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Discuss Earthing an Outbuilding in the Electrical Forum area at ElectriciansForums.net

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This is my first posting here - I am not an electrician so please bear with me.
I am trying to establish the best method to provide earthing of a 60A supply to a new build garage / workshop to be taken from a domestic residence. The incoming supply to the house is TN-C-S with a supplementary earth rod. An isolator and Henley blocks have been installed at the head end ready to connect a 60A switched fuse for the garage sub-main cable which is a 14m length of 16mm2 3-core SWA cable laid underground between the buildings. An earth rod and pit has been installed adjacent to the garage in case it is required. Inside the garage is a 10-way RCD consumer unit. The Part P registered electrician who will be responsible for the inspection/testing is of the belief that the garage must have its own separate earthing system i.e. it needs to be TT whereas if compliance allows I would prefer to extend the equi-potential zone of the house and PME it. Hence work on the installation so far allows for either eventuality. There are no water, gas pipes or metal framing in the garage but there is a substantial steel door frame and vehicle lift which could be bonded. From what information I can find, there are instances where 'exporting' the earth is acceptable and others where TT earthing is a requirement. My difficulty is establishing the rules for compliance and where there is room for judgement based on circumstances. My main reason for wanting to extend the EPZ is that extension leads could be run from the house or the garage to anywhere around the buildings possibly at the same time - which with separate earthing seems dangerous to me. If PME of the garage is acceptable, the earth conductor in the sub-main is of adequate csa to serve as CPC and main bonding conductor. I would propose to also connect the new earth rod to the garage earth marshalling terminal as a supplementary earth for added protection. I would welcome the advice and opinions of professionals familiar with this type of installation.
 
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telectrix

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there's no reason not to use the existing PME. adding rods is a bonus to minimise danger if the supply Neutral is ever lost. tell your sparks to read the regs. and get on with it. you seem to have a better grasp than he does.
 

telectrix

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just wondering if the 16mm is overkill or future proofing.
 
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Thanks for the replies guys. My background is mechanical engineering but I like to read up on things outside my field which can be dangerous! From what I have read this is a widely debated issue and lots of sparks including mine have been taught separate building - separate earth without taking distance into account.
The sub-main rating calcs take into account one circuit will feed a summer house about 30m away. I also have a lot of machinery in the workshop which could be used simultaneously. 10mm2 cable would have just sufficed but I prefer to have a margin.
 

DPG

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It's nice when someone takes an interest in something that is outside of their normal field of expertise.
 

Wilko

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Hi - if you would like to read further on the subject may I suggest the IET publication Guidance Note 8? This has examples of how earthing systems work and covers this exact situation. You’ll then be able to explain to your Man why it can be ok :)
 

davesparks

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The Part P registered electrician who will be responsible for the inspection/testing is of the belief that the garage must have its own separate earthing system i.e. it needs to be TT whereas if compliance allows I would prefer to extend the equi-potential zone of the house and PME it.
You have a 16mm copper earth in the SWA which is larger than the required 10mm for main bonding so there is no reason not to use the PME earth.

Suggest to your electrician that they read guidance note 8 and learn the requirements for earthing and bonding instead of believing rumours and what their mate told them in the pub.
 
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Thanks for the information guys. I have ordered a copy of Guidance Note 8 so that we can both read up.
 
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