Posted by: @jamespa

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The issue of whether they should anyway be upgraded is separate, the concerning thing (if indeed they do need to be upgraded) is that this wouldn't have arisen had I not been considering islanding, and seriously there must be hundreds of thousands+ of houses with this arrangement.

Indeed. There are thousands of houses which have substandard electricity supplies!

Fault designations for electricity supplies to premises are covered on the MOCOPA Guide, as used by Meter Operators.

I suggest you download and keep a copy of that illustrated MOCOPA Guide, which can be understood by anyone dong a visual inspection.

The later/current version issued by the Retail Energy Code Company (RECC) has ceased to be a useful reference tool
and is now a set of written rules describing who is at fault when something isn't done correctly.

Posted by: @transparent

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Whether the outer metallic braid of the mains incomer is covered by an insulated layer (Polyethylene or Bitumen), it remains the case that the actual location of the earth-tie is at the substation transformer.

Thanks.  I know and can see that the tie from earth to neutral is at the local substation transformer.   

However its not this that matters for safety, its the tie from the customer protective earth to actual earth.  The point at which the customer earth is connected to actual earth will surely depend on the insulation of the incomer.  If the incomer is insulated throughout its entire length (including any joints) and therefore the tie is is also at the substation transformer but the fault current isnt via the neutral, what is to prevent a situation (when the grid is connected) where the metallic braid has an interruption (eg at a joint) and thus the supply, but not the earth, remains connected?  You would have no way of knowing this.  Obviously this wont happen if the cable is cut, but if it corrodes or something disrupts continuity at a joint - what then?

And also in this case what is the material difference between TN-S and TN-C-S and is either actually ever satisfactory?  Something still isn't quite making sense.

I dont disagree, btw, with your comments on the weakness of the current arrangement, (but both are easily fixed), however I have now developed an interest in getting to the bottom of understanding this.  Dont feel obliged to answer!

Posted by: @transparent

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Posted by: @batpred

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I also found the reason for the multiple categorisations for our earthing arrangement. It is TN-S, but the head was upgraded as in TN-C-S, causing the confusion.

Please define what you're referring to as "the head".

When you used it previously I thought you were referring to the test point on the top protruding section of an earth rod.
But now I think you mean the Service Fuse - which is often called the Cut Out by DNO Engineers.

I used the term head like I heard the electricians use it. A year ago, I had some issue (some loose contact that was causing it to heat). I had to call the DNO myself and the sparky I had called (that I subsequently found out is not registered) told me to describe it as the head (referring to the fuse block). 

I found what I hope is a useful summary of the regulation requirement for earthing arrangements in the website of an NICEIC registered electrical company. Unfortunately both EICR reports I had done last year lack these measurements. I reproduce them below. I plan to have it measured, but I believe the capacity of the DNO conductor to the supply point would also need to be considered? 

External Earth Loop Impedance (Ze)

Regulation 313.1 requires a number of characteristics of the supply to an installation to be determined, including the nominal voltage to earth (U₀), the earth loop impedance of that part of the system external to the installation (Ze), and the prospective short-circuit current at the origin of the installation.

The value of external earth loop impedance (Ze) measured or otherwise determined in accordance with Regulation 313.1 may differ from the applicable typical maximum value declared by the electricity distributor, which is usually:

• 0.8 Ω for TN-S system
• 0.35 Ω for a TN-C-S system
• 21 Ω plus the resistance of the installation earth electrode for a TT system.

On closer observation, I had missed that the latest EICR I had from December is showing a Ze of 0.17 Ω, so the installation is compliant. 🙂 

Still, would the capacity of the DNO conductor to the supply point not need to be able to take 36.4Amps (8Kw) for safety?

Posted by: @jamespa

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what is to prevent a situation (when the grid is connected) where the metallic braid has an interruption (eg at a joint) and thus the supply, but not the earth, remains connected?  You would have no way of knowing this.  Obviously this wont happen if the cable is cut, but if it corrodes or something disrupts continuity at a joint - what then?

Then you've just discovered the reason why Electricians put stickers on Distribution Boards which recommend a check every 10 years!

It all comes down to how your single-phase supply cable has its outer braided earth connected to the earth of the 3-phase cable 'in the road'.
If your electrician notices a marked increase in earth resistance since your previous test, then he'll notify the DNO.

They will send out a team to dig up the highway and inspect that joint.

There might've been a 'standard' brass connector which was used for connecting braided earths on cables of different diameters.
After all, we have such connectors for SWA cables today.

I don't know if TN-S supplies are installed any more.

The new-build estates all seem to have TN-C-S, which uses the Neutral to provide the earth.