Why 10mm² for main bond cable ?

[phatboy]

Do the JEC still provide Economy 20 supplies?

Yes, and they now have E20Plus as well, that allows that supply to run 24/7, but paying peak prices for the non-E20 hours

 

[bernardgreen]

Where does your 10V figure come from?

I found at the last house that the voltage between the Neutral and True Ground would at times be fluctuating around 8 volts.

When wiring this cottage I was advised by DNO that there could be "significant" poterntial differences between my CPC ( new PME supply from the street ) and the CPC of the adjacent retail unit which is TT and from a different street cable. The advice was that depending on the impedance of the ground rod in the retail unit any connection between the two CPCs could carry high currents in an unfused circuit. Hence it was necessary to ensure ther was no possibility of the CPCs coming in contact either directly or via earthed or bonded metal pipe work.

( Back ground ) The cottage and the retail unit were until 2011 owned by the same person and all the electric came from the TT supply to the retail unit. I had a new electrical supply laid in to the cottage. The water supply to the retail unit is still from my meter and an isolating section of plastic pipe has to be maintained between the cottage plumbing and the retail unit plumbing.

 

[JohnW2]

I found at the last house that the voltage between the Neutral and True Ground would at times be fluctuating around 8 volts.

That sounds very credible. It presumably depends upon (amongst other things) the number, location and impedance of the "ME"s of PME (westie used to tell us that "M" was often only 2, with one earth at the tranny and the other at the far end of the main) and, indeed, the same for bonded extraneous-c-ps. In a 'fully stretched' distribution circuit, with about 250V at the tranny and about 220 at the farthest consumer (during peak load), the VD along the N from transformer to farthest consumer would presumably be about 15V (but estimates are complicated because of the 'M').

However, as I wrote in post #10, are you not overlooking the impedance of the earth rod at the tranny? You said that 10V relative to true earth would result in a current of 100A flowing into an extraneous-c-p with an impedance of 0.1Ω. However, as I wrote, does not that current also have to flow through the resistance of the earth rod at the tranny before it gets back to the N of the tranny? If so, and if (as we have been told in the past) that rod may have an impedance of 'several ohms', then the current resulting from that 10V might be well under 10A, rather than 100A, mightn't it?

Kind Regards, John

 

[bernardgreen]

and if (as we have been told in the past) that rod may have an impedance of 'several ohms',

The target value for the effective resistance at a substation supplying the LV networks is less than 1 ohm. This may involve several rods and in some locations "deep drilled" rods have to be installed

 

[JohnW2]

The target value for the effective resistance at a substation supplying the LV networks is less than 1 ohm. This may involve several rods and in some locations "deep drilled" rods have to be installed

That may be the 'target' but westie seemed to indicate that such was often not the case, and I seem to recall that he said that <10Ω was 'acceptable'.

In any event, even if it were around 1Ω (people rarely try too hard to appreciably exceed 'targets'!), the possible current (with a 10V PD) would be around 10A, not 100A, regardless of how low was the impedance (to true earth) of the bonded extraneous-c-p

Kind Regards, John