures. Even 60°C would mean the fuse was fine in normal use and handling, but would melt if the plug overheated enough to be a fire risk.   

       //beautiful piece of engineering// Ah, but you're so right. Very solid, very safe - at least when competently wired. They're also easier to use than most other designs - the longer earth pin, with its chamfered edges, means that you can easily insert the plug by feel when it's out of sight. And the cord exits downward from the plug, meaning that it doesn't stick out unnecessarily from the wall.   

       US and European hardware seems a bit flimsy and ill- designed.
MaxwellBuchanan, Oct 17 2014

       //The fault was in the plug, not the dishwasher.//   

       Unless you installed an after-market plug, the fault was the dishwasher because the plug is part of the dishwasher.   

       If a toaster plug got scotched in this manner, I don't think I'm going out on a limb to say that most people would just buy a new toaster. Perhaps the dishwasher people think the same way.
the porpoise, Oct 17 2014

       The plug was indeed aftermarket.   

       But would most people buy a new appliance because of a fault in the plug? If so, this explains why the world is going to, if I may say so, shite.
MaxwellBuchanan, Oct 17 2014

       Why not just include a small stink bomb contained within a low temperature melting capsule? Problem solved!
xenzag, Oct 17 2014

       So you return from work to a pile of ashes and a bad smell?
MaxwellBuchanan, Oct 17 2014

       //But would most people buy a new appliance because of a fault in the plug?   

       A small one, yes. Though I'm with you on preferring to fix what I can.   

       //If so, this explains why the world is going to, if I may say so, shite.   

       It's as if you just noticed.   

       Anyway, I thought a quick search would turn up something, but it doesn't. Not even in patents, which means your idea is really new or really old. Closest I can find is about installing a thermal cut off fuse in a clothes dryer or in one of those awful plug-in scent atomizers. So bun for you.
the porpoise, Oct 17 2014

       //So you return from work to a pile of ashes and a bad smell// Long before a fire would start, the smell would alert anyone to the existence of a problem. I have seen plugs virtually turned to carbon and no fire was started.
xenzag, Oct 17 2014

       There are some appliances (e.g. microwave ovens, clothes dryers, industrial/medical equipment) that have a thermal fuse or thermally protected metal-oxide varistor (TMOV) to cut off the circuit if things get too hot. If you really want overkill, you can pot the circuits with non-flammable potting compound or fill the electronics cavity with an inert material (e.g. sand) for extra fire prevention.
Spacecoyote, Oct 19 2014

       //smell would alert anyone//
This requires the presence of an anyone. A thermal fuse does not.
I think this is a brilliant idea. I've only met a few plugs with an integral fuse; the rules here are for circuit breakers (on the main distribution board) for everywhere; but they don't help with a little (but still problematic) heating.
Unfortunately, this only solves issues with-in the plug. What about making the actual wires from a low-melt-temp metal? Any bad connection or over-heating anywhere, things melt and stop. Not so easy to replace as a fuse, of course...
neutrinos_shadow, Oct 19 2014

       Maybe a guy named Noah for arc detection?   

       I wonder if some sort of whole house arc detection, would give the homeowner an edge. " You got a problem, just don't know where it is."
popbottle, Oct 21 2014

       Alternatively, a plug with a small external Tesla coil connected to the inside of the plug and to the earth wire. With a midi chip, it'll play the any number of fire-related songs on arcing/overheating/short circuit..
not_morrison_rm, Oct 21 2014


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