Earth continuity

[Guest77]

Another possibility, is that there is an RCD socket or perhaps an item of equipment connected which is skewing the results.

or dodgy connections

 

[happysteve]

Out of curiousity what is the formula for calculating length of run? Cheers

Table I1 in the On-Site Guide (p190 in the yellow one) gives resistance per metre (for a single conductor, e.g. end-to-end on a ring) and (R1+R2)/metre (if using method 1 for continuity of protective conductors).

The row that says "1.5" then a "-" is the resistance per metre of your cpc: 12.1mΩ
The row that says "2.5" then a "-" is the resistance per metre of your live conductors: 7.41mΩ

To work out the theoretical length of the cable (based only on measured resistance) you take your measured resistance and divide by one of those numbers. You can either convert your measured resistance from Ω to mΩ, or convert the above numbers from mΩ to Ω, or just use both numbers "as is" and multiply the answer by 1000.

So: ( 0.8 / 7.41) x 1000 = 108m (for your L and N end-to-end),
and: ( 0.6 / 12.1) x 1000 = 49.6m (for your cpc)

If 108m seems a bit long for your RFC, then a couple of poor connections on your live conductors is more likely.

If 50m seems a bit short for your RFC, then a parallel path on your cpc is more likely. This might be supplementary bonding, two water heaters / boiler connection, or even something bizarre like a switched FCU that forms a link between the two legs of the RFC about half way round when switched on (but not when switched off), and will always form a link for the cpc (I have seen a level 2 student terminate a S-FCU like this, bless!).

Handy reference for resistance/m:

1.0mm: 18.1 mΩ/m
1.5mm: 12.1 mΩ/m
2.5mm: 7.41 mΩ/m
4.0mm: 4.61 mΩ/m
6.0mm: 3.08 mΩ/m
10mm: 1.83 mΩ/m <--- main equipotential bonding conductors (usually)
16mm: 1.15 mΩ/m <--- earthing conductor (often)
25mm: 0.727 mΩ/m

Handy reference for (R1+R2)/m

1.0+1.0mm: 36.2 mΩ/m <--- older lighting radials (and sometimes strappers, e.g. with 3x1+1mm)
1.5+1.0mm: 30.2 mΩ/m <--- standard lighting radials
2.5+1.5mm: 19.51 mΩ/m <--- standard socket radial (protected by 20A OCPD)
4.0+1.5mm: 16.71 mΩ/m <--- standard socket radial (protected by 30A or 32A OCPD)
6.0+2.5mm: 10.49 mΩ/m
10+4.0mm: 6.44 mΩ/m

 

[captainkev]

OP says #21 that it is a new property, so we assume new wiring, so likely also new (possibly cheap) sockets which could be giving poor connection on L & N

 

[Risteard]

OP says #21 that it is a new property, so we assume new wiring, so likely also new (possibly cheap) sockets which could be giving poor connection on L & N

I thought his reading was based on the end-to-end resistances, which wouldn't be affected by the socket contacts. Presumably the actual terminal screws are tight in the socket outlet.

 

[rattlehead85]

I thought his reading was based on the end-to-end resistances, which wouldn't be affected by the socket contacts. Presumably the actual terminal screws are tight in the socket outlet.

I think he means that the terminals on the sockets may be of poor quality i.e don't clamp down well. That Would most definitely affect end to end results, however, to have both line and neutral readings exactly the same rules that out bar an absolute freak circumstance.

 

[Hellmooth]

I thought his reading was based on the end-to-end resistances, which wouldn't be affected by the socket contacts. Presumably the actual terminal screws are tight in the socket outlet.

End to end resistances wouldn't be affected by the socket contacts, am I missing something?

Ah right I've got you now, you mean the actual contacts and not the terminal screws, will pay more attention next time!

 

[cprfenom]

If the building has block work and the sockets have a fly lead it is possible that you are getting parallel path via the building?