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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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    davezawadi (David Stone):

    There is no problem having a TNC-S supply and an Earth rod, this is "protective multiple Earthing" in reality. If the Earth wiring in the property is not continuous, this is a problem as it is not permitted to have multiple Earthing systems in one installation. The alleged TT part will be very obvious by the provision of RCD protection at the DB, but a simple continuity test will immediately tell you what is happening. If two Earthing systems I would code C2 as it could be dangerous.


    Many old Norfolk installations were originally TT earthed. I have come across old earth rods buried under new extension foundations. The house has then been converted to P.M.E. but the rod left in by the builders.


    Most modern P.M.E. installations have a notice informing us of such by the main cut out.


    I have recently seen an old installation converted to P.M.E. but have left in two original earth electrodes as there is no harm in doing so, but every benefit in leaving them. The big house used to be two homes but is now just one after conversion.


    Main bonding will cover any differences in potential of conducting pipes etc. within an installation if required.


    Z.


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  • 0
    davezawadi (David Stone):

    There is no problem having a TNC-S supply and an Earth rod, this is "protective multiple Earthing" in reality. If the Earth wiring in the property is not continuous, this is a problem as it is not permitted to have multiple Earthing systems in one installation. The alleged TT part will be very obvious by the provision of RCD protection at the DB, but a simple continuity test will immediately tell you what is happening. If two Earthing systems I would code C2 as it could be dangerous.


    Many old Norfolk installations were originally TT earthed. I have come across old earth rods buried under new extension foundations. The house has then been converted to P.M.E. but the rod left in by the builders.


    Most modern P.M.E. installations have a notice informing us of such by the main cut out.


    I have recently seen an old installation converted to P.M.E. but have left in two original earth electrodes as there is no harm in doing so, but every benefit in leaving them. The big house used to be two homes but is now just one after conversion.


    Main bonding will cover any differences in potential of conducting pipes etc. within an installation if required.


    Z.


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