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Earthing for detached garage

Alan B

Looking for some advice on what is required for earthing arrangements on a detached garage. I am in two minds about the need to convert the garage to a TT island.

House supply is TNS, but I probably have to assume it may be converted to TNCS in the future.

The garage is about 1.5m from the house, it has 2 double sockets and an LED light installed.

Feed to the garage is via steel conduit running down the house, through a concrete path and then up and along the outside of the garage. Then through the wall to a steel junction box and then plastic conduit to metal clad fittings.

If earth connection is disconnected from the steel conduit, testing with a continuity tester shows it to be isolated from earth, haven't tried an insulation test so far. There are no other services entering the garage and it's brick built without a steel frame.

At the moment I have the steel conduit connected to the cpc of a 2.5mm T+E cable and then single core 2.5mm earth cable continuing the earth from the steel junction box to the garage installation.

If an EV was connected to a garage socket for charging in the future it would have to be via a 15m cable. I think it would be far more likely that a EV charger would be put on the outside of the house close to where the car will be parked.

So can I say that as :-

The steel conduit is isolated from earth and therefore not an extraneous conductive part, no other extraneous conductive parts.

Sockets unlikely to be used for EV charging

Therefore OK to use TNS earth from the house without bringing 10mm earth bonding cable from the house???

Alternatively 

I could isolate the garage and put a an earth rod in

I could probably get a 10mm earth bonding to the conduit at the house side but getting the cable to the garage would require replacing the conduit which will be quite expensive for the customer on a job that's gone way over budget already due to a multitude of problems hidden under loft insulation and inside walls.

 

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    With such a short distance between the two buildings, my initial thinking is that it's best for them to share the same earthing arrangements.

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    I would suggest that a bare steel conduit running in an outdoor concrete path must be an extraneous conductive part. How could it not be?

    The conduit can't be an extraneous-conductive-part - as it's part of the electrical installation, and the definition explicity excludes things that are “part of the electrical installation".

    You do raise a good point though - there comes a point where an exposed-conductive-part (or many such parts connected together) might have such good contact with the Earth that you'd want to connect it to the MET using something closer to the size of a main bonding conductor than a flimsy c.p.c. - to cope with diverted N currents if nothing else. That seems to be a bit of a hole in BS 7671 (almost the reverse of the one that requires us to use a full sized bonding conductor on some trivial extraneous-conductive-part that never going to have a resistance less than hundreds of Ohms to Earth).

     

    As an aside, the buried section of the steel conduit might have been treated in some way to better preserve it from corrosion - wrapping with Denso tape perhaps - which might co-incidentally have reduced its resistance to Earth quite significantly.

        - Andy.

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    Thanks all for the usefull input. I will leave things as they are. I have learnt a lot through this discussion and thinking about the issues involved.

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    AJJewsbury: 
     

    I would suggest that a bare steel conduit running in an outdoor concrete path must be an extraneous conductive part. How could it not be?

    The conduit can't be an extraneous-conductive-part - as it's part of the electrical installation, and the definition explicity excludes things that are “part of the electrical installation".

    You do raise a good point though - there comes a point where an exposed-conductive-part (or many such parts connected together) might have such good contact with the Earth that you'd want to connect it to the MET using something closer to the size of a main bonding conductor than a flimsy c.p.c. - to cope with diverted N currents if nothing else. That seems to be a bit of a hole in BS 7671 (almost the reverse of the one that requires us to use a full sized bonding conductor on some trivial extraneous-conductive-part that never going to have a resistance less than hundreds of Ohms to Earth).

     

    As an aside, the buried section of the steel conduit might have been treated in some way to better preserve it from corrosion - wrapping with Denso tape perhaps - which might co-incidentally have reduced its resistance to Earth quite significantly.

        - Andy.

    My previous post:

    "And if it runs in an outdoor concrete path an extraneous-conductive-part as well. Oh ek. It is only half of an extraneous-conductive-part as it may introduce a potential into an installation, normally an earth potential, but according to the definitions it is part of the electrical installation so is not fully an extraneous-conductive-part. “……not forming part of the electrical installation.”

    But having said that, an exposed conductive part “can be touched and which is not normally live, but can become live under fault conditions”. But the steel conduit can't become live as it is earthed and a fault will cause the supply to be swiftly disconnected thus rendering it safe."

    Z.

     

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    But the steel conduit can't become live as it is earthed and a fault will cause the supply to be swiftly disconnected thus rendering it safe.

    I think you've got the head on the wrong end of the pantomime horse there … ADS works by earthed exposed-conductive-parts becoming made live by a fault - if it wasn't there's be no earth fault current and nothing to trigger the protective device to open. 

    Even if it only becomes live for a tiny fraction of a second (or anything up to 5s) it still becomes live - the definition doesn't depend on it remaining live indefinitely.

       - Andy.

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    AJJewsbury: 
     

    But the steel conduit can't become live as it is earthed and a fault will cause the supply to be swiftly disconnected thus rendering it safe.

    I think you've got the head on the wrong end of the pantomime horse there … ADS works by earthed exposed-conductive-parts becoming made live by a fault - if it wasn't there's be no earth fault current and nothing to trigger the protective device to open. 

    Even if it only becomes live for a tiny fraction of a second (or anything up to 5s) it still becomes live - the definition doesn't depend on it remaining live indefinitely.

       - Andy.

    So Andy, just what is the true definition of “live?”

    Thanks,

    Z.

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    So Andy, just what is the true definition of “live?”

    BS 7671 doesn't say - so it must be obvious to the likes of us.

    It was the definition of an exposed-conductive-part I was referring to.

       - Andy.

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    “Live” means normally a current carrier of part of its intended function . So Line Load Phase & N all qualify Except CNE (by convention). I would say

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    ebee: 
     

    “Live” means normally a current carrier of part of its intended function . So Line Load Phase & N all qualify Except CNE (by convention). I would say

    That's a “live part” - which an exposed-conductive-part wouldn't be … but the definition of an exposed-conductive-part says it can “become live under fault conditions” - so there must be a slightly more subtle meaning of the word “live” alone. 

        - Andy.

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    Ha right I see what you mean AJJ

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