Insulation testing earth fault... strange one

[Dane]

Have a new build to finish of etc

Doing the testing

Got a ring main, with a short circuit to every earth including main earth and bonding except the ring main earth

Continuity tests .... All good

End to end tests ..... All good

Cross connects .... All good

Every other circuit all dead tests, no issues what so ever.

All circuits pass insulation tests when testing seperatly.

Just a measure, carried out a global insulation test and this fault cropped up.

Was narrowed down to the ring main.

Removed the ring main, tested it all fine. Once the ring is put into the board it fails tests insulation resistance to any earth is around 0.04M ohm

It is a new build, I installed all wiring and everything went in as it should be.

The earthing system is PME

80amp main fuse

Any ideas?

Might not be clear above, but its a bit hard to explain.

should i just power up and see what goes ............. bad day explode

 

[sarni31]

might be a trapped live when fitting a socket front ?

Guinness

 

[Lurch]

Nail through a cable, just shorting the phase to adjacent earth of something else?

 

[Dane]

Neither of them would explain the poor results when testing on

The 1 bit of cable we narrowed it down to and main earth only, nothing else connected.

Its very hard to explain though, ive tried to explain the best i can.

 

[Evans Electric]

Presume you mean it reads to the bonding when the ring earths are not connected to the earth bar , otherwise you have a worse problem. Sounds like what Lurchio says to me, perhaps in contact with a pipe.

 

[Dane]

Forgot to mention there is no metal pipes or structure.

 

[flyingspark]

Perhaps melted or damaged sleaving & insulation between live and an extranious/exposed conductive part.

Sorry if you have done this, but have you tried narrowing it down by process of illimination with the main protective bonds to gas, water etc?

 

[flyingspark]

ok, was too slow, ignor my last reply!

 

[NozSpark]

I had a IR fault on an existing ring the other week,

At first I thought that I'd put a screw through a cable when I put up some mini trunking.. I measured the resistance and it came up as 11k ohms (low ohms range),, each screw that I'd put in had the same readings...

After chopping out a good bit of lime plaster I found that I'd not caught a cable.. in the end I found a taped joint burried in the wall and the plaster must have been damp..

Which leads me to.....

Has the plaster dried out properly???

 

[Evans Electric]

So the live or neutral of the ring is reading to the earth bar, its own earth wires are clear,so need to disconnect the earths from your earth bar and see which one is reading , then take it from there.

 

[Dane]

So the live or neutral of the ring is reading to the earth bar, its own earth wires are clear,so need to disconnect the earths from your earth bar and see which one is reading , then take it from there.

the one that its getting a reading on is the main incomer. Already narrowed it down.

I even got my self a bit of 10mm Earth

Checked it to that (2m length) one hand held in air knowing it will be fine... it was

I then rammed the end into the ground...

and got the bad readings again.

The plaster is 100% dry, its been sat 3 months bone try, water tight.

 

[Evans Electric]

Are we looking at the live or neutral having a screw / nail/metal sheathing, into blockwork then , picking up a reading through the general mass of earth ? Or trapped in a KO box .

 

[Lurch]

Bizarre! I stick to my original theory. Phase on ring caught with something that is grounded, though not neccesarily a cable or earthed thing, just somehow connected to the earth, IYSWIM (assuming it is definitely not in any way damp anywhere).

I think the ring needs ripping apart to find the leg that is causing it, might be one of those things someone else would walk in and spot a culprit straight away, not a usual fault by the sound of it.

 

[Dane]

Not usual at all. It threw myself and another completley at first.

Was loosing light by 4.30 so had to pack up and got to go back in the morning.

Going to rip the ring main apart like you say lurch....

Some how, some where that 1 piece of cable is going to earth via the building. Again, there are no other metal parts within the builing.

Cables are all in plastic capping... yes it could be a capping nail! But its must be a very wet building on the structure side to get a reading through the building like this surely?

 

[batty]

Only thing I can think cable has lost insulation through hole in joist rubbed against bonding cable which is also bare hence low reading but clutching at straws here.

 

[Dane]

Only thing I can think cable has lost insulation through hole in joist rubbed against bonding cable which is also bare hence low reading but clutching at straws here.

That would be clutching at straws...

Reminded me something else that may help though

Bonding cables removed for the tests

 

[jl-heating]

all end to end tests on the ring are ok..

all ir on the ring are good when the cables are removed from the board.

all Ir tests on all other circuits are good removed from the board

ir test on board with neutral and earth clamps fitted and go down the breakers show a dead short.

remove 1 leg from the ring on the earth and the readings are fine ... remove the other leg and the reading fail ..

this is what i got when i spoke to dane ealier .. correct me if i am wrong dane .

ze has not been checked yet as the incomers are not connected yet this is only a dead test before inisialation of the board.

 

[batty]

Is it a metal consumer unit?

 

[Dane]

Is it a metal consumer unit?

simple answer

No

Its a standard 5 + 5 wylex split load

 

[The Godfather]

Any ideas?

OK, Dane:

You say the Ring tests fine when not connected to the CU?

You say Bonding removed for purpose of test?

Have you checked for continuity to another circuits CPC and or Neutral?

Don