Charred wire puzzle

[Omahn]

I am not an electrician, nor do I intend to be and I'll be getting a qualified electrician to look at this next week. But, my curiosity has got the better of me so I thought I would ask my questions here.

If you have a look at my CU unit shown in the picture above, you can see a charred wire near the right hand side of the picture. The trip switch did throw itself but only after suprising amounts of smoke.

I've removed the offending wire from the MCB (and insulated as appropriate) with the idea that when I turn the power back on, I'll find some sockets or appliances without power and so I'll know where the problem lies. But strangely, I've turned the power back on, and nothing is off. Everything is working fine. Would anybody be able to explain why? The house is fairly new (10 years old) so I wouldn't expect any redundant wiring. The MCB in question is simply labelled 'Sockets', yet it seems to supply power to the garage and shed also. Is this normal practice? Seems a little suspect to me?..

 

[BSBS]

yes what you have described would be normal.. There appears to be 3 wires connected to the MCB two of these wires are in essence the same cable becuase the the sockets would be wired in a "Ring" essential what you have done is disconnect one end but nothing will have lost power becuase it is still being fed from the other cable still connected to the MCB.. the third wire in the same MCB i would imagine is a "spur" which may feed a seperate socket.

The problem with disconecting one end of the "ring" as you have done is that now the cable may become overloaded becuse 2.5mm cable is not rated to carry 32A. When you wire the circuit as a ring you essential double the CSA (cross sectional area) of the cable 2x 2.5mm =5mm which can carry 32A

Burning on the end of the cable like that is usually a sign of a poor connection into the MCB.

 

[Omahn]

Thanks for the quick reply, BSBS.

yes what you have described would be normal..

snip

The problem with disconecting one end of the "ring" as you have done is that now the cable may become overloaded becuse 2.5mm cable is not rated to carry 32A. When you wire the circuit as a ring you essential double the CSA (cross sectional area) of the cable 2x 2.5mm =5mm which can carry 32A

I feared that might be the case.. (increasing the loading of the existing cable.) I'll make sure we're very careful with the appliances on that circuit until the electrician visits this week.

Thanks again.

 

[Adam_151]

Just switch it off, and cut back the cable, re-strip it, and put it back in the mcb and ensure that you get it tight, then switch back on again.

On another note, WTF is that Wylex breaker doing in a MK board?

EDIT: You are lucky that this occured at the CU and not at a junction box in the loft, the reason the breaker tripped is because the heat generated from the bad connection caused the insides of the breaker to heat up, and that triggered the thermal part of the breaker, if it had been further down the line, there would be nothing that would trip out at all

 

[andy]

if there is a problem on the ring and you have disconnected 1 leg, then you could have put a dangerous load on the other leg which can easily cause a fire. never disconnect it unless your 100% that both legs of the ring are disconnected or neither are.

altho havin said that, it might be for a spur where you have too much plugged in. or it could just be a dodgy connection, in which case it simply needs cut back and re-connected

 

[Omahn]

Just switch it off, and cut back the cable, re-strip it, and put it back in the mcb and ensure that you get it tight, then switch back on again.
On another note, WTF is that Wylex breaker doing in a MK board?

I think I'll leave it to the electrician as the insulation around the wire in question has turned to ash over the last couple of inches and chopping it off before the 'burnt' part doesn't leave enough slack to get it into the breaker.

That Wylex breaker was put in when we suspected a fault on the existing breaker, B&Q assured me that it would be fine. Is this the case? If not I'll get the electrician to sort that out too.

EDIT: You are lucky that this occured at the CU and not at a junction box in the loft, the reason the breaker tripped is because the heat generated from the bad connection caused the insides of the breaker to heat up, and that triggered the thermal part of the breaker, if it had been further down the line, there would be nothing that would trip out at all

How often should I be checking these connections? Does the connection get looser over time?

 

[Omahn]

if there is a problem on the ring and you have disconnected 1 leg, then you could have put a dangerous load on the other leg which can easily cause a fire. never disconnect it unless your 100% that both legs of the ring are disconnected or neither are.
altho havin said that, it might be for a spur where you have too much plugged in. or it could just be a dodgy connection, in which case it simply needs cut back and re-connected

Point taken. I'll be leaving it off. Not sure about the loading. (Didn't even realise the garage was on the same breaker.) At around the time it tripped, it had a washing machine, dishwasher and a freezer on it, turned on. Nothing else though.

 

[andy]

How often should I be checking these connections? Does the connection get looser over time?

would have been a dodgy connection when first made. very rare for them to work loose, unless often moved (i.e caravan wiring, or wiring subject to vibration)

 

[Adam_151]

That Wylex breaker was put in when we suspected a fault on the existing breaker, B&Q assured me that it would be fine. Is this the case? If not I'll get the electrician to sort that out too.

While DIN rails are standard, buss bars are not, MK consumer units are designed for, and tested with MK devices, wylex with wylex devices, ad nausum, it may well be the case that your wylex breaker mates perfectly well with the buss bar in your CU, but its also possible that it does not (without looking at them closely I couldn't say, and even if it does, it won't be certified, and the CUs standards approval might be void), if you can get the correct parts, its better to use them rather than messing around and doing something which is really a bodge.

 

[andy]

you can see from the pics that the wylex is a different size. have problems getitn the cover on?

 

[HDRW]

...
Not sure about the loading. (Didn't even realise the garage was on the same breaker.) At around the time it tripped, it had a washing machine, dishwasher and a freezer on it, turned on. Nothing else though.

It would be nice to know what the three wires are - probably a ring and a radial, but better to know for sure. The garage may be the radial?

As for the load when it tripped, a washing machine and dishwasher on at the same time could happen to have 2 x 3kW heaters on at once, so that's about 26A. Not enough to trip the breaker, but it's a heavy load, and it they are close to that end of the ring that is the charred wire, and it wasn't as tight as it should be, that would cause the heating. The fact that the end of the wire is charred but firther down it isn't, means it must have been the termination that was the problem, not an overload of the wire itself.

Cheers,

Howard

 

[Omahn]

you can see from the pics that the wylex is a different size. have problems getitn the cover on?

Nope, straight on, although I'll still be getting it replaced given the advice from Adam_151.

 

[Omahn]

As for the load when it tripped, a washing machine and dishwasher on at the same time could happen to have 2 x 3kW heaters on at once, so that's about 26A.

Sounds logical, the washing machine and dishwasher were fairly early in their cycle so both will of been using the heating elements. (Just before the breaker went.)
I've left the power off for now to that breaker, just hope I can get an electrician out in the next few days. (No washer, dryer, fridge or freezer.) (Not to mention the dishwasher!)

 

[ban-all-sheds]

That Wylex breaker was put in when we suspected a fault on the existing breaker

What was going on before that made you think there was something wrong with the breaker?

B&Q assured me that it would be fine.

Was this the guy that advised you?:

How often should I be checking these connections? Does the connection get looser over time?

They don't work looser - almost certainly the loose connection was done by the person who put the breaker in..

 

[Omahn]

That Wylex breaker was put in when we suspected a fault on the existing breaker

What was going on before that made you think there was something wrong with the breaker?

Another electrician had only recently replaced the pull cord switch to a shower, yet it failed shortly after. Turned out to be the switch anyway.

Was this the guy that advised you?:

Yeah, that's the guy!

Electrician is coming round later today to test and sort it all out. Thanks for everyones advice.