O PEN Devices and Supplies

The big problem with our network is the mishmash of earthing, you can go down a terrace and find properties still wired to the 15th edition right next to a recent rewire with a combination of all three earthing styles at different properties every 3 to 4 meters all the way down the street, in this situation is TT really TT if they have bonding on a gas pipe shared with TNCS gas pipe next door?

I could not agree more.  But the mish mash underground is not fixed either - round our way they have been breaking up the old cast iron gas mains as they have inserted cheerful yellow plastic, so now that bond to the gas pipe is not sharing with next door in a solid metallic sort of way, but more like a horizontal electrode from the house to the first cracked pipe in the street where they made a gap so the yellow pipes could be fusion welded, so now about ten ohms loop instead of previously something too low to measure..
In the same way the addition of a water meter has added an impromptu break of about a foot of plastic in the water pipes, so the scope for parallel support to a dicky neutral with the sub-ohm metal services is not it once was.
The electrode fields of influence certainly overlap at the level of tens of ohms, so any attempt at 'TT' is certainly fault current limited alright, but not exactly pure like the text book diagrams of electrodes in isolation and circular voltage contours radiating outwards  either.

It may be simpler on new estates with all plastic services and pure PME, but give it a few years of hacks and changes and it won't be, and there may be scope for a few nasty accidents when things like water meters are replaced .

Mike.

The big problem with our network is the mishmash of earthing, you can go down a terrace and find properties still wired to the 15th edition right next to a recent rewire with a combination of all three earthing styles at different properties every 3 to 4 meters all the way down the street, in this situation is TT really TT if they have bonding on a gas pipe shared with TNCS gas pipe next door?

I could not agree more.  But the mish mash underground is not fixed either - round our way they have been breaking up the old cast iron gas mains as they have inserted cheerful yellow plastic, so now that bond to the gas pipe is not sharing with next door in a solid metallic sort of way, but more like a horizontal electrode from the house to the first cracked pipe in the street where they made a gap so the yellow pipes could be fusion welded, so now about ten ohms loop instead of previously something too low to measure..
In the same way the addition of a water meter has added an impromptu break of about a foot of plastic in the water pipes, so the scope for parallel support to a dicky neutral with the sub-ohm metal services is not it once was.
The electrode fields of influence certainly overlap at the level of tens of ohms, so any attempt at 'TT' is certainly fault current limited alright, but not exactly pure like the text book diagrams of electrodes in isolation and circular voltage contours radiating outwards  either.

It may be simpler on new estates with all plastic services and pure PME, but give it a few years of hacks and changes and it won't be, and there may be scope for a few nasty accidents when things like water meters are replaced .

Mike.

but give it a few years of hacks and changes and it won't be, and there may be scope for a few nasty accidents when things like water meters are replaced .

I’ve got a very thorough plumber, his apprentice once asked him why he uses a set of crock clips to bridge the gap before removing a section of the water pipes. Obviously the lad was confused as to how this helps prevent electric shock if there’s an earth fault. “Go and ask the sparks” I drew him a little picture with a very unhappy plumber holding onto one piece of pipe with one hand and the other piece with the other during fault conditions! I hope he remembers that in years to come when he’s feeling lazy about going to the van for his temporary equipotential bonding clamps.

perhaps he should carry a clamp meter. Still, a jump lead is a good idea working on an installation of unknown condition - which it probably most. I suspect though such caution is the exception, rather than the rule.

Mind you fast forwarding to about 60 seconds into this video after the long talking intro about chat rooms for gas engineers , that image should hammer the point home to anyone who doubts.  Would you wish to undo that main bond without gloves ?

(a clearer demonstration of PME arcing problems I have not seen . An AFDD or indeed switching off at the main will probably not help a fat lot here either.)

Mike.