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I have found myself reading reports of fires lately, including some that started due to high-resistance connections in electrical outlets and switches. A load on a high-resistance connection can generate enough local heat to ignite building materials, while not tripping any upstream safety devices. This is a problem that may not manifest until years after the original installation.

(Having not thought of this before, obviously I am now scared out of my mind...)

Why not integrate a thermal fuse into the outlets? It seems like a cheap way to prevent some fires. Is there some technical flaw in the concept? Or is this just an actuarial cost-benefit computation?

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  • \$\begingroup\$ I believe that UK socket plugs (not the power point itself) do actually contain a thermal fuse, and I understand that in the past, buying an appliance also meant having to put your own plug on it. Disclaimer: I saw that on a youtube video. \$\endgroup\$ – AndrewP Sep 13 '16 at 3:04
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    \$\begingroup\$ UK plugs have a standard fuse in them, not a thermal one, but both the plug and the socket are made of (at least) self extinguishing plastics. \$\endgroup\$ – Dan Mills Sep 13 '16 at 3:41
  • \$\begingroup\$ Don't know the answer. I guess maybe the building code and inspection process are supposed to make sure this does not happen or doesn't happen very often. But sometimes mistakes are made. For example with aluminum wiring in homes in the US. It was thought to be safe and was somewhat common for a while. But it is now regarded as fairly hazardous. Nobody wants to buy buildings with aluminum wiring. \$\endgroup\$ – mkeith Sep 13 '16 at 4:46
  • \$\begingroup\$ Rent or purchase a thermal camera. Plug a fan heater into your outlet one at a time and after 5 minuted check with thermal camera in plug point shows larger than average heat signature, if so then investigate further else move onto next point and test. If you pull a used heater load out of a socket and the prongs are too hot to touch that is a warning sign. \$\endgroup\$ – KalleMP Dec 9 '18 at 17:39
  • \$\begingroup\$ @AndrewP UK appliances did indeed often require the end-user to fit a plug many years ago (UK plug Law 1994). However, even with ready-fitted plugs, I still take them apart to confirm they have been wired correctly, or for moulded-on plugs, that the fuse is of a suitable rating and fitted correctly (e.g. no 13 A fuse in a 25 W hot glue gun). \$\endgroup\$ – Andrew Morton Feb 8 '19 at 19:24
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The other answers seem to be misinterpreting the 'thermal fuse' part of your question. A 'thermal fuse' is an electrical overload sensor that uses heat as an indicator of an electrical overload. It sounds like you are asking about a thermal cutoff like the kind included in motors to which open a circuit when the locally detected heat (ie. not generated by the electrical current itself) exceeds a set parameter.

The reason this is not included in electrical outlets has to do with the cost-benefit of including such a complex sensor (~$0.75) in such a simple and inexpensive device like an outlet (~$0.30). Electrical codes require all wiring devices to be installed in a UL listed box or enclosure.

The same codes require these boxes to be flame resistant. The idea is that the effects (heat, fire, etc.) of the high resistance connection will be limited to the box. Fires certainly do occur as a result of this but this is infrequent compared to the much more common ways electrical fires start.

Codes are updated every couple of years and are getting better and better at addressing less and less common occurrences. For example the 2014 NEC requires AFCI (Arc-Fault Circuit Interrupters) in many locations that do a much better job of detecting events on the more dangerous, fire-starting end of the spectrum of 'high resistance events' you are describing.

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There is a very simple reason why this is not used. The main function of the fuse or circuit braker is the overcurrent protection of the enitire wire section. Wires with certain cross section are protected at the node where a higher cross section is distributed in many sections with smaller cross section wires. All sbsequent devices like wall sockets, connecting cables,...etc need to have the same nominal current capability. In case that an electric device has internaly smaller cross section than connecting cable, then it needs another fuse inside.

With your proposal, even if the wall socket had a fuse, this won't prevent the wiring taking on fire, in case that wire cross section is too small.

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    \$\begingroup\$ That's not really the situation I'm describing. \$\endgroup\$ – Stephen Collings Sep 13 '16 at 12:47
  • \$\begingroup\$ I see. Well, in most cases the fire is caused by improper fuse rating, by use of inadecute prolong cables, and for loose tightened wires. Your proposal is to mount a thermal sensing device that would detect rise of the temperature at electric contacts and then a circuit braker, all in one wall socket. Seems a good idea, but rather impossible to acheive in such small space, and the price would be high. All this for detecting a worn/defective outlet socket or plug. \$\endgroup\$ – Marko Buršič Sep 13 '16 at 13:01
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    \$\begingroup\$ I'm not proposing a circuit breaker, I'm proposing a thermal fuse. They cost well under a dollar in quantity, are less than 4x9x11 mm, and will directly and permanently interrupt the power feed to the load if their temperature rating is exceeded. en.wikipedia.org/wiki/Thermal_cutoff \$\endgroup\$ – Stephen Collings Sep 13 '16 at 19:45
  • \$\begingroup\$ @StephenCollings That's a switch for small electric motors mounted into the motor's windings. Certainly you can't glue it on socket contacts rated for 16A or more. \$\endgroup\$ – Marko Buršič Sep 14 '16 at 6:51
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    \$\begingroup\$ The size and cost I was quoting was one rated for 15 amps and 250 vac... \$\endgroup\$ – Stephen Collings Sep 14 '16 at 11:26
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The use of thermal fuses in domestic electric systems does not help to improve safety.

In general the design of switches and outlets is done in such a way that any temperature rise due to the flowing current is absorbed by the mass or contacts and connections. This is possible since the dissipated energy is very small (I2R). Considering that outlets and switches are properly designed and if professionaly connected the risks here are very limited. The connections in junction boxes also can form a risk if the junctions are not professionaly made in a diy situation. If an outled or switch has a bad connection the device gets hot and gets burned long before there is any fire hazard. If an outlet is burned it should not be used and replaced by a new one. If such an outled remains in use the risk of fire increases

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  • \$\begingroup\$ This assumes the process happens slowly enough for anyone to notice. It sometimes doesn't. \$\endgroup\$ – Stephen Collings Sep 13 '16 at 12:47
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The easy answer is cost, and the main reason is standards and certification bodies.

Cost of mass produced items has to be minimized, adding a safety feature that is not required by certification and increases production costs can be seen as engineering malpractice. Cost is one of the constraints for an engineer.

Standards and certification bodies rely on engineers and subject matter experts, that generally have to come from the same industry they are regulating. They don’t want standards to become prohibitively onerous, and they have to justify and document their decisions.

Placing a 5km/hr speed limit would considerably increase safety, but would also be unnecessarily burdensome to the same society that it’s intended to protect. A cold actuarial compromise has to be used.

No system is 100% safe. The scenario you describe implies a degradation of the plug and/or socket that puts it way out of the limits of the required standard. It is a very rare scenario, that could be caught in time by an informed consumer and is commonly included in device documentation.

Consumer education (particularly lack thereof) is the main part of the equation.

There are inexpensive electronic safety devices that can be added to table saws that prevent them from slicing through your fingers. But these are not required. And consumers pay more attention to the item price than to a safety feature that dares to imply they are clumsy, that enters the capitalistic equation.

But standards and regulations can be changed. And modifications are made all the time to include new causes for concerns. It just needs people championing them. And that generally requires an informed consumer.

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