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Discuss What should i do with this one? in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. egginyourface
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    egginyourface Electrician's Arms

    The other week i got a call from a little granny who had a leak in the loft which caused some of the lights to trip out and sockets etc.

    I went there and isolated the affected circuits. I said i would go back in a week after everythings dried out and recconnect.

    Any way, i though i would do a little zs on some of the sockets. Results i was getting was 2.19 ohms.

    So i did a Ze test at origin and found just over 2 ohms. Checked main incomer and found there was no earth coming from the head.

    We called eon and when they came round to have a look they told us that it wasnt there problem and it was up to the electrician to supply earth via earth rod.

    The property is not in a rural area its on a normal street.

    The board is 16th edition board, so was thinkng i would put a couple of rcbos in for lighting and sockets and stick an earth rod in.

    Is this ok, would i need to put an rcd before the fuse board??
     
  2. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    if you are providing a TT earthing arrangement, you need all circuits to be protected by 30mA RCD. easiest and cheapest way would be to replace main switch with RCD, but not compliant with 17th as regards splitting of circuits to minimise inconvenience.
     
  3. ash_s3
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    ash_s3 Electrician's Arms

    Do all circuits need RCD protection even if BS7671 maximum Zs readings are met?
     
  4. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    Location:
    cheshire/staffordshire
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    Telectrix
    doubt very much if you'll get acceptable Zs readings on a TT. bonding will bring them down, but you can't rely on bonding for disconnection.
     
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  5. SPARTYKUS
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    SPARTYKUS Trusted Advisor

    Location:
    Shropshire
    Egginyourface

    Where was the 2ohms coming from if you did a Ze and it wasnt coming from the service head?
     
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  6. ash_s3
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    ash_s3 Electrician's Arms

    Thanks, i was just wondering if you actually needed to RCD all circuits if max zs was met, obviously where cables are >50mm, socket outlets etc..
     
  7. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    did you disconnect the main earth to test Ze? or is that 2 ohms through the bonding?
     
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  8. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    In general, if the CU is plastic, and the tails are sheathed, then only the circuits fed from the CU require RCD protection.
    If the CU is metal, then a stand alone RCD will be required between the meter and the CU.
    You might be able to get away with using a plastic stuffing gland where the tails enter the CU.
    If the tails are not sheated, then they will require changing, including any that belong to the DNO.
     
  9. chronyx
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    chronyx Regular EF Member

    Location:
    Surrey, UK
    Where do the regs state that?

    (I am not being a pedant, I am genuinely unsure and interested!)
     
  10. DNS1
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    DNS1 Trusted Advisor

    Are all the other properties on the street on a TT system?

    What is the main earth at the moment? If it's not coming from the DNO, presumably it's a rod, but 2 ohms is pretty unrealistic for a TT isn't it?
     
  11. GLENNSPARK
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    GLENNSPARK Electrician's Arms

    Location:
    west yorks.
    is it a terrace?..or a semi?......as 2 ohms is way too low for a TT......just wondering if theres a shared supply here and theres a secret earth someware going to next doors service head.......
     
  12. GLENNSPARK
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    GLENNSPARK Electrician's Arms

    Location:
    west yorks.
    just because theres a cutout here doesn`t mean it doesn`t come from next doors cutout......
     
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  13. trev
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    trev Regular EF Member

    2 ohms too low for a TT?
    The best I've ever had was 0.29. Mind you I didn't just mess about with one twig sized rod, I went very very deep.
     
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  14. ipf
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    ipf Trusted Advisor

    Had a similar situation a couple of years ago. Contacted DNO and they were there within the hour. TN-C-S provided. Presumably there is evidence of an existing earth connection at origin ? If so and not TT, I would contact them again being more insistent.
     
  15. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    In general, there are two methods used to provide fault protection in domestic installations: ADS and double insulation.
    Section 411 provides information on ADS, Section 412 provides information on double insulation.
    There are other methods for providing fault protection, but they are not really practicable for dometic installlations.
    With a TT installation, ADS can be provided by use of an RCD, however unless the supplier provides such a device, there will always be a part of the installation (between the cut-oyt and your RCD) which will not be protected. That part can be protected by use of double/reinforced insulation.
     
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