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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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    Thanks all for the responses so far, in some ways reassuring to see that the question has caused some debate and I am not asking stupid questions.

    Thinking about it I have only done a continuity test from the conduit to MET in the house so far. I have not carried out an actual loop test which may give me different results.

    As some additional information the buried section of conduit is only about 1.2m long, not sure how deep it is but it could be just near the surface of the concreate which would put it just above ground level.

    I think if I convert the garage to TT the biggest risk in an open PEN condition would be someone using a tool powered from the garage and then coming in to contact with the conduit which could be at the same voltage as the PEN.

    If I isolate the conduit from the house I get the same problem with someone using a tool powered from the house.

    Based on the current owner today I believe that either of the two above situations are more likely than an EV being plugged in to the garage.

    There is also the added complication of a gas storage tank mounted on the outside of the garage with the gas pipe bonded in the house. The gas pipe is non insulated and within reach of the conduit. Thinking about this element the conduit has to be at the same potential as the PEN and connected to the house MET.

    I think I am going to leave it as it is, if practical bond the conduit at the house end.

     

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  • 0

    Thanks all for the responses so far, in some ways reassuring to see that the question has caused some debate and I am not asking stupid questions.

    Thinking about it I have only done a continuity test from the conduit to MET in the house so far. I have not carried out an actual loop test which may give me different results.

    As some additional information the buried section of conduit is only about 1.2m long, not sure how deep it is but it could be just near the surface of the concreate which would put it just above ground level.

    I think if I convert the garage to TT the biggest risk in an open PEN condition would be someone using a tool powered from the garage and then coming in to contact with the conduit which could be at the same voltage as the PEN.

    If I isolate the conduit from the house I get the same problem with someone using a tool powered from the house.

    Based on the current owner today I believe that either of the two above situations are more likely than an EV being plugged in to the garage.

    There is also the added complication of a gas storage tank mounted on the outside of the garage with the gas pipe bonded in the house. The gas pipe is non insulated and within reach of the conduit. Thinking about this element the conduit has to be at the same potential as the PEN and connected to the house MET.

    I think I am going to leave it as it is, if practical bond the conduit at the house end.

     

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