Add earth rod to existing house earth

Discussion in 'Electricians' Talk' started by jfostern, Jul 19, 2021.

  1. jfostern

    jfostern New Member

    I have a socket in my garden fed from house wiring via a swa cable, but only two core,the swa itself is used as the earth back to house socket.Unfortunately this earthing has too high a resistance for rcd,and I realise the swa cable should have been a three core,including an earth core.

    As it stands can I supplement the earthing by adding an earth rod near garden socket(paralleling up the earth rod with existing house system earthing) and would this be sufficient to give me a good earth loop test
  2. nigel willson

    nigel willson Screwfix Select

    No. Find out why the ELI is too high. Needs tested and inspection by sparks to find out why
  3. jfostern

    jfostern New Member

    I have carried out a test and know that the swa as the only earth return is not good enough and that by running a temporary copper earth cable between garden socket and house makes the earth loop acceptable!….do not want to do this in practice since quite a long exposed route for earth cable..reason for considering earth rod!
  4. nigel willson

    nigel willson Screwfix Select

    When you say not good enough? Numbers! Have you checked all joints, how is armour earthed?
  5. jfostern

    jfostern New Member

    Metal clad box in conservatory where armour is correctly glanded to metal clad box in garden also correctly glanded .In conservatory a plug and flexible cable connect to house/conservatory socket from box mention previously… garden is socket connected through garden box.
  6. Ind spark

    Ind spark Screwfix Select

    Using the swa as the earth is acceptable, maybe you have an incorrect fitted gland.
  7. jfostern

    jfostern New Member

    Using the swa as the earth is acceptable, maybe you have an incorrect fitted gland.

    Thanks,…I thought it was, but it hasn’t given me a good earth return…In addition to the glands I am going to try bonding the armour to the metal clad boxes, to see if this improves things!..

    Is it known that armouring alone should give a good earth return?
  8. nigel willson

    nigel willson Screwfix Select

    Use earthing nuts. Probably not going through the finish on the the boxes. What readings do you have!
  9. jfostern

    jfostern New Member

    Tried scraping earth location on boxes,….I am using an earth loop socket tester which does not give a value, but a green neon Indic.equal to less than 1.8ohm and ok for correct rcd operation,however my reading is an amber neon indicating 1.8 to 92 ohms, which suggests needs checking!
    My efforts are to try and get a green neon is the case for all house sockets!
  10. adgjl

    adgjl Active Member

    As others have said, you need an electrician with the proper test instruments. Depending on the earthing type, a value up to 200 ohms may be acceptable. You need the proper test kit to determine where the problem (if there is one) lies, with the experience to tell what earthing type you have.
  11. Tony Goddard

    Tony Goddard Screwfix Select

    What test equipment have you used and how did you test??, a multimeter won't do this test, you need either an Earth Loop tester or a multifinction tester with loop test facility

    (edit, just read above)
  12. Tony Goddard

    Tony Goddard Screwfix Select

    What earth system do you have, if it's TT you are within the acceptable range. Clearly a range of 1.8 to 92 isn't useful as a test, its as accurate as the latest SAGE predictions!!
  13. MGW

    MGW Screwfix Select

    Do not connect TT and TN systems together, I have seen the result, copper balls on the ground under where the earth wire was, the multiple earths in a PME system are fitted by the DNO not you.

    For the RCD to not work as said means an earth loop impedance of over 200 ohms, even then it would still likely work, but is considered as unreliable, to get that with a SWA means the cable has been damaged and the armour rusted through. I have as a temporary measure disconnected the SWA and used an earth rod and RCD, while waiting for a window when the concrete could be dug up and new cable laid.

    It is some debate as to if out buildings should be TN or TT when I wire my garage it will be TT so ready for EV charging.

    I have seen large resistors used to connect DNO earth to site earth, and only once, at a gas terminal where the earth was extremely good, 100's of earth rods used, it was in a box around 2 meters long, meter wide and meter tall, I am sure there was similar boxes on other sites, but that one at Point of Aye is only one I have seen, and never seen them used with domestic.
    Last edited: Jul 19, 2021
  14. The Happy Builder

    The Happy Builder Screwfix Select

    It would be a pretty impressive earth rod that gives you a green light on your tester.

    We could try getting really technical about this, but there is not really an issue with the amber light lighting up, because it’s RCD protected, presumably a 30 mA RCD?

    There could be other issues, but the amber light on the tester isn’t one.
  15. Bob Rathbone

    Bob Rathbone Screwfix Select

    To digress slightly, I did a test for Ze at my sister in law's house with an old Robin meter. the ones in the yellow plastic cases. The results were high, 7 or 8 Ohms on a TNC-S. I checked all connections and decided to test at the cut out, now Ze was the expected 0.3 Ohms. It was the smart meter causing the issue.
    As for our asker, if it is to be a non 'get an electrician' route, then I would suggest changing the outlet for one with an RCD and fitting an earth electrode, do not use the amour as earth. I do not know the supply arrangement, it may be TNC-S in which case the supply earth should not be used outside the zone protected by the equipotential bonding, ie outside the house.
  16. Ind spark

    Ind spark Screwfix Select

    Alright Bob.
    Do you fit an earth electrode with every outside socket if the supply is tncs ?
    Iv never seen it done.
  17. Bob Rathbone

    Bob Rathbone Screwfix Select

    I am being specific to this post and trying to see a way for the asker to solve the issue posed in the post.
    My view on this is that any provision for an electrical supply that is outside of the zone protected by the bonding should not have the earth associated with the bonding and the house installation exported to it as this will create a possible failure in the testing routine created by mixed disconnection times. Let me explain, in a house the maximum disconnection time for fixed equipment is 5 seconds if the wiring system is surface or protected by a metal sheath. All equipment outside the equipotential zone must disconnect in a much shorter time, 0.3 seconds or faster in certain conditions. This disconnection time cannot be achieved if a CPC from the 5 second zone is used in the 0.3 second zone as during a fault in the 5 second zone, the CPC and the class 1 equipment in the 0.3 second zone may remain live to touch for up to 5 seconds. That is clearly a failure.
    A further issue arises if the CNE conductor on the TNC supply is broken or becomes discontinuous, all of the neutral current and hence associated voltage will appear on any earthed metalwork of the installation. Inside the zone protected by the bonding this is not an issue, but at the transition from this zone to outside it and outside the zone, their is no protection against this voltage to 'real' earth. This is why on a TNC-S system, the CPC should not be exported outside the equipotental bonding zone.
    I do not propose a separate electrode for each socket, but rather a well tried and proven solution that follows the practice used with marinas, caravans and camp sites. The installation outside the zone protected by the euipotential bonding should be supplied from a CCU protected by a 30mA RCD with an associated earth electrode.
    The reason you do not see this done is that many electricians do not fully understand the implications of exporting the CPC.
  18. Ind spark

    Ind spark Screwfix Select

    Thanks for the explanation.

    How about if someone uses an indoor socket for their outdoor class 1 equipment.
    I know the electrician can't forsee everything but it could happen.
    So all outdoor sockets need an electrode really, wonder how many of them do.

    Edit: sorry I'm going off track this doesn't help the op.
    Last edited: Jul 20, 2021
  19. Comlec

    Comlec Screwfix Select

    No. What is required, when installing a circuit or an addition to a circuit, is a design that takes account all the information available. This will include earthing arrangements in the existing installation.
    So you can get the situation where a outside socket on a house wall is supplied from an internal RCD protected power circuit, but the shed 100m down at the bottom of the garden has a socket with an earth rod. This is why many folks engage the services of an electrician to design, install, test and certify new work or properly test existing installations.
  20. Ind spark

    Ind spark Screwfix Select

    I mean tncs.

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