Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 5 replies
  • Subscribers 32 subscribers
  • Views 3434 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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.
  • 0
    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.
  • 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.


  • 0
    Thanks for the replies.  


    I'll do a bit more testing/investigating on the bonding to see what connects to what and make sure that I don't have 2 different systems and otherwise not worry about it too much.  No notices at the incomer.  There is a 30mA RCD in a separate enclosure to the old-style Wylex re-wireable fuse CU which has at least been upgraded to use the plug-in MCBs.


    Difficult to be sure, but the earth rod is 'conveniently' in a corner between the side and rear extension, and has a curly earth cable that seems to go through a hole in the extension wall, so probably put in when the extensions were done.  May be fun if it is to be retained when at a future date the location where it is sited gets built over as part of further development....


    Jason.


  • 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
  • 0
    jbrameld:

    Thanks for the replies.  


    I'll do a bit more testing/investigating on the bonding to see what connects to what and make sure that I don't have 2 different systems and otherwise not worry about it too much.  No notices at the incomer.  There is a 30mA RCD in a separate enclosure to the old-style Wylex re-wireable fuse CU which has at least been upgraded to use the plug-in MCBs.


    Difficult to be sure, but the earth rod is 'conveniently' in a corner between the side and rear extension, and has a curly earth cable that seems to go through a hole in the extension wall, so probably put in when the extensions were done.  May be fun if it is to be retained when at a future date the location where it is sited gets built over as part of further development....


    Jason.


     


    So, as it is, just one single earth fault on just one final circuit will trip off everything. Not perhaps dangerous, but not reliable. You wouldn't want to be plunged into darkness if in the bath or shower, or half way down the stairs,  if an appliance like a washing machine become faulty and trips off the lot.


    Z.


Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines