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TN-C-S and TT in the same house?

jbrameld
I have across a property in Norfolk that looks like it has both TN-C-S and TT earthing.  I haven't fully investigated what is connected to what yet, but the incoming supply is on an overhead line, and there is an earthing conductor from the company head into the CU, so this is assumed TN-C-S.


When digging out for a new patio in the back garden we found a connected earth rod - just buried under the grass, no enclosure etc,  No obvious sign of that appearing at the CU either.


The property was extended around 20 years ago and the earth rod is just outside the extension at the back of the house


Incoming gas has a bond at the meter box, but I haven't yet traced if this is connected to the earth rod or to the CU.  Incoming water is also bonded but again I need to check where to.  The incoming water would have been moved from the original location when the property was built (1940s/50s at a guess) as it now appears in the rear extension.  I'm not convinced the incomer is metallic but I need to pull the washing machine out to check.  If not then no bond required anyway.


So accepting that more investigation is required, does anyone have any theories as to why we may have potentially 2 earthing systems in one house?  No powered outbuildings so no need for TT islands and I wouldn't expect an extension in the property to be done as an island.


Thanks,


Jason.
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    You do not want two different earthing systems overlapping in a way that brings two lots of exposed metalwork one on each system within reach of each other, so no nearer than about 2m.  Unless you have a car charger or a swimming pool, the easiest thing is to join it all together so you have PME with an auxiliary electrode, and not to worry too much.  It may already be like that. A quick check with a wander lead from the rod in a window and back to the CU will tell you.

    In many of our continental neighbours regulations, the provision of an electrode, either rod or a loop of buried tape in the foundations of a building is an essential condition of receiving a PME supply. The loop type electrode, or wires to two separated points of attachment to rebar or whatever, allows a ring-round test to verify the underground joints have not failed.

    Mike
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  • 0
    You do not want two different earthing systems overlapping in a way that brings two lots of exposed metalwork one on each system within reach of each other, so no nearer than about 2m.  Unless you have a car charger or a swimming pool, the easiest thing is to join it all together so you have PME with an auxiliary electrode, and not to worry too much.  It may already be like that. A quick check with a wander lead from the rod in a window and back to the CU will tell you.

    In many of our continental neighbours regulations, the provision of an electrode, either rod or a loop of buried tape in the foundations of a building is an essential condition of receiving a PME supply. The loop type electrode, or wires to two separated points of attachment to rebar or whatever, allows a ring-round test to verify the underground joints have not failed.

    Mike
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