FIRE RISK PROTECTION:- MELTING SOCKET - LOOSE WIRES ?

[jnchapple]

My dishwasher stopped the other day because the the neutral side copper connector inside a standard 13 amp socket had literally melted away, leaving a very badly charred and burnt socket. I tend to use the dishwasher overnight, so I am lucky that a fire did not start.

The intense heat was obviously caused by arc-ing beween the neutral pin on the plug and its socket. Both were effectively destroyed and I have taken photos of the damage.

As far as I can see, there would have been no leak from neutral to earth in this case - just meltdown. So am I right that an RCD will not detect problems of this kind.

I notice too that mains wires in plugs tend to become loose over time. I suspect that the pressure of the screws is gradually reduced as the copper strands gradually give way. This has also caused arc-ing from time to time.

Can you install surge detection centrally ie like an RCD but which can detect arc-ing problems in a ring main ?

 

[plugwash]

this type of fault is pretty much impossible to detect and automatically disconnect

the only noticeable effect it might have is an increase in harmonics but with any significant electronic equipment around those are likely to be high anyway

 

[felix]

Your best chance of detecting this kind of fault is to look for high frequency modulation of the current at the consumer unit. To prevent false tripping caused by devices such as dimmer switches you would have to filter out all the harmonics of 50Hz and detect only random noise pulses. I don't know of any device that does this. Perhaps you'ld like to invent one!.

 

[plugwash]

thing is an arc will generate mostly harmonics of the mains freqency (like almost any non liniar device will)

i just can't see this working (mains is noisy as hell anyway)

if there was a known reliable way to stop theese kinds of faults im 99% sure it would be a regs requirement by now.

 

[Adam_151]

I know it isn't british, but this: http://www.iaei.org/magazine/00_d/gregory.htm might be worth a read for anyone interested

 

[mapj1]

Arc fault detectors are not perfect, but as the article describes they can perform a useful function protecting heating and lighting circuits.
Obviously you can't have one feeding all loads, as they will false trigger on loads that are intrinsically noisy (arc welding gear comes to mind!).
In general they are seen in the 110V countries where high current (low voltage) arcs are more common. The simple ones are a high pass filter and a threshold detector, but I belive some electronic ones are more sophisticated.
Incidentally in some countries regs a 60V arc voltage is assumed when calculating the Earth loop fault current, leading to rather different assumptions about over current and disconnection time.
regards M.

 

[ban-all-sheds]

The intense heat was obviously caused by arc-ing beween the neutral pin on the plug and its socket.

Why do you say "obviously"? Could it not have been a loose connection in the plug or the socket?

How old was the socket?

I notice too that mains wires in plugs tend to become loose over time. I suspect that the pressure of the screws is gradually reduced as the copper strands gradually give way. This has also caused arc-ing from time to time.

Regular maintenance would seem to be a Good Idea™.....

 

[Damocles]

I don't know if you were lucky or not. Quite possibly these things are designed to be sufficiently heat resisting that they will melt quietly into total failure before catching fire. So in fact luck might not come into it at all. Just fail-safe design.

 

[mapj1]

Not always clever design- a fully loaded moulded plug is a particularly bad contender for the booby prize as the connections are often onto slightly oxidised wire to start with and well lagged, there is nowhere for the heat to go. Not unknown for them to get so hot they catch fire. For some reason tumble driers are particularly bad. Also the fuse terminals lose their spring temper once they have been cooked, making matters worse there too.
regards M.

 

[jnchapple]

Thank's everyone for your input on fire risk protection.
I've followed up on arc fault circuit protection and the "Fire Guard" cutler hammer product range. Terminology is a problem. the US seems happy with AFCI (arc fault circuit interrupter), but in Europe this term isn't used. I have found the equivalents for the UK yet but am looking.
Maybe Siemens or Hager have equivalents but their literature is a headbanger maybe because they think in German and write ambiguous English - Aarghhh !

 

[kendor]

as an arc effectively acts as a rectifier (remember the old mercury filled glass bubble rectifiers of old) a DC content detector could maybe be utilised?