Low R2 and where to start

[topquark]

Hi all, I'm testing an old ring final circuit, circa 1996 (upstairs, kitchen+garage) on a TNCS (PME) earthing system, and I'm getting the following initial readings:

R1 = 0.42 Ohms
R2 = 0.19 Ohms
Rn = 0.36 Ohms

That's enough to see I already have a couple of potential issues to track down; in that the cpc resistance is too low (run in 2.5mm/1.5mm twin/earth) and so should be around (0.42+0.36)/2*2.5/1.5 approx 0.65 Ohms give or take a little. So probable multiple earth path problem.

On digging further I am getting 0.11 and 0.10 Ohms (from either end of the cpc) to the main bonding for the water! Any ideas why there would be any connection here? (tested to the bonding cable when removed from the MET). Oh and there are no sockets in either the bathroom (or ensuite) that would require connecting in such a way.

No signs of any additional cables in any of the sockets on the rings (oh and the fly leads are all missing to the metal knock out boxes!).

The other problem is a possible loose connection on the run for the line (R1 and Rn a bit too far adrift and R1 is higher).

What am I missing, apart from a few brain cells :-D

Thanks
Martin

 

[Sparky Ninja]

Parallel paths can come from many more areas than that. Simple site fabric can create a parallel path on occasions. As long as you've checked polarity, and measured your Zs at these sockets, I cannot see why there are any problems.

 

[tony mc]

Any supplementry bonding from a socket to a water pipe in kitchen etc?

 

[Deleted member 9648]

Wot Widdler said ...plus.

Parallel path to main bonding......Boiler on the ring?...Immersion on the ring?

As long as other readings are satisfactory not worth bothering with.

 

[topquark]

Hi Widdler, thanks for the reply. Just a bit nerbous about the parallel path disappearing and leaving a poor connection (I'd be happier if I could find it :-D).

Hi Tony MC, thanks, there don't appear to be any extra cable connections in any of the sockets (or knock out boxes), so no :-(. I think as Widdler is suggesting this must be part of the fabric but can't for the life of me think what it might be.

 

[nickblake]

What you could do is check the R1R2 where you cross the tails of the ring live of one leg to cpc of the other leg and check at every socket , the readings should increase till you get mid way through the ring then start to decrease ,you may have a cross connection on the ring basically a ring in a ring, agree with widdler and wirepuller too

 

[topquark]

Wot Widdler said ...plus.

Parallel path to main bonding......Boiler on the ring?...Immersion on the ring?

As long as other readings are satisfactory not worth bothering with.

Thanks Wirepuller, a useful edit there. The boiler and immersion have a dedicated circuit but must admit I haven't checked that electrically

Bet that's it, I'll be happy if it is!

Cheers
Martin ...

 

[Sparky Ninja]

Hi Widdler, thanks for the reply. Just a bit nerbous about the parallel path disappearing and leaving a poor connection (I'd be happier if I could find it :-D).

I would expect that if the parallel path was later removed then you would still maintain an acceptable r2, you have no reason to think otherwise.

If you are lucky enough to have a client where you can keep digging then do so, you'll find it very informative. I'd take an approach like wirepuller says and check for things like immersion etc first.

Following from that, you could split the ring and check with an R2 lead the resistance from point to point working away from the CU. Strictly speaking the x1.67 rule should apply and you will notice as you go along when the R2 drops 'the' or 'a' parallel path will be located.

 

[ezzzekiel]

the 1.67x is very rarely accurate and figures can vary depending on a lot of things is one clip tighter than other, one particular socket terminal not as tight, cpc bit thinner/thicker than should be, providing all figures are not a million miles out and all other test results come out ok then i wouldnt worry about it either.

always find it a bit strange when looking at other peoples test sheet where all values for r1 rn r2 and crossover readings are exactly perfect yet mine very rarely are..........

 

[topquark]

I'm used to things not being quite "exact" due to component tolerances etc (I come from a electronics/computing background). So useful to get some real world input. I'll check the boiler and immersion tomorrow to see what circuit they're really on.

 

[telectrix]

and it is not necessary to fit cpc fly leads to knock-out boxes any more, but IMO it is good practice

 

[chiplard]

I'd bet on supplementary bonding.

 

[IQ Electrical]

Is someone paying you to find why the r2 reading is so good?

Part of the skill of periodic inspection is to know when a problem exists and also to anticipate where results on a real-world installation will differ from electrically perfect text book calculations.

Without this experience, we'd never finish an inspection, parallel paths are prolific, even on an 'all insulated' domestic installation.

Unless the installation is using conduit/trunking as the CPC then there is no requirement for fly leads on back boxes although it is good practice.

 

[tonys]

If your that concerned and want to get to the bottom of it, then break the ring down and test between each leg.

 

[nickblake]

and it is not necessary to fit cpc fly leads to knock-out boxes any more, but IMO it is good practice

Agree but only as long as the back box has one fixed lug , and also light switch back boxes need earthing

 

[malcolmsanford]

Agree but only as long as the back box has one fixed lug , and also light switch back boxes need earthing

Is that something new that's come out that as slipped by me without me noticing Nick?

 

[IQ Electrical]

Is that something new that's come out that as slipped by me without me noticing Nick?

Me too, Nick is quite correct on the '1 fixed lug' part but there is no distinction made between socket and lighting back boxes in BS7671:2008 or Guidance Note 8 (earthing and bonding).

 

[nickblake]

Knowing me im reading to deep into the regs but page 27 of the on site guide, the purpose of protective earthing is to ensure that in the event of a fault,line conductor to exposed conductive part sufficiant current flows to operate the protective device in the time required .as we all know. so correct me if im wrong , but there is no earth connection on light switches so when the face plate is screwed back with a solid lug on the back box then if the live wire of the switch breaks off or pops out as the do and touches an unearthed back box it wont blow the protective device but the face plate screws will be exposed and live ,so you can see my point .. unless its an earthed metal face plate then the same should apply as socket outlets because at the end of the day they are back boxes single or double

 

[Amp David]

Have they had a conservatory or extension built and the ring extended into it?

 

[topquark]

The boiler (downstairs in utility) was on the upstairs ring . Even after taking the bonding out of the equation there is still an extraneous connection (via the pipework itself I guess) giving a better than (I) expected R2.

All in all, was a useful exercise though. Was doing a check prior to changing the CU.

Is someone paying you to find why the r2 reading is so good?

I wish , no, this was for my elecsa inspection tomorrow, just me being a bit too picky

Last minute change of CU as the job I was going to use is going too slow (not due to me though ). Had to pay the LABC £141 for the privilege as well

 

[IQ Electrical]

Good luck for the assessment, let us know how it went!

 

[ringer]

... the purpose of protective earthing is to ensure that in the event of a fault,line conductor to exposed conductive part sufficiant current flows to operate the protective device in the time required...

I concur.
Reg 411.3.1.1 "Exposed-conductive-parts shall be connected to a protective conductor....."
couldn't be bothered to type it all in, but you can read the rest for yourselves