Very odd setup....

[1john]

Had a small job today, just install an outside socket, when I went to price the job all looked good, split load CU, main bonding in place, lovely. Upon testing the CU seemed to be a fairly pro install, even numbering on the neutral and line conductors, although two sets of 1,2&3 as they had counted from #1 from the main switch and #1 on the rcd instead of continuing the numbering right through (hope that makes sense) although all the CPC's were bunched into about 4 terminals instead of spread out in corresponding terminals. Anywho, insulation resistance was good, RFC loop readings all good, r1r2 readings all good, Ze out... strange, had a reading of 53 odd ohms, very strange as all the properties in the area are TNS, on poking my head around the corner to where the main supply cable comes in I notice that the 16mm main earth cable doesnt go to the earth clamp on the main cable but goes down some trunking that is housing the main supply cable and then pops out through the wall to an earth rod. Someone has wired the installation to be compliant to the 16th ed but TT'd a TNS supply. With a test on the suppliers earth I get a good reading of 0.13 ohms.

Why oh why has someone done this? I cant for the life of me begin to understand.

 

[nelectric]

Has the property always been TNS? Perhaps it previously needed to be TT and the DNO upgraded their supply after the install.

 

[1john]

Always, all these supplies were put in at the same time, this are terrace houses in town center. the main earth from the supplier had been used on the previous installation, there were old cables that had been disconnected from the connection, notibaly one going to the incorrect side of the stop tap.

 

[nelectric]

Can only think then that there was previously some issue with the DNO's earth and previous installer just whacked a rod in to save the hassle of dealing with an unresponsive DNO. Have you asked the owner for certs/info about the last installation?
Not quite sure what you mean by wrong side of the stop tap - what stop tap?

 

[spark1]

only the Spark who installed it can answer your question, however, some Sparks actually prefer a TT installation as opposed to a TN. Reason being ,they have control over the continuity of the means of earthing and any loss of the earthing provision from the DNO [ which does happern ] will have no effect on the safety of the installation.

 

[Spark123]

Were all the ways RCD protected?

 

[1john]

Nah, it was a traditional split load, so lights and central heating on main switch, EVERYTHING else on RCD. The DNO had installed a main switch for the installation, all tails and cables in the harmonised colours too which makes it a fairly recent install. Aparantly it was a 'proper sparky' and she had 'some papers' although I am not conviced of either claim.

by stop tap, i mean the suppliers side of the water stop tap.