Neutral pin on plug melted - what happened?

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In my computer room I have the pc, monitor, tv and various other bits and peices plugged into a 4 gang extension lead, which is plugged into a single wall socket placed behind the desk, under the monitor. The last few days I've heard a few breif pops and ticks which I thought were coming from behind the monitor. The psu on this monitor failed about 2 years ago and it was repaired and ok till now. I thought "Oh well, looks like the monitor is on its way out again", but as you do you, you hope it will be alright so you carry on using it. Last night I was on the pc when everything connected to that 4 gang switched of for about 3 seconds, can't rememeber if there was a pop or tick just prior to this cos it happened out of the blue. Anyway, when the power came back on there was a loud crackling coming from the plug in the wall socket, accompanied by obvious burning. Panicing slightly I hesitated diving straight for the socket switch as is was right next to the red hot plug, although I did switch the monitor off. The power was still on during this cos the TV came back on. After about 5-6 seconds the cracking stopped, still supplying power to the tv, so then I flicked the switch on the socket and the plug was extremely hot to the touch. When it cooled down enough I pulled it out and the neutral pin had melted the plastic around it, and also the neutral hole on the socket. I took the plug apart and from what I could tell the live wire was undamaged but the blue and earth wire insulation had burn away and had obviously made a short. My house still has an old style consumer unit with the old style wired fuse plugs and no rcd or trip switch that I can see.

As I said these pop noises I assumed were coming from the monitor but could have been coming from the plug as it is behind the monitor just above floor level.

So the question is can anyone throw any light on what might have happened here? Is it possible for a monitor failure to end up burning out the plug in the wall socket? Why didn't it damage its own plug which was in the 4 gang? Is the monitor a red herring and is it just a fault with the 4 gang, which must be at least 10 years old. Is it a fault somwhere else in the circuit? Why didn't any fuses blow? Would any rcd protection had prevented the potential for a fire?

I'm going to replace the wall socket but I'm a bit apprenhsive about plugging things back in, is there any test equipment which can test the safety of the monitor, etc?
 
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The most likely cause is a bad connection (pin not clean, screw not tight etc). This would cause the connection to be high resistance which would generate heat (as you've found). I'd be tempted to replace both the plug and the socket.

Regards - JB
 
This sort of thing is almost always due to a poor contact, it could have been a worn socket, dirty pin on the plug, poor connection in the plug, or even poor connection in the socket. This thing is actually more normal on the live in plugs due to poor connections with the fuse, but in general neutrals do seem to burn up more than lives, and I'm not sure there is a sensible explanation other than uninformed people viewing the neutral as less important and not tightening the terminals properly

RCDs and MCBs wouldn't have helped you really, except the RCD would probably have tripped when the neutral melted through to the earth (but it is pure chance that they melt together or not)

When you replace the socket, cut back any heat damaged wireing if there is enough slack (if there isn't then depending on how much there is, the wire could be spliced in the back of the socket box with a crimp, or you might even have to go further back) and use a decent socket, like MK or Crabtree

Same with the 4 gang, cut the flex back to where it isn't heat damaged and install a new plug, or just throw it all away and buy a new one
 
Thanks for that, the strange thing is though this plug has virtually never been unplugged so its not as though its become worn or anything. Now that I think about it I've been noticeing a funny "uriniey" smell when I get sat in my computer chair for the last 4 or 5 days. Ruling out myself as the source :) I thought it was either coming from the lav next door or one of my cats had done the business nearby, didn't really investigate it too much to be honest. It will be interesting if that has gone when I get home tonight, since everything is unplugged in that room at the moment.
 
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btw regarding extending wires in backboxes would soldering on a short length of suitable sized flexible wire (either stripped from flex or bought seperately) be acceptable, using flexible wire it should be much easier to get a good solder joint and should also place far less stress on the joint when the socket is taken in and out of the backbox.
 
I'm no trician but that to me sounds a bit dodgy, seeing as its part of the ring main the stranded wire could in theory take up to 30A. And how good would a stranded wire soldered to a solid wire be jointwise? Have you seen the size of immersion heater cable? And thats only supposed to take up to 11A I think. I did think however you could use 30A terminal blocks rather than crimp connections to extend the wire using normal twin and earth, as long as it doesn't become excessively scrunched when put back in the cavity.

Anyway just got back from lunch and I had a look behind the socket, there doesn't appear to be any heat damage to the neutral or earth wire/insulation so I think I will be ok. The smell sems to have gone too. Just a bit worried that this could have happened when I wasn't in the room and been much more serious. Initially I'm going to look for a socket with and RCD built in just for this room then think about updating my CU. So hopefully in the meantime this will guard against earth/neutral shorts, and live/neutral shorts will be handled by the fuse. Plus I think a mass plug terminal tightening excercise is in order.
 
Bubo said:
I'm no trician but that to me sounds a bit dodgy, seeing as its part of the ring main the stranded wire could in theory take up to 30A.
ring cables are only rated at 20A you know and you can easilly get flex with the same conductor area so thats not an issue.

Bubo said:
And how good would a stranded wire soldered to a solid wire be jointwise?
if you tightly twisted the stranded wire onto the solid wire and then soldered it would be very good. solid to solid is harder to get a good joint because its difficult to ensure very close contact before soldering.
 
plugwash said:
btw regarding extending wires in backboxes would soldering on a short length of suitable sized flexible wire (either stripped from flex or bought seperately) be acceptable, using flexible wire it should be much easier to get a good solder joint and should also place far less stress on the joint when the socket is taken in and out of the backbox.

I can't see a problem with that method. Bearing in mind, (I don't know about now) we were taught to do "married" and "T" soldered joints before crimp connectors were available.
 
Bubo said:
Now that I think about it I've been noticeing a funny "uriniey" smell when I get sat in my computer chair for the last 4 or 5 days. Ruling out myself as the source :)
Hmm. Are you sure? I mean, if you are sat at the computer for 4 or 5 days, something's bound to happen...
 

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