Low R2

[Malcolm]

Hi. I don't know if anyone can help or shed any light on this one.
I installed a new circuit in my sons garage. 2.5 T&E clipped direct, 4x metal clad sockets and back boxes.
I haven't terminated it into the CU yet as I need to get a RCBO.
I just got everything in place and did dead tests.

End to end L 0.21Ohm
N 0.21Ohm
E 0.02 Ohm

IR >1000 Mohm

There is nothing connected in the board so no parallel paths.
I had my yearly assesment today and I asked the assessor and he couldn't think of why it would be so low.
Any one any ideas?
Thanks

 

[Pete999]

Hi. I don't know if anyone can help or shed any light on this one.
I installed a new circuit in my sons garage. 2.5 T&E clipped direct, 4x metal clad sockets and back boxes.
I haven't terminated it into the CU yet as I need to get a RCBO.
I just got everything in place and did dead tests.

End to end L 0.21Ohm
N 0.21Ohm
E 0.02 Ohm

IR >1000 Mohm

There is nothing connected in the board so no parallel paths.
I had my yearly assesment today and I asked the assessor and he couldn't think of why it would be so low.
Any one any ideas?
Thanks

Poor connection on your MFT?

 

[Malcolm]

Nope. Checked it half a dozen times. same result. Checked it with my other mft, same result.

 

[hightower]

Excuse me for silly suggestion (just a trainee) but no parallel paths like cable armouring (I know you said T+E, just trying to have a go at helping) :willy_nilly:

 

[stevethesparks]

How far away is the CU from the sockets, and what are they fixed to?

 

[hightower]

What does the end to end test as on the rest of the unused cable drum?

 

[joehat]

Read the meter wrong?

 

[malcolmsanford]

They haven't put in a 10mm CPC in the T+E have they?

Honestly this defies logic. You obviously have a ring on your r1 rn and so by rights your smaller CPC should be higher on r2.

All I can think of it start to break the ring down, it's only 4 sockets and test the cable from CU to Socket 1 and then Socket 1 to 2 and go round the ring

 

[Malcolm]

Thanks for the replies.
(Hightower) No parallel paths. End to end on drum, I will try that. It was a new 100Mt roll.
(Steve) It's a double garage, the CU is in the garage. so total ring about 15/20mts? They are fixed to a breeze block wall.
(malcolm) Will do, I will be going back there next week to finish off, so I will try that.

 

[Pete999]

Thanks for the replies.
(Hightower) No parallel paths. End to end on drum, I will try that. It was a new 100Mt roll.
(Steve) It's a double garage, the CU is in the garage. so total ring about 15/20mts? They are fixed to a breeze block wall.
(malcolm) Will do, I will be going back there next week to finish off, so I will try that.

Metal sockets? if so remove the front plates from the back box and dis the fly lead (if you have one) and re test r2

 

[Malcolm]

Right just checked the rest of the roll.

L 0.37
N 0.37
E 0.67

Which is what you'd expect. So why it is down on the install I'm stummped.

 

[Malcolm]

Pete, yeah will try that also.

 

[Pete999]

Right just checked the rest of the roll.

L 0.37
N 0.37
E 0.67

Which is what you'd expect. So why it is down on the install I'm stummped.

Have you responded to post 10 yet?

 

[Malcolm]

Sorry wrote it out then got sidetracked.

Pete, yeah will try that also.

 

[Pete999]

Sorry wrote it out then got sidetracked.

Pete, yeah will try that also.

Please do I'm clutching at straws but I may be onto something

 

[Malcolm]

It looks like I might be up there on Saturday so will try things out then.
I will post back
Thanks

 

[Deleted member 26818]

Screwed through into the earth of another cable when fixing the sockets to the wall?

 

[hightower]

Screwed through into the earth of another cable when fixing the sockets to the wall?

Thing is that's an end to end reading and it's not connected to the db so this shouldn't have an affect

 

[Octopus]

Is the T&E in metal conduit?

 

[Deleted member 26818]

It's fixed direct.

 

[Dave OCD]

I reckon the wall is damp and the low reading is via the 'fabric of the building' with metalclad sockets being used.

 

[Pete999]

I reckon the wall is damp and the low reading is via the 'fabric of the building' with metalclad sockets being used.

See post 10 I asked the OP to remove socket fronts and fly leads and retest he is going back Saturday to do just that.

 

[mattg4321]

Damp walls is my guess too. Had this before.

 

[Malcolm]

Hi all.
Didn't have time to do any electrical work on Saturday. Was helping him move all his stuff from the old house.

Dave & Matt. Yes I wondered if it could be the building that was bringing it down, but I had never come across it before.

 

[Adam W]

I agree with Pete and Dave - the sockets and backboxes will all be earthed, but also there will be some kind of parallel path via the mass of the building, like with a TT system. The building won't even need to be particularly damp for this either. As Pete says you could disconnect all the flyleads and leave the socket fronts hanging in mid air and test to prove this, but a low R2 is a good thing.

 

[Lucien Nunes]

Damp? What's it damp with, mercury? A dripping wet wall might get you 0.02 MΩ but not 0.02Ω. Taken at face value (which is not always right with 1 s.f. on low resistance tests) 0.02Ω is equivalent to about 1.5m of 1.5mm², so the socket boxes are probably connected directly together with a large chunk of metal. Perhaps the fixing screws have gone through metal studwork or a plaster bead or similar.