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Topic Title: plug pin getting hot and melts fuse carrier
Topic Summary: 3kW oil filled radiator
Created On: 07 January 2013 11:31 PM
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 07 January 2013 11:31 PM
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psychicwarrior

Posts: 220
Joined: 18 October 2010

i knew i had something concerning (to me) to ask from the last few days:

inspected a delonghi 3kW oil filled rad (with digital this that and the other controls) and found the fuse carrier in the plug had melted (13A fuse still continuous though) and strong smell of melted plastic etc - radiator still operating.

before switched off, it was plugged into a socket on a ring final and the socket face was scorched brown, bubbled - though no signs of scorching internally (looking through the shutters)

fitted new socket plate.

suspecting an overcurrent, I cut plug off rad and fitted another plug and plugged it back in and tested current draw - consistent 12.7A (ran it for 5 mins or so) in line with rating on rad.

have to say live pin on new plug got a bit warm again (maybe not melting temp though) to say the least.

tried another socket, same thing it seemed.

tested current draw at cu - 12.7A (9.4 on one leg and 3.3 on the other)
tested continuity etc. all ok

i cant fathom why the plug got hot and melted the original plug and new one seems to get a bit warm too.

Is this thing normal on these appliances; have i missed anything obvious as i cant think what it might be?

thank you for any input comments, advice.
Habs.
 07 January 2013 11:41 PM
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DOUGIE1000

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prob wired in 1mm flex also,

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Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 07 January 2013 11:47 PM
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mikejumper

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Originally posted by: psychicwarrior
i cant fathom why the plug got hot and melted the original plug and new one seems to get a bit warm too.

Is this thing normal on these appliances; have i missed anything obvious as i cant think what it might be?

Was it a moulded on plug?
I had a similar problem with a washing machine.
The overheating was caused by a bad weld inside the plug where the wire is joined to the plug pin.
 08 January 2013 12:29 AM
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whjohnson

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Joined: 24 January 2009

Just had a phone call today - similar thing with a dishwasher installed in a kitchen/diner extension I wired 2 years ago.
They report a burn mark on the socket - I'll be checking tomorrow, but it does seem to me that the loads for so-called portable appliances like these do seem to be somewhat high for something which connects via the humble 13A plug.

A 20A DP switch and 2.5 HR flex would be a stouter solution.

-------------------------
Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.
 08 January 2013 01:32 AM
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spinlondon

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Could it be that corrosion inside the socket caused a high resistance which then caused the overheating?
Plugs and sockets operating near to their rating would be expected to get warm.
 08 January 2013 06:39 AM
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normcall

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Good quality plugs and sockets will not cause this problem.
You can replace to your hearts content rubbish with rubbish and the problem will not go away.
Good engineering principles rule, not price (or in todays world, name)!

-------------------------
Norman
 08 January 2013 08:22 AM
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colinhaggett

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Originally posted by: normcall

Good quality plugs and sockets will not cause this problem.

You can replace to your hearts content rubbish with rubbish and the problem will not go away.

Good engineering principles rule, not price (or in todays world, name)!


Spot on, their is so much junk being sold today.
 08 January 2013 08:43 AM
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psychicwarrior

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The house is one of those expensive but quickly thrown up houses which seem to suffer from the 'throw it up quickly' syndrome with one or two consequences in quality.

That aside, I did notice that on some of the the back-boxes I looked at are quite shallow in depth cut into block and the socket faces may be argued to be pressing the incoming cables to some degree.

How likely is that to cause such a significant problem I described?
... and to seemingly only affect the plug (which appeared to have scorched the plate externally as I said) and not the socket plate internally or the t&e ?!

I feel like re-cutting the back-box just to see - but there could be a few like this in this 'modern expensive' house.

On the subject of crap products, what brand of socket face plates would you recommend as 'well engineered' given I though compliance with BS ought to make sure all those with that standard are good enough for the the job... or is it a case of there may be one or two good ones in a batch of the less fancied brands, but more with suspect quality.
 08 January 2013 09:04 AM
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broadgage

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13 amp plugs and sockets do not cope well with being fully loaded, even reputable makes get alarmingly hot, and cheap ones often melt.

Kettles seem OK as they are only used for a few minutes at a time, but space heaters that may run for hours are problematic if they use more than about 10 amps.

MK plugs and sockets are IME the best of a bad lot.
Still would think twice about long term operation at a full 13 amps though.

Whilst in theory compliance with the relevant Brittish Standards should ensure an acceptable product, in practice I would take this with a large pinch of salt in the case of cheap imports.
 08 January 2013 09:13 AM
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AJJewsbury

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My guess would be initially a loose connection - probably in the plug, less likely the socket, or as Spin suggests the contact between the two - probably on L. Together with the heat from the fuse, it can quickly run-away, the heat increasing the resistance of the joint further, which produces more heat, which ...

It's not that uncommon to see scorch marks around the L pin on 13A plugs/sockets.

As Norm says, replacing with good quality components, carefully assembled, will usually solve the problem.

- Andy.
 08 January 2013 10:05 AM
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colinhaggett

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Isn't the British Standard just for the dimensions?
 08 January 2013 10:51 AM
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alancapon

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I would have to agree. Many seem to go for cheapness over quality these days, and the results really are as expected. IME the MK range are still about the best quality, albeit not quite as robust as they used to be. They are however made from plastics that mark with heat, rather than melt, as well as being tested in excess of the BS requirements. I still prefer a good quality 13A plug to a moulded type, particularly where loads are high and for a period of time.

Regards,

Alan.
 08 January 2013 11:13 AM
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AJJewsbury

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Isn't the British Standard just for the dimensions?

I've got a strong feeling that BS 1363 specifies performance requirements for sockets and FCUs (temperature rise for given currents at given ambient temperatures) - I'd be surprised if it didn't include plugs in that too.
- Andy.
 08 January 2013 12:21 PM
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rogerbryant

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It appears that the problem is (almost) always with the live and hence fused pin. This appears to be a flaw with the concept of a fused plug. I have not seen it in lands with unfused plugs.

A fuse is designed to get hot (in fact to reach the melting point of copper) and I suspect that the heat generated tends to weaken the contact springs resulting in more heating. Cheaper plugs will most probably contain less metal to conduct the heat away and the contact springs will be made of lower grade materials.

Best regards

Roger
 08 January 2013 03:03 PM
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alancapon

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Originally posted by: rogerbryant
. . . A fuse is designed to get hot (in fact to reach the melting point of copper) and I suspect that the heat generated tends to weaken the contact springs resulting in more heating. . .

I also wonder whether it is something to do with the "soft plastic" used for moulded plugs versus the "hard plastic" for the re-wireable type. There also seems to be a lot more "thermal insulation" round the fuses in the moulded type, and less air round the fuse as well.

Regards,

Alan.
 09 January 2013 10:07 AM
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rogerbryant

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Alan, there is certainly less air flow in a moulded plug and as the conductor is welded to the fuse carrier there is also less metal to act as a heatsink than with a screw terminal.

Best regards

Roger
 09 January 2013 10:30 AM
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Ricicle

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In my opinion the use of plugs and sockets for continuous loads at or approaching 13A is a no-no. I have had umpteen occasions where catering equipment (Ban Maries) in our canteen kitchen have their plugs burning up - mainly located around the fuse and/or L pin.
The load is a near constant 12.7A. I have replaced plug and socket each time with MK ones.
I think the design of the fuse carrier is poor as it cannot sustain amperages towards 13A.

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 10 January 2013 11:23 AM
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gkenyon

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Going back to my appliance repair experience.

In a moulded plug, the clip in the fuse holder connected to the L pin is often riveted to the conductor going to the L pin.

Given the soft plastic, if there is a slight problem with the riveting, i.e. not enough pressure, or small change in the use of the rivet material, this can cause slightly loose connection, and eventually overheating.

I've seen this on appliances at around 1.5 kW too.

There have also been problems with the brazing/welding/crimping procedure on some moulded plugs, where strands of the flex are broken off in the process. This can cause localised heating, and may affect L and N (and of course lead to higher E continuity resistance too) - it's Hobson's Choice which one you get!

Both of these can cause problems for single phase induction motors where the start-winding remains in-circuit at all times ("capacitor run"), if the volt-drop to the appliance becomes too high (tumble dryers on extension leads, or in detached garages fed from the "2.5 sq mm spur off the ring-main", are the biggest casualties it would seem).

Note: not advocating the "2.5 sq mm T&E spur off the ring-main" solution, which is often not conformant to hte Wiring Regs

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 10 January 2013 01:54 PM
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Parsley

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I guess thats why there's a note in Appendix 15 regarding cookers over 2KW, immersion heaters, space heating and similar loads not being fed from the ring circuit.

I've seen this damage to the 13 Amp plugs in commercial kitchens with toasters, we have changed the plug/socket for a 16Amp commando type and put the toaster on a dedicated radial circuit.

Regards
 10 January 2013 11:49 PM
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gkenyon

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Originally posted by: Parsley

I guess thats why there's a note in Appendix 15 regarding cookers over 2KW, immersion heaters, space heating and similar loads not being fed from the ring circuit.



I've seen this damage to the 13 Amp plugs in commercial kitchens with toasters, we have changed the plug/socket for a 16Amp commando type and put the toaster on a dedicated radial circuit.



Regards
The reason for that, is given in the sentence preceding the list in Appendix 15: preventing exceeding load current in any part of the circuit for long periods.

In a larger kitchen, certainly there could be a problem.

Discussion point: In a small galley-kitchen, say 2.7 kW Oven (gas hob), Kettle, washing machine, dishwasher, microwave, 4-slice toaster, and a little "under unit" lighting totalling 150 W spurred off the ringmain, couple of extra socket outlets for food processors, cordless phone charger etc, is there really much risk of "overloading for long periods" ?

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Eur Ing Graham Kenyon CEng MIET TechIOSH
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