Wall plug a fire hazard - Page 1
Forum section: electronics / general

author: Unknown
author_rank: Unknown
timestamp: February 07, 2012, 08:46:12 pm
content: Quick story share. Just now I had a 3kW electric heater connected to a plug strip which is in turn connected to the wall socket.The heater would cut out for no obvious reason and at random intervals.Check the heater cabling, fine. Checked the plug stip cable, fine. The plug of the strip that connects to the wall was warm. Took that apart and to my surprise all 3 (!) screw terminals were loose. There was heat damage on the wiring. Fixed all that and it works fine now. This was a new strip I bought personally and the wall plug has only been connected once. I can only guess that the screws were never fixed properly at the factory and the issue manifested itself now that I used the high power heater. This is a plug with all the approvals...an excellent ignition source.Stay safe.

author: IanB
author_rank: Super Contributor
timestamp: February 07, 2012, 08:53:29 pm
content: Yikes! What bothers me is that plugs are often moulded on to the cable these days so you can't see inside them to inspect them even if you want to.

author: G7PSK
author_rank: Super Contributor
timestamp: February 07, 2012, 09:01:34 pm
content: I have noticed plugs getting hot at nowhere near the rated 13 amps, always it has been on equipment which came with plugs fitted and the plugs made in China. I suspect that they do not conform to the standards that ore marked on them. I have a number of old MK plugs made in Britain that get recycled onto newer equipment when the old stuff goes, they never warm even with an overload.

author: PStevenson
author_rank: Regular Contributor
timestamp: February 07, 2012, 09:29:17 pm
content: I did a video for exactly the same reason (and to promote a cover of umbrella I did) but if you watch the first few seconds of the video I took a picture of a plug on something I was fixing for someone, if you think loose screws are bad you haven't seen anything yet!http://youtu.be/2-HX92KcFX0

author: IanB
author_rank: Super Contributor
timestamp: February 07, 2012, 09:50:24 pm
content: I'm not so sure about tinning wires that go into screw terminals. I think the screw really needs to compress the copper strands into a tight bond. If you tin the wire the solder may slowly yield under the screw and give way over time leading to a loose connection, making the fire hazard you were trying to avoid.

author: PStevenson
author_rank: Regular Contributor
timestamp: February 07, 2012, 09:56:08 pm
content: Quote from: IanB on February 07, 2012, 09:50:24 pmthe screws in UK plugs are pointed so you don't get a good enough bond unless you tin them, new appliances now come with little brass thingies over the ends of the wire.the plug in that video was pure shit, usually there is a band over the input hole that stops the wire being pulled loose - sorry for my lack of proper words, my head is buggered from lack of sleep and I just cannot think of the right words for anything at all

author: Unknown
author_rank: Unknown
timestamp: February 07, 2012, 09:59:33 pm
content: I agree with IanB, I wouldn't solder wires involved in mechanical collections (screw or crimp) or wires that have to flex (which is why soldering connectors is usually a bad idea). Solder removes the elasticity of the wires and is deformed plastically by the pressure, resulting in loose wires and fire hazards. Using a ferrule can be a good idea for stranded wire, although terminals in plugs should be designed to accept bare stranded wire. Who wires removable plugs with solid wire?

author: Bored@Work
author_rank: Super Contributor
timestamp: February 07, 2012, 10:07:15 pm
content: Here local regulations prohibit tinning wires that go into screw terminals. Previously it was allowed to just screw in the bare stranded wire. These days, however, they must have ferrules.

author: PStevenson
author_rank: Regular Contributor
timestamp: February 07, 2012, 10:39:59 pm
content: well as I said, the screws are pointed and what happens is that when you screw them in, the strands just snap off. I do agree that tinning reduces elasticity but in this case it's the best of a bad situation.and yes, "ferrules" thankyou, now I can buy some haha

author: IanB
author_rank: Super Contributor
timestamp: February 07, 2012, 10:53:25 pm
content: Maybe the pointed screws are designed for use with ferrules? I don't remember pointed screws being that much of a problem in the past. I have the odd new and old plug at home, I'll have to take a look when I get home from work.

author: pikuorguk
author_rank: Newbie
timestamp: February 07, 2012, 10:57:39 pm
content: My girlfriend has an electric bike, you'd love the charger that comes with it, it's finest quality Chinese with fake 'CE' sticker on it. The wires in the plug worked themselves loose so I took the plug apart to screw them back in. Someone in the factory had got the live/neutral swapped and had a nice chunky 13A fuse in there too.The best bit though is the battery (24VDC) connects to the bike through a standard IEC C13/C14 PC-style lead. There's the same connector on the DC end of the battery charger too.So yes, if you were stupid you could try to run your PC off the charger... or you could hook the battery directly up to the mains.

author: Bored@Work
author_rank: Super Contributor
timestamp: February 07, 2012, 11:05:28 pm
content: Tinning is actually considered the worst thing to do.First there is the process of cold flow. The tin becoming soft under pressure over time and the connection therefore becoming lose. IF you have pointy screws than it is easy to imagine that you have a rather high pressure on the tin, which should facilitate cold flow.Then there is the issue of the tin having a much lower melting point then copper. When a connection is a little bit lose and some sparks happen the sparks do melt small points of the tin away easily and the connection becomes even more lose.

author: Kibi
author_rank: Unknown
timestamp: February 07, 2012, 11:24:44 pm
content: I've been assigned to do some PAT testing now because the normal PAT testing guy is useless.The PC power cords are rated at 6A (0.75mm2 cable), however, the moulded plugs are fitted with 10A fuses. This would probably never be a problem, but I still have to replace the fuses with 3A or 5A ones.The PC's are a reputable brand, supplied by a reputable supplier.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 07, 2012, 11:40:00 pm
content: I would not advise using a 3kW heater via a plug strip in any case,as the current is getting a bit high (12.5 A).I wouldn't be allowed to do so in Australia in any case,as our normal GPOs are rated at 10A max,so we would have to have a 15A outlet fitted.Heaters with that sort of rating would probably be hardwired.You are not supposed to use solder in Oz,either,but a sneaky trick I have seen,is to solder just the tip of the wires to help prevent unravelling.

author: IanB
author_rank: Super Contributor
timestamp: February 08, 2012, 12:32:06 am
content: Standard household outlets in the UK are rated at 13 A and many devices (electric kettles, room heaters) are designed at 3 kW to take advantage of that. I get very bored with the wimpy slow 1.5 kW electric kettles here in the US that are at the limit of what the 120 V outlets can supply. I would import a 240 V 3 kW kettle from the UK if there was some convenient way of getting a 240 V outlet in my kitchen. Unfortunately I'd have to rip the walls apart to run a new circuit and I don't fancy doing that.You probably shouldn't run a 3 kW load on a plug strip in an ideal world, but most of them are also rated at 13 A and should be able to take it.

author: westfw
author_rank: Super Contributor
timestamp: February 08, 2012, 08:47:06 am
content: I think a lot of high-amperage appliances in the US come with warnings NOT to use any sort of extension cord.(and 3kW@220V is the same as 1500W@110V, amps-wise. And it's the I^2R heating in the wiring/connectors that causes problems.)

author: Rerouter
author_rank: Super Contributor
timestamp: February 08, 2012, 08:54:57 am
content: our gpo's are rated for 10A, but any good sparky would run wire rated to 13A, then again, they are hard to find today, heck its hard to even find ones with a valid or non expired one these days,I have seen some of the dodgier ones running 7.5A rated for GPO's, so yeah, if you need 3KW, use a 15A gpo, worst case they run 13A rated wire, and that still covers 3KW

author: slateraptor
author_rank: Frequent Contributor
timestamp: February 08, 2012, 09:18:03 am
content: Quote from: Alex on February 07, 2012, 08:46:12 pmThat's a pretty good way to start an electrical fire. Was there not a clearly visible sticker on the power cord explicitly warning against the use of extensions and multi-plugs?

author: Unknown
author_rank: Unknown
timestamp: February 08, 2012, 10:24:48 am
content: Quote from: slateraptor on February 08, 2012, 09:18:03 amIs it? A plug rated at 13A should be able to take 13A at 100% duty cycle. Nothing to add to that. The strip also had a small linear psu for a cordless phone. The total current is below 13A. Yes, I will have losses with any extra extension lead, but I should not have fire hazard issues no matter how many strips I decide to connect in series, provided of course that the rating of one at the start does not exceed 13A (and even if it does, I should still not have a fire hazard because of the fuse).This was a quality control failure at the plant which caused problems when the plug was pushed close to, but below its rated current.

author: slateraptor
author_rank: Frequent Contributor
timestamp: February 08, 2012, 11:06:43 am
content: Quote from: Alex on February 08, 2012, 10:24:48 amDidn't realize you're from the UK.It's not uncommon to find plug strips without any overload protection mechanism in the US, although typical outlets are tied to 15A circuit breakers; I don't know how UK goes about their business. I've also personally seen a small electrical fire started when a friend decided to de-ice the front end of his Jeep with a 1.5kW space heater and 50-ft 13A-rated outdoor extension cord. About half of the cord was coiled in a bucket where the fire started so it was easy enough to contain, but that technically shouldn't have happened in the first place.

author: amyk
author_rank: Super Contributor
timestamp: February 08, 2012, 11:31:34 am
content: Quote from: slateraptor on February 08, 2012, 11:06:43 amExtension cords are rated for operation in open air, not coiled up in a bucket. That decreases cooling enough to be dangerous.

author: slateraptor
author_rank: Frequent Contributor
timestamp: February 08, 2012, 11:51:42 am
content: Quote from: amyk on February 08, 2012, 11:31:34 amThat's pure assumption; you'd be hard pressed to find any sort of derating specification on any commercially available extension based on physical orientation. Suppose I decided to use an outdoor extension cord fully stretched out on 125F pavement, let alone the sub-freezing ambient temperature when the electrical fire occurred.

author: JuKu
author_rank: Frequent Contributor
timestamp: February 08, 2012, 12:33:25 pm
content: Really, don't tin wires. There is a very good reason why the practice is illegal in EU.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 08, 2012, 03:13:32 pm
content: QuoteI don't know about the US but in the UK I would normally expect to see extension cable reels to be explicit that their rating is unwound from the reel.Sometimes the lower rating when wound on the reel will also be specified - e.ghttp://www.screwfix.com/p/masterplug-switched-4-socket-cable-reel-work-power-30m-13a/67915Note the rating is given as "Unwound: 3120W, 13A, 240V. Wound: 720W, 3A, 240V", and there is a substantial difference (13A vs 3A).The example where a cable overheated and caught fire mentioned that it was "wound in a bucket" - it should have been de-rated because of this, sounds like the user didn't realise.

author: IanB
author_rank: Super Contributor
timestamp: February 08, 2012, 03:26:07 pm
content: Extension cords in the US come with a big warning on the packaging not to use them coiled up. Of course nobody tends to read the packaging, but they should.

author: SeanB
author_rank: Super Contributor
timestamp: February 08, 2012, 04:06:22 pm
content: Shoot, then you should not see the stuff that passes for wiring here. No plug, no problem, we have matches! Wiring using single core phone wire, or steel fencing wire. Glow in the dark illegal connections, complete with the earth return, and the tree insulators at chest hight using now bare wire.There is a manufacturer of extension cords that has a reel that is rated for 15A fully wound, using a special cable with a spiral ridge on it to allow heat to dissipate and to allow air in to keep the cable cool. Comes with a 15A thermal breaker as well, and well regarded by the tradespeople, even if it is more expensive.As to plugtops, there are cheap and nasty ones ( made from rolled sheet nickel plated brass) that I do not trust at over 2A, and other cheap ones that will happily handle 15A, even though they are rolled sheet as well, but thicker and with a better cable clamp and screw terminals. Others are solid brass ( drilled in the centre to save cost) with a decent screw connection. The moulded ones are notorious for cooking internally, and some of the chinese cords are made using 1.5mm wire for the supply leads and 1mm for the PE conductor ( 1.5mm on a good day, but generally 1mm plus a hair) that break after a short period of use.I tend to make my own, using cable, a plugtop and an industrial socket outlet in a steel box, with a proper cable gland ( not that funny style the USA uses, these are actually waterproof to a degree) to keep the cable neat.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2012, 11:21:44 pm
content: Quote from: slateraptor on February 08, 2012, 11:51:42 amIt's no assumption, it's true and PVC cable is normally rated at something like 85oC with a typical extension lead only being rated to its full load at 30oC.

author: PetrosA
author_rank: Frequent Contributor
timestamp: February 09, 2012, 02:47:45 pm
content: Wind a cable up and it becomes a transformer. Simple.

author: slateraptor
author_rank: Frequent Contributor
timestamp: February 09, 2012, 03:49:17 pm
content: Quote from: PetrosA on February 09, 2012, 02:47:45 pmI was thinking more inductor, but yeah, makes sense how there's no possible way an extension cable's rating can hold in both wound and unwound unless worst case was taken into account...although I still maintain that I've never seen a derating spec. My reasoning was def out of line.

author: Unknown
author_rank: Unknown
timestamp: February 09, 2012, 07:06:02 pm
content: I believe the inductance is pretty insignificant at 50/60 Hz (feel free to do the math or measure it if you're curious). It's all about the lack of dissipation.

author: IanB
author_rank: Super Contributor
timestamp: February 09, 2012, 07:59:03 pm
content: Since the cable has out and back conductors attached side by side most of the inductive effects would cancel out anyway.

author: SeanB
author_rank: Super Contributor
timestamp: February 10, 2012, 04:09:15 pm
content: If you think it is inductance then take a reel of cable ( a full reel of 1mm wire still in the shrinkwrap) and pass 5A through it from a 12V battery. Use a 55W headlamp bulb as a load, and leave it for a hour before measuring the temperature rise inside the bundle. That will show you just how much heat is generated in wiring in normal use.

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2012, 05:37:53 pm
content: If you think it's the inductance you need to brush up on your electrical principles: remember inductance doesn't dissipate any power, it only stores energy. If the cable were a superconductor it wouldn't make any difference to the temperature rise whether it's tightly coiled or unrolled. It' only heats up more when tightly coiled because there's nowhere for the heat to escape to.

author: SeanB
author_rank: Super Contributor
timestamp: February 10, 2012, 08:36:29 pm
content: Precisely, it is heat that is generated from I2R losses that is the problem, the inductance will be negligible, as the 2 cores are pretty much doing common mode rejection so the only inductance would be a very low leakage inductance. That is why I said use a 12V DC source, as the inductance will be of no consequence with a long time of constant current flow. resistive losses will predominate.

author: G7PSK
author_rank: Super Contributor
timestamp: February 10, 2012, 10:13:37 pm
content: There is another source of heating with a coiled wire, if the reel or container is metal you could get eddy currents.

author: GeoffS
author_rank: Unknown
timestamp: February 11, 2012, 05:59:04 am
content: Quote from: Rerouter on February 08, 2012, 08:54:57 amGPO's in Australia are required to have the same size cable whether they are 10A, 15A or 20A. Cable size is 2.5mm2

author: IanB
author_rank: Super Contributor
timestamp: February 11, 2012, 06:48:15 am
content: All this talk of GPO's is confusing me. Back in the UK that always used to stand for General Post Office

author: GeoffS
author_rank: Unknown
timestamp: February 11, 2012, 07:26:54 am
content: It means the same things in Australia. Say GPO to most people and they think you mean the post office.If on the other hand you say power point, they think you're talking about a software package.

author: IanB
author_rank: Super Contributor
timestamp: February 11, 2012, 07:45:16 am
content: In Britain we just call them "sockets".

author: vk6zgo
author_rank: Super Contributor
timestamp: February 11, 2012, 09:29:55 am
content: The name used by the licencing authorities in Australia is GPO,which is an acronym of "General Purpose Outlet".There are some strange names around;The company I worked for as a lad called the ordinary mains plugs that you wire onto appliance cords ,"Plug Tops"!I don't know how many younger Australians do know the other meaning of GPO,as the post office has been "Australia Post" for nearly 40 years,& that organisation hasn't placed much emphasis on anything that sounds like "post office"----they're "Australia Post shops"!Oddly enough,the term GPO in Oz meant a building which was the central post office for each Capital City,rather than the name of the whole organisation which ran the post offices & the 'phones,as in the UK.The name of the organisation in this country was the Postmaster General's Dept ,& the acronym was the "PMG".OK,I know, "Postmaster" is one word,I didn't come up with it!The PMG was split into "Australia Post" & "Telecom Australia" back in the 1970s

author: ciccio
author_rank: Frequent Contributor
timestamp: February 11, 2012, 12:46:50 pm
content: Back to the original post..Once I was testing a "medium size" power amplifier, about 800 VA consumption, when I notice a strange smell: burnt plastic, and what we call "cooked copper".I switched it off, and checked every component, but everything was OK.The smell came from the mains cable, a standard pre-made molded IEC socket labeled at 10A, with a cable labeled as 3 x 0.75 mm2, and standard Italian 10 A plug.The cable's outer insulation was melting and it damaged even the bench-top (a black scar is still there).I disassembled the cord and checked the cable conductors' copper: it was not 0.75 mm2, but less than 0.20..The cord came with a PC power supply, and maybe it would operate correctly with a 400 watt load, but it was a real fire danger, being it interchangeable with other devices.This is not the first time I encounter Asian-made mains cables where the manufacturer was saving on copper size: copper is expensive.

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2012, 06:20:44 pm
content: Quote from: SeanB on February 10, 2012, 08:36:29 pmIt woudn't make any difference if you used one reel for the live and one for the neutral, the extra inductance would not increase the power dissipation at all.Quote from: G7PSK on February 10, 2012, 10:13:37 pmNo the common mode rejection would prevent that, the live and neutral conductors are close enough to cause the magnetic fields to cancel one another out so the eddy current will be negligible.Quote from: ciccio on February 11, 2012, 12:46:50 pmEven 0.75mm2would be too small, for 10A you need at least 1mm2cable.

author: Unknown
author_rank: Unknown
timestamp: February 11, 2012, 07:46:01 pm
content: The standard C13 IEC power connector is rated for 10 A, so a mains lead with an IEC connector should have at least 1 mm2conductors. I don't believe there's a grounded connector rated for 6 A in that series, which is why you see plenty of equipment with very modest power draw equipped with these sockets. You sometimes see the C6 connectors (rated for 2.5 A) for portable equipment, but most scopes, for example, draw way less than 10 A but still ship with C13 sockets.I'm not surprised that companies ship cords with thinner conductors with these products, although it makes it very confusing for the consumer who assumes that the cords should be interchangeable.

author: IanB
author_rank: Super Contributor
timestamp: February 11, 2012, 09:48:51 pm
content: My Rigol came with a very fat power cord, making it look like it was designed for some huge and power hungry piece of equipment. However, I suspect mechanical strength and integrity is part of the safety equation. It is not only about conductor size.