Adding a thermal cutout to a 13amp plug?

[Tim+]

EV ?granny leads? plug into a standard 13 amp socket and draw about 10 amps continuously. Because of the prolonged nature of the load, the plugs usually have overheat protection built it.

This is fine and dandy unless you find yourself in a situation where you have to use an extension lead. Now the thermal protection is in the middle of the chain so to speak (and probably outdoors where little harm can result) but now the plug on the extension lead is unprotected.

So, I was wondering if such a thing as either a plug with a built in thermal cutout exists or whether it?s possible to add a self-resetting thermal cutout into a conventional 13amp plug.

Does this sound possible? What would be a ?safe? temperature limit for such a plug?

Tim

[Max Demian]

You either need better quality connections or to reduce the load. 13A plugs, sockets and leads [1] certainly should take 13A continuously; people do/did it for 3KW electric heaters. If they don't they're not "fit for purpose".

[1] Assuming the lead isn't wound onto a reel.

[Dave Plowman (News]

I'm not sure 13 amp was designed for long term continuous loading at peak.

Hence things like permanent heaters normally being hard wired.

[Tim+]

You might not have a choice if charging away from home.

Charging at 10amps is quite slow enough thank you. Some 13amp sockets and plugs *do* overheat with a continuous 10 amp load though, hence my question.

I?m well aware of that issue.

Tim

[Theo]

Many 3kW electric heaters don't take 3kW continuously. They do when they're first turned on, but after a while the room gets up to temperature and they cycle on and off on their thermostat.

I had a 3kW kettle boil itself dry the other day (the cutoff failed). After about 10 mins of taking 3kW the plug was really hot and the cable quite warm.

There's a reason 3kW immersion heaters are typically on a hardwired connection rather than a socket - because they can run at full load for long periods.

Theo

[Martin Brown]

Unless you put it under a duvet I can't see that there should really be a problem. You can buy overheat thermal cutouts quite cheaply but I don't know of any integral to a 13A plug.

Actually it has been found that 13A continuous load on a nominal 13A socket is somewhat problematic and life shortening for both the plug and the socket. That is one of the reasons why many portable heating appliances have been derated to 2.4kW maximum (ie 10A). You can get away with it for a kettle that only runs for a couple of minutes but not for a room heater that might draw 3kW for many hours continuously.

I'd have thought most decent plugs and sockets could stand 10A current draw almost indefinitely. The plug *will* get warm to the touch though due to thermal dissipation in the fuse. Fuses typically will blow in just a few minutes at twice their nominal rated current.

[Theo]

One way to do it:

Theo

[ARW]

I was called to a hospital kitchen last week as a 13A plug on a plate warmer was getting hot.

After I had run my hand under cold water for 5 minutes after trying to unplug it (yes it was that hot) then I swapped the plug.

The only source of heat was the fuse or the fuse holder "push to fit terminals" (I have some photos).

Make your own extension lead?

[Tim+]

<sigh>

They might not be, but thermal overheat protection makes them safer if you do find yourself in the situation on needing to use one.

Tim

[Tim+]

That?s interesting but I suspect no use for a standalone extension lead. In a simple extension lead I really want something that can switch the full 10 amps without using relays etc (or whatever is in the granny lead control box.

I suppose I could try and add a relay in the lead somewhere in another enclosure though?

Tim

[Tim+]

I have what I think is a fairly decent heavy duty extension lead with an RCD device that was salvaged from a hot tub pump and heater device. I?m just thinking about how to add an extra level of safety to the system to recreate what I?d have with the granny lead alone.

Tim

[Tim+]

There are any number of pictures of burnt out sockets and plugs on EV forums. Saying that you don?t think it?s a problem isn?t helpful.

Hence my question.

Tim

[Theo]

Could you fit something like this in a 'wall wart' style plastic enclosure?

would bolt it to the live pin for good thermal contact and then loop the circuit through.

(other temperatures/currents available)

You would have to check the wall wart enclosure and its pins are rated for full current though. Many of them are thin thermoplastic (ABS?).

Theo

[Chris Bacon]

The only times I've had problems with 13A plugs getting hot is when there has been a bad connection to one of the pins (not the earth one). They don't have to be running at high loads for that effect to happen, either.

[The Natural Philosopher]

Whilst a properly wired brand new plug in a brand new socket my be perfectly fine at 13A, over time this ceases to be the case. I remember my late mothers Hoover Keymatic, after about 10 years of use, had a smoking 13A plug. Pure corrosion on I think the fuseholder had increased the resistance of the contacts.

The socket in the wall was pretty sick too. Lot of scorching

[John Rumm]

They are, however time, dirt and corrosion[1] can mess with the best of them.

The thermal loading/stress test in BS 1363-1 specifies a test where the plug is loaded with a 14A load for 60 mins, and then allowed to cool for

60 mins. The cycle is then repeated 200 times, before a final test at 14A when the temperature is taken directly after. The maximum permitted rise being 37 deg C at the pins, and 52 degrees at the terminals and the external surfaces. (tests conducted at 20 degrees ambient) [1] There are also fairly stringent corrosion tests in the spec as well.

[Dave Plowman (News]

It is going to depend on the condition and quality of the socket, as well as the plug. Of course something in part of the bit you do control could be a stopgap.

But the real answer is to use a connector rated for the job in hand - not one designed long before EVs were even thought of.

Or, of course, reduce the load to the point where any 13 socket will be happy delivering long term.

[John Rumm]

So take your own good quality extension lead as well. Then it's only the state of the socket you need to worry about.

(there are plenty of 3kW Immersion heaters out there running for years on a 13A plug without any problem)

[Dave Plowman (News]

It's often the fuse connection that causes them to run hot - depending on plug design. Unless both new, worth making sure they are clean and tight.

[John Rumm]

If it was like most chinesium kettles, the flex CSA was probably undersized for anything but a very short term load.