Melted socket, plug and wiring

[strength_10]

Hello. Long time reader but not many posts. After more of a bit of insight than anything else as I have asked for a local electrician to come and have a look next week (unless anyone reading is near BS39 in Bristol/Bath and wants to have a look).

I have a washing machine (couple of weeks old as previous one had bearings failing) and a tumble dryer in my utility room. Both appliances are individually plugged into single sockets under the counter which are separately fed from 2 FCU above the worktops. Both FCU are fitted with 13A fuses. Have been happily (I think) like that for 6 years since the house was rewired and had new consumer unit fitted just after we moved in and had solar panels installed.

Couple of days ago washing machine stopped working. Assumed it was an unlucky new appliance to Samsung sent out an engineer. He spotted (quickly) that the plug was damaged and the socket starting to melt through. He then spotted that tumble dryer plug was even worse and socket properly melted through. Good news is the new washing machine doesn't seem to be broken after all. Bad news is both sockets and both appliance plugs are broken. I removed the fuses from FCU, isolated the MCB and unscrewed the sockets to find wiring inside is blackened and a bit melted.

Obviously I'll need the wiring and sockets replaced and the circuits tested but I'm after a bit of insight (I appreciate it's a bit of guesswork from afar) as to:

a) what causes this sort of damage?
b) why it may have happened to 2 separate sockets and plugs being fed from 2 different FCU feeds in short succession?

Thanks in advance,
Cameron

 

[marconi]

This damage is caused by Ohmic heating (IsquaredR), the heating effect of a current passing through resistance. Wires, contacts, terminals, fuses all have some resistance. Unless the heat can be dissipated the temperature of an element of resistance will rise and rise. What happens tough is that as the temperature rises do does heat loss through convection, conduction and radiation so some temperature is reached when heat generated equals heat loss. Stuck behind your two machines there is also the problem of heat being generated by the tumble dryer and at times from the washing machine during its hotter cycles.

The Ohmic heating then depends on the square of the current and the resistance. Current first - the current drawn at any instant depends on the voltage of the supply and the impedance/resistance of the load. An increase in supply voltage will cause more current to be drawn. Where you live the supply voltage may be at the higher end off its range. It may also be that your PV solar system is raising noticeably the voltage off your home's wiring especially during the current long spell of very sunny clear blue weather. The same effect can be happening through other folk's pv systems if there are a number of them nearby to you and you are on the same phase. Resistance second - metallic components have a positive temperative coefficient which is a convoluted way of saying that the resistance of them increases with their body temperature. This one can have increasing Ohmic heating as the resistance rises. This is particularly so for the cartridge fuse. The poor fuse struggles in moulded on plugs to dissipate its heat because it is thermally insulated by the surrounding plastic and front face of the socket.

There is a final effect not yet mentioned by visible in the photo of the plug around the neutral pin and neutral side of the fuse. As the plastic of the plug heats up it begins to break down. Plastics are made of carbon. As the plastic breaks down regions of carbon 'track' between the neutral and live fuse because of the electric field between them. Eventually a conductive path forms - sometimes only on the surface but also sometimes within the plastic - so an electric current flows directl between line and neutral which inevitably creates Ohmic heating. I also suspect, but have not been able to confirm, that the silvery plug pins have higher surface contact resistance and higher body resistance the nickel brass pins. Last, before the use of insulated pins, the pin was of uniform cross section. The addition of insulated pins has meant that the pin's csa is much reduced along the section of the brown/blue plastic- so again more resistance and harder to dissipate Ohmic heat.

Regarding the socket. The cheap brands are often greatly stressed by prolonged high currents. It is instructive to dismantle a good and a cheap socket and compare the innards sometime. The socket has its part to play in dissipating heat. Heat flow by conduction depends on a temperature gradient. There is the equivalent in classic heat theory of thermal resistance Rth viz; H = (Tin-Tout)/Rth - I leave you to work out the consequences of this (applies to plug too). The standard method of inserting a straight length of stripped back 2.5mm2 L or N into the socket terminals may provide an adequate electrical connection but it is not the best way of minimising the thermal resistance of the connection. I like to 'Barrymore' a single conductor - that is bend it over on itself - before inserting in the socket terminal to maximise not only the electrical contact but also to help increase thermal conduction of any heat generated away via the live and neutral.

My perfect solution would be to replace the appliances plugs with black (higher IR radiation) pottery MK 13 Amp ones. I would plug these into metalclad MK 13 Amp single sockets without a switch. I would also relocate the sockets from behind the appliances to a cupboard at the side of them so that they are better ventilated, away from the heat generated by the appliances and can be kept an eye on.


A long post but I have too much time on my hands at the moment!

 

[marconi]

duplicate

 

[Midwest]

My mistake if i misread the Op's first post, I was making the point that if the washing machine is brand new then check the warranty as some of them have some interesting clauses...
If the plug is damaged from the melting/sockets then obviously change it. but if you cut off the plug on some appliances and wire direct to a flex outlet then i have personal knowledge of the manufacturer/retailer refusing to honour the warranty as they cut the sealed plug off and used it in a way not as the manufacturer designed it.
Had a customer who had a dish washer that was playing up and currys / knowhow refused to honour the warranty as she had removed the sealed plug and used a flex outlet. maybe if she pursued the issue she might have won but currys were having none of it

I did some extensive research sometime ago about moulded plug tops and white goods or kitchen appliances. I emailed several of the major appliance manufacturers, asking specifically if removing their plugs from their appliance leads would invalidate their warranty, and posted their replies in a thread. Only one replied back that it would. Unfortunately, not being a full time researcher, I never booked marked my thread, and my PC with their emails went u/s , so I can't remember or say who that was. I did install a Bosch appliance recently, and the instructions stated that only a Bosch replacement lead & plug should be used, if the original was damaged. So I guess, reading the product manual is important, before making this decision.

Notice, I never suggest you sell your tools

 

[MJPD29]

LAP & generic items sold by reputable wholesalers will be made to the same standards and EU regs as other manufacturers, they will therefore be up to 'standard' otherwise they would not be on sale. Quality of different manufacturers and cost of components is a different issue entirely. However I normally find that poor workmanship is to blame rather than material failure, loose cables in terminals etc etc.

Regardless of whether they are supposedly made to the same standard I have seen LAP sockets fail out of the packet, they have a cheap feel to them and are definetly not made to the same standard as more expensive ones. Maybe fit for use, but definetly don't have the same life span

 

[Strima]

Cheap accessories will be made to, and pass the relevant standards, however they'll just pass whereas more expensive accessories will normally greatly surpass the relevant standards.

 

[sham]

Cabtree and MK any good ?

 

[Lucien Nunes]

Yes, both good, although many would argue they aren't what they used to be.

Looking at the pictures of the socket backs, it's impossible to say with certainty whether the burnout was caused by poor quality socket contacts, or bad workmanship by the installer. Because they are unswitched singles, the terminals and socket contacts are joined directly together and heat flows readily between the two. Also, the burnt line conductor was pressed against, and melted into, the back moulding directly above the contact. But there are hints that the heat was generated by loose connections rather than slack contacts. Bad workmanship may well be the root cause, but there can be other factors such as excessive thermal cycling of the terminal due to the heating of poor quality, high resistance contacts.

Re. cutting off plugs, there is a UK-specific instruction that used to be standard, maybe it still is, I haven't noticed recently, advising users that if the plug didn't match their socket outlet, it should be cut off and destroyed, a suitable plug fitted and the correct fuse fitted in the DB. It's a hangover from the days when there were still some round-pin and non-standard sockets in use. There are still situations where that applies (clean-earth supplies in commercial buildings with T-earth variant BS1363 sockets etc). It would be ludicrous if removing and replacing the plug with one of the same type invalidated the guarantee, while fitting a different type did not.

Re. causes of plug heating, while all of Marconi's points are valid, in practice some of them are much more important than others and some can usually be discounted. The greatest variable, the contact resistance, trumps all the others under bad conditions.

Somewhere near the head of my list of things for the new video blog will be stuff about heating in contacts and terminals. In 1882, Musgrave Heaphy who wrote the first wiring regulations, the Phoenix Rules, was aware of how often bad connections caused fires. 136 years later we are still struggling to get to grips with this.

 

[Santa]

All of this suggests that putting the sockets for high-power appliances above the work surface where they can easily be seen is a sensible option.

 

[Gavin John Hyde]

All of this suggests that putting the sockets for high-power appliances above the work surface where they can easily be seen is a sensible option.

try telling the customer that when they ask for a kitchen rewire, so much is out or sight out of mind now, all clean smooth lines. bare minimum on display.

 

[123]

try telling the customer that when they ask for a kitchen rewire, so much is out or sight out of mind now, all clean smooth lines. bare minimum on display.

I'd love to see customers face is their stone worktop had to have a hole in it with plug and flex for dishwasher coming up through it

 

[Strima]

Having just spent a couple of grand on granite worktops I can, hand in heart, say no fecking chance of cutting holes in it for cables to come through.

I’m waiting for someone to suggest fitting 16 amp commandos next.

 

[static zap]

...... where they can easily be seen is a sensible option.

We are meant to question
"strange smells in the kitchen / laundry room "
..unless you really love fish !
(unless your neighbors are always burning rubbish/scrap in town)

 

[rolyberkin]

On the cutting plugs off issue I was on a kitchen install last week when kitchen fitters quoted Neff as saying that cutting a plug off invalidated the warranty. I called their customer service on speaker phone and they told me the same. I then asked to speak to a named person in their technical department who would tell me why this was the case and provide me with the relevant technical data or provide reasons as to why this would cause the warranty to be invalidated. After about three minutes on hold they then backtracked and told me that this in fact would not invalidate the warranty unless I wired it incorrectly and that issue caused the fault, I then cut four plugs off in front of the kitchen fitters.;-)

 

[Midwest]

I had another experience with Velux windows, and their electric windows. On one install, I rang them (speaker phone in front of customer) and they said cutting off plug would invalidate warranty, then a few months later, did the same again and they said it wouldn't

 

[Octopus]

Get a few old mk single sockets off eBay as above and problem solved

Definitely don't think new mk are any good.

3 G switch last week, unable to do up 6 of the screws..... Off shore manufactured carp