How to Find earth Break in ring main

[richardhill84]

Hi,

I have just had a new CU fitted by a local sparky. The new CU was fitted because i have a new extension and the one fuse box wasn't large enough and was a bit long in the tooth.

On testing the ring main before commissioning the new CU, the sparky detected that there was a break earth on the ring. He proceeded to check every socket and nip up the earths (in case one was loose) but the problem persisted.

To get around the problem, he converted the ring to two radial circuits (20Amp MCB).
After doing so, every socket worked in the house (and tested correctly work plug earth tester) on just one radial MCB. Thus the earth break must be between the last socket and the CU.

The sparky said that i/we really need to try and find the fault and to give him a ring when i'm decorating the front room and ripping up the carpets etc.....

But my question are (if you can help).....

1. Is there anyway of determining/detecting the last socket on a curcuit to make tracing the fault easier?

2. If he had tested every plug with the earth tester when it was still in a ring main configuration, could he not had deduced the same conclusion (i.e. there was an earth break between the last socket and the cu)

I'm not questioning the sparky as he seemed good, i'm just interested in how it all fits together and why.

Thanks

 

[Spark123]

I take it he must have found the offending length of cable before connecting to the two MCBs as he must have disconnected the lives and neutrals to separate the circuits out.

 

[richardhill84]

not sure....
All i know is that on the old fuse box there was just one 32Amp fuse for the entire ring.
On the new CU, there are two 20 Amp MCB's one for each rdial. But as i said, just one radial mcb can power all the sockets in the house.

I guess he just split the ring return into a seperate curcuit. thus creating two radials???? (just a guess)

 

[Steve]

So turning off one of the MCBs leaves all the sockets energised? Odd he didn't find that.

If you know the route of the wires from the CU, you will be able to trace the fault and fix it yourself.

I had a similar situation recently. Plasterer took a socket off, and the wires fell out. I took it off completely, and was mystified that half the ring now didn't work. Of coruse, there was a break upstairs in an FCU (the screws were completely loose). I had to do a bit of detective work to find that though, looking which way the wires went from the socket in question, and which sockets didnt work. You have it easy - its close to the CU!

 

[securespark]

If your circuit covers 50m² or less, and the EFLI at the destination of each radial is OK, why not leave it as radials?

 

[dingbat]

not sure....
All i know is that on the old fuse box there was just one 32Amp fuse for the entire ring.
On the new CU, there are two 20 Amp MCB's one for each rdial. But as i said, just one radial mcb can power all the sockets in the house.

That is dangerous. You don't have two radials, you still have a ring, with an incomplete circuit protective conductor, but now energised from two sources. Get him back to properly separate each leg. (If he'd done this in the first place then he would already know where the break is!)

 

[richardhill84]

Yes, Turning off one MCB keeps all the sockets 'energised'. I know thats it must be a break between the last socket and the CU, i just don't know which is the last socket. I guess it could be any in the house.
Do you know if the last is typically upstairs or downstarirs????

securespark: I don't want to leave it as is as just one radial is powering all sockets and it is only a 20Amp MCB. I could draw more than 20Amps if i have the lots of appliances on and trip it out.

 

[luminaire]

not sure....
On the new CU, there are two 20 Amp MCB's one for each rdial. But as i said, just one radial mcb can power all the sockets in the house.

Sounds to me as though he must have an interconnection somewhere.

I had a similar thing once and I ended up up connecting one lead of the continuity tester to the phase on the MCB and a wander lead going from one socket to another throughout the house until I found which socket was on which circuit.
Be sure to switch off at the mains if you are going to attempt anything like that.

 

[richardhill84]

not sure....
All i know is that on the old fuse box there was just one 32Amp fuse for the entire ring.
On the new CU, there are two 20 Amp MCB's one for each rdial. But as i said, just one radial mcb can power all the sockets in the house.

That is dangerous. You don't have two radials, you still have a ring, with an incomplete circuit protective conductor, but now energised from two sources. Get him back to properly separate each leg. (If he'd done this in the first place then he would already know where the break is!)

You're getting me worried now. Are you saying he should have split the ring thus creating two radials? How would this be done typically. Would he have to disconnect one side of a socket thus breaking the ring. I'm pretty sure he didn't do that.

 

[Steve]

Oh I understand now. If its only the earth thats broken, then you'll still have a ring on the L and N wires. Which is dangerous as it is. Turn off one of those MCBs now! And get another electrician in! You could pull 40 amps as it stands! The electrician should have tested all 3 conductors!

 

[richardhill84]

Oh I understand now. If its only the earth thats broken, then you'll still have a ring on the L and N wires. Which is dangerous as it is. Turn off one of those MCBs now! And get another electrician in! You could pull 40 amps as it stands! The electrician should have tested all 3 conductors!

I should have mentioned that he has said to leave the second MCB off until the fault is found.

 

[Steve]

Why fit and connect the 2nd MCB then?

 

[dingbat]

You could pull 40 amps as it stands!

Er, no... either MCB would still trip at its usual rating. What's potentially dangerous is the dual source of supply.

But just to confirm, Richard, does either MCB energise the entire circuit? I'm guessing from your last reply that it does and that, therefore, your 'electrician' is incompetent. Get it fixed properly.

 

[richardhill84]

You could pull 40 amps as it stands!

Er, no... either MCB would still trip at its usual rating. What's potentially dangerous is the dual source of supply.

But just to confirm, Richard, does either MCB energise the entire circuit? I'm guessing from your last reply that it does and that, therefore, your 'electrician' is incompetent. Get it fixed properly.

I'll check it this evening and let you know. Based on your last response i'm hoping it doesn't.
Are you saying he should have physically broken the ring somewhere in the middle to create the two seperate radials? I guess he can't do this at the CU?

 

[richardhill84]

You could pull 40 amps as it stands!

Er, no... either MCB would still trip at its usual rating. What's potentially dangerous is the dual source of supply.

But just to confirm, Richard, does either MCB energise the entire circuit? I'm guessing from your last reply that it does and that, therefore, your 'electrician' is incompetent. Get it fixed properly.

I'll check it this evening and let you know. Based on your last response i'm hoping it doesn't.
Are you saying he should have physically broken the ring somewhere in the middle to create the two seperate radials? I guess he can't do this at the CU?

Out of interest, why would a dual source of supply dangerous. Isn't the same as a ring main. Thats dual sourced (i.e. i.e Current will go clockwise or anticlockwise around in a ring to the socket.)