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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.
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'Cos Big Dishwasher would rather sell you the new model than a new fuse. |
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The fault was in the plug, not the dishwasher. |
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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? |
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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. |
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Alternatively it might be good to include the
thermal fuses in the outlets which would work
here (USA) as well as other locations. |
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// 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. |
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//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. |
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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*. |
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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. |
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*The noble, ambitious and futile endevor is a British speciality. |
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//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. |
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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. |
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//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. |
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US and European hardware seems a bit flimsy and ill-
designed. |
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//The fault was in the plug, not the dishwasher.// |
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Unless you installed an after-market plug, the fault was the dishwasher because the plug is part of the dishwasher. |
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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. |
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The plug was indeed aftermarket. |
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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. |
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Why not just include a small stink bomb contained within a low temperature melting capsule? Problem solved! |
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So you return from work to a pile of ashes and a bad
smell? |
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//But would most people buy a new appliance because of a fault in the plug? |
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A small one, yes. Though I'm with you on preferring to fix what I can. |
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//If so, this explains why the world is going to, if I may say so, shite. |
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It's as if you just noticed. |
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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. |
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//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. |
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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. |
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//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... |
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Maybe a guy named Noah for arc detection? |
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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." |
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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.. |
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