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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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    AJJewsbury: 
     

    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.

    Not according to Electropedia (IEC 60050): definition of ‘live’ 151-15-60 and definition of live 581-21-22

    live adj qualifies a conductive part intended to be energized in normal operation

    c.f. definition of energized

    However, just to throw a spanner in the works … PE and PB conductors may carry protective conductor current (intended current flow) in normal operation, or even be used for other functional purposes (e.g. EV Pilot function) … which is of course very different to “leakage current” which is an “unwanted or unintended current” resulting for example from capacitive effect or as a result of the [perhaps changing] properties of insulation.

    So, is PE now ‘live’ ?

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    Not by convention Graham ?

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    live adj qualifies a conductive part intended to be energized in normal operation

    so how's that squared with the definition of an exposed-conductive-part which isn't (traditionally) energised in normal operation but would “become live under fault conditions” - perhaps it would be better if they'd said “become energised under fault conditions”…

       - 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.

    When I first read your post, I agreed with your reasoning i.e. the part first becomes live, then the ADS kicks in.

    However, this definition from 60598-1 takes a different view (assuming the pdf on the internet is correct. I don't have access to check the pukka copy at the moment. )

    3b75eda4f77b5b4d5b9275415ae91c12-original-class-1.png

    But, for context, the standard does have its own definition of live part, which effectively means live in normal operation - so ignoring short term (hopefully) faults.

    1abf153c496d7c5772ba95d4363ad137-original-live-part.png

     (Annex A basically deems a part live if ≥0.7mA flows through 2kohm to earth)

    So, whether the part becomes live or not depends upon what the definition of live is.

    In a standard such as 61010-1 it would refer to not becoming ‘hazardous live’. i.e. it may become live, provided the combination of voltage and duration (and available current, frequency etc) ensure it isn't 'hazardous live'.

     

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    AJJewsbury: 
     

    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

    That's no good at all. A definition is required. What may be obvious to you may not be the same “obvious” to somebody else. Science needs precise definitions. What would happen if we were left to producing our own personal definitions in science and engineering. Chaos.

    To what level must something get to be defined as being electrically live? Is a 12 Volt car battery terminal live? Is an E.L.V. conductor live? Is a metal fence post live? I think we should be told.

    I have only just seen 2400's post above which is very helpful in this matter. His reference seems to be talking about the Voltage of an AA size primary cell upwards.

     

    Z.

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    That's no good at all. A definition is required.

    Unfortunately BS 7671 doesn't give us one - so we have to use our initiative … and take account of the wording of the various definitions we do have. (Also keep in mind that such definition we do have are written by failable humans on committees…)

       - Andy.

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