Spark between neutrals

[Jim77]

Hello
I've just been replacing a twin 13a socket in a bedroom on upstairs ring and when I removed the two neutral wires there was a small spark when they touched together (The mains was still on). When I measured the voltage between the two black wires it was 245V. What's happening here? Is there a live/neutral cross somewhere? Why did the main fuse not blow? The wiring has been in place 29 years (new house in 1980). The socket has been used many times for vacuum cleaner etc.

Thanks for any replies
Jim

 

[ricicle]

It sounds pretty much like there is a borrowed Neutral on the circuit somewhere although there should not be a substantial voltage between the two Neutral conductors if it is a ring final circuit. How did you test that the socket was isolated. This fault could have given you a severe shock or worse.
Are you competant to fault find and do you have the appropriate test equipment. If not then this needs investigating by a competant electrician.

 

[rebuke]

You were replacing a socket with the power still on, or do you mean that other circuits still had power?

If the circuit was still powered up (a very scary thing to do while changing a socket!), my best guess is that one end of the ring doesn't have the neutral connected (or it's not a ring at all, but a long radial), so you had return current from one side, and the actual neutral at the other.

Alternatively, you might have a borrowed neutral, i.e. the neutral in the ring has been used for a lighting circuit somewhere, so there is a return current from that on the ring. If the ring was properly connected however you wouldn't measure a voltage neutral to neutral, so again it implies there's either a break in the ring somewhere, or it's a radial circuit...

How confident are you working inside consumer units, if you're happy to (and have a means of safe isolation - remember even if you turn the CU main switch off there is still power at the input to the switch), then you could disconnected the two neutrals for that ring, and meter them to check the ring continuity etc...

 

[Jim77]

Thanks for your replies.
Yes I was changing a socket with power on I was being very careful

Perhaps I'm not competent to test this after all - I can use meters etc, however I'm not qualified to work on domestic electrics nor am I fully familiar with the latest regs.

I think I'll take your advice to have it checked. Any rough idea on cost to check the electrics in a three bed house in Scotland?

Thanks again
Jim

 

[ColJack]

no, you're an idiot..

you NEVER work live.. even qualified and time served professional electricians don't do it unless it's a life and death situation...

it's also not a ring..

you've something plugged in upstream and the returning neutral current jumped the gap back to the supply..

 

[bongos]

Yes I was changing a socket with power on I was being very careful

It would have been simple enough to isolate the circuit. Never take the risk of working on a live setup.

As for the cost of a PIR - it will depend on the limitations of the inspection. Are all fittings etc accessible? I assume you won't want floorboards being pulled up all over. Best bet is to ring a few local sparks or get recommendations from friends/family and ask how much spark will charge, agreeing any limitations before the PIR commences.

 

[Jim77]

no, you're an idiot..

you NEVER work live.. even qualified and time served professional electricians don't do it unless it's a life and death situation...

Yes, I keep forgetting how times have changed!

it's also not a ring..

It's labelled upstairs ring at CU

you've something plugged in upstream and the returning neutral current jumped the gap back to the supply..

If I unplug everything on the circuit should the fault clear?

It would have been simple enough to isolate the circuit. Never take the risk of working on a live setup.

A much nicer way of saying it - thanks.

And I'll follow your advice, bongos, and contact a local electrician.

Thanks
jim

 

[ColJack]

the only acceptible time to work live is if turning power off would endanger lives, such as where ventilators are inuse etc..

It may well be labled upstairs ring, but then any idiot with a pen and a laymans knowledge would call the sockets in a house a "ring"..

there is no "fault".. you shouldn't work live then you wouldn't get sparks..

you do however have to test properly to ensure that you have continuity along the entire length of the socket circuit.. this means testing at the board.

the sparky will test it for you, and if you mention the suggested possibility of borrowed neutrals, he'll check for those too..

 

[electronicsuk]

t
there is no "fault".. you shouldn't work live then you wouldn't get sparks..

Although it's obvious that the OP working live is bound to lead to a little sparking on the conductors, the fact that 245v is present between the two neutral conductors does seem a little off. After all, splitting the ring effectively leaves you with two radial circuits, and you wouldn't expect to read 245v between the neutrals of two separate radials.

Perhaps this suggests a neutral break elsewhere in the circuit with the live still intact. That lack of a return neutral path on one leg of the ring and splitting the neutrals at a second location could lead to full phase voltage appearing on one neutral conductor (via a plugged in, switched on appliance) when measured against the other neutral or earth, which presumably still have continuity back to the CU. It would also explain a larger than expected spark between the neutrals - even in a heavily loaded ring, I wouldn't have expected a huge P.D. between the two conductors.

 

[ColJack]

until it's confirmed as a ring by testing at the CU, you can't trust that it is what it says..

it may very well be a radial that someone has taken a spur off at the breaker, leaving you with 2 sets of wires there that someone can easily mistake for a ring..

the test at the CU will also confirm whether it is supposed to be a ring and that it has a broken neutral..

 

[sparkyspike]

Hello
when I removed the two neutral wires there was a small spark

Maybe you should have disconnected the two live wires first...

But then, you may not have suspected a fault and subsequently, not come on here.

 

[securespark]

If it hasn't been stressed enough already, can I make everyone reading this acutely aware of the importance of safe isolation?

I get a bit of a bee in my bonnet about this because I have seen the consequences of not isolating safely & it is extremely grim.

Please, you MUST practice safe isolation before working on an electrical circuit.

The only live working you should do is testing. It is best to wear gloves when testing to avoid any possibility of contact with live parts.

First obtain a two-pole voltage detector and test it on a known source.

Test the circuit you are working on to see if it is dead. Test between
L & N, L & E and N & E.

Then test the device again on a known source to prove it is still working.

Lock off the device if possible. If the fuse board is in a garage or outhouse that is lockable, lock it and keep the key with you.

If you are removing a Wylex-type fuse or MCB, buy a blank fuse carrier & fit that in place of the fuse, then a fuse cannot be re-inserted by accident.

I know you'll be thinking, that's a bl**dy faff just to change a socket, but, believe me, it's better safe than sorry.

Do not put yourself or your loved ones at risk and never fall victim to complacency!

 

[ricicle]

And normally the accidents happen when you are doing what you think is the most menial of tasks.

NEVER WORK LIVE

 

[sparkyspike]

Or,

NEVER WORK, LIVE!

 

[Jim77]

Thanks guys

I really have got your message and I promise that I will NEVER WORK LIVE again.

Jim

PS Is it OK to remove the fuse from the CU without switching off? Or is that classed as working live too?