Ring Main Tripping RCD, Line to Earth reads 0

[JimmyB1988]

Hi All,

Will post an introduction on the new members forum but need to blurt out my question while its still in my head.

Me and Mrs JimmyB1988 have moved into our first house, it has a split consumer unit, half on RCD half not, my sockets were on the half without an RCD. One breaker does all the sockets including the Kitchen. So I moved the sockets breaker over to the RCD side, moved the neutrals too over to the RCD neutral bar. But it keeps tripping the RCD, all works fine on non protected side, i insulation resistance tested the circuit and reads 0Mohms between line + neutral, neutral + earth, line + earth? Confused?

Can anyone think of anything obvious?

Thanks Guys/Girls

 

[uksparks]

Sorry can you just clarify the readings as not 100% sure about what you have written.

Line to Neutral =
Line to Earth =
Neutral to earth =

Also can you provide

r1 =
rn=
r2 =

 

[jimjimjim]

Unplug stuff

 

[Clive P]

Hi,
Firstly I see your qualifications are in Engineering, although you may have electrical experience.
You need to double check the cables you have moved across are definitely the Line Neutral & EARTH for that circuit.
Secondly , unplug everything from the Ring Main to check whether it may be an appliance fault.
Finally, If you are using an IR tester it may show a dead short, but can be higher due to the resolution of the meter.

This should get you started anyway.

Regards.

 

[Megavolt]

Just see it now. Boiler, tv ect now not working for some strange reason. 1000 of them

 

[JimmyB1988]

Hi,

Line - Neutral 0.00 Mohms
Line - Earth 0.00 Mohms
Neutral - Earth 0.00 Mohms

I have unplugged everything in every room, and the boiler is on a spur from the ring which i've knocked off. Cooker is on the ring, but i knocked that off too. (As I type I realize the extractor is on the ring too so i will try again with that disconnected)

I switched off the main switch at the c/u, I disconnected the ends of the ring from the breaker and neutral and earth bar.

R1+R2 is 0.32 ohms

 

[Plonker 3]

Them IR readings are a dead short, looks like someone had a fault and changed it non RCD protected to stop it tripping, or you haven't actually disconnected the cables from the DB.

 

[JimmyB1988]

Yep I'm a maintenance guy, so i have a bit of knowledge about electrics and access to meters etc from work. But i'm no domestic electrician.

 

[JimmyB1988]

I thought they read as a dead short, but if they did it wouldn't it trip the MCB when on the non RCD side?

 

[Plonker 3]

Not if the earth path is faulty or has a higher resistance than the neutral.

Have you disconnected EVERYTHING?

 

[Silly Sausage]

Them IR readings are a dead short, looks like someone had a fault and changed it non RCD protected to stop it tripping, or you haven't actually disconnected the cables from the DB.

Re think that Dilb!

 

[Octopus]

Yep I'm a maintenance guy, so i have a bit of knowledge about electrics and access to meters etc from work. But i'm no domestic electrician.

Why do people write this? the theory and practice of electrics doesn't differ much between commercial and domestic, especially when it comes to this stuff!

 

[Plonker 3]

Re think that Dilb!

Well you knew what I meant

 

[Marvo]

When you did the IR test what test voltage did you have your tester set to? If there's still something connected such as an electronic ballast or TV amplifier if might give you low IR readings due the test voltage exceeding the clamping voltage of the built-in surge arrestors.

 

[Silly Sausage]

Well you knew what I meant

Hahaha...we don't want to send the innocent up the wrong garden path now...

 

[HandySparks]

You need to repeat the tests on a low ohms range to get a proper picture of the state of this circuit.

Edit: 0.00Mohms could be as much as 5k ohms.

 

[Plonker 3]

Hahaha...we don't want to send the innocent up the wrong garden path now...

I thought we could, until the thread got moved into the DIY board

 

[Silly Sausage]

No it hasn't, I can't post there.

 

[Marvo]

The opening post had good concise info and there's obviously a good degree of competence so I've left the thread in the General forum.

 

[Plonker 3]

Strange, I thought the mods were bored, as just about every other thread today has been moved lol

 

[Octopus]

The opening post had good concise info and there's obviously a good degree of competence so I've left the thread in the General forum.

Are you sure??

 

[Nightmare1308]

when u done a insulation resistance test did u remove the cables from the consumer unit ?
i see it hard to believe that u didn't have a problem before u moved it to the RCD side ? also like others have said have u removed all items from the sockets and isolated all spurs ? plus is there any neons on the circuit what could give u false readings ?

 

[Marvo]

Are you sure??

Hmmm, we'll see how it goes...

 

[Octopus]

Hmmm, we'll see how it goes...

I'm no mod, but I think there are too many buzz words used!

 

[DPG]

L/N/E all shorted together? Could be I suppose. Or is the OP mis-reading the tester and actually seeing an infinite resistance reading? Daz

 

[foresite123]

Hi chaps , first post so hello to everyone.

One explanation could be a neutral to earth fault and the tester is picking up a reading through the connected load.

Not all switched spurs are double pole and can leave the neutral connected in which case if there was a neutral to earth fault then it will show up on L/E test as well.

Could be a problem with the boiler or maybe a some lighting taken off the ring through a spur somewhere.

Regards

 

[JimmyB1988]

Hi All,
Had another crack at it this evening

Tryed to be systematic, so identifyed one outgoing cable's conductors as L1, N1 and E1 using tape, other obviously is L2, N2 and E2
Checked the end to end resistance of each conductor on low ohms
L1 - L2 0.27 ohms
N1 - N2 0.27 ohms
E1 - E2 0.42 ohms

Then IR tested between L, N and E again.
L - N 0.00Mohms
N - E 0.00 Mohms
L - E 0.00 Mohms

As suggested then tested the same conductors using the low ohm setting.
L - N 28.8 ohms
L - E 30.1 ohms
N - E 0.14 ohms

Worrying figures, theres a short somewhere i think.

So next i split the ring, I disconnect the socket at the top of the landing, making two radials and from the DB i test each radial

L1 - N1 >99.99 Mohms
N1 - E1 >99.99 Mohms
L1 - E1 > 99.99 Mohms

L2 - N2 93.1 Kohms
N2 - E2 0.41 ohms
L2 - E2 93.2 k ohms

So my Cable 2 leg holds the fault, what do you guys think seems most likely from those readings?

 

[Megavolt]

Remove all sockets on suspect radial and test each leg till you find it.

 

[R G]

Now you will have to narrow it down by dividing that leg , and testing as you do so , until you narrow it right down to one leg between 2 sockets , the fault could well be at one of the sockets ..

 

[Octopus]

Had one of these on Monday, split the ring until I was able to identify 1 link between 2 sockets with an almost dead short L - E and on another link a IR reading of 0.35 N - E. Happy days. These things take time and I was doing this in a converted barn where the circuit was not laid out logically so it took a while to trace the links!

 

[telectrix]

from the figures you have given, it looks like a N-E fault/short. as said above, keep splitting the faulty leg down.