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Bonding

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My house was rewired for me 20 years ago by an NICEIC registered firm of electricians. Concurrently a new BS7870PVC concentric cable was run down a pole on my land from overhead cables, then underground and finally up a hockey stick into a DNO head in a new external meter cabinet. Separately a 10mm cable from a new earth rod was run up an adjacent tube, to a MET in the same cabinet.  A conventional TT system, I presume. 

 

The (then) new internal CU (less than 2m from this cabinet) was fitted with 2 x 30ma RCDs between them feeding 14 circuits. One of these (protected by a 16a MCB) goes to a detached fuel store (copper tube connects a steel oil tank to the central heating boiler within the house). Another (also protected by a 16a MCB) goes to a detached now metal framed greenhouse and adjacent twin garage. Sub distribution boards in each outbuilding are fused down to 15a radials to every twin socket and to 5a for lighting. There is no EV charger yet. 

 

2.5mm 3core SWA cables connect the main CU to the sub distribution boards.

 

Does the armour in the SWA qualify as 10mm bonding ?

 

Should separate earth spikes be driven in, near the sub boards in each outbuilding?  If so, how should the (3 core) SWA best be terminated at origin (CU) and at each outbuilding?  

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    What is the impedance of the earth rod you already have ? It may be enough on its own. Also the wire to the electrodes can be quite a lot thinner than the typical TN-x woud require - the rod resistance to ground means that the current is limited and even 4mm would be ok for those connections. 

    It is perfectly OK to have one large TT system with interconnected CPCs and several electrodes in many places, but equally islands each with an RCD and electrode fed by 2 core supply is also OK 

    In general the armour of a 10mm SWA can be taken as comparable to the copper cross section you will need in most cases. 

    There is somewhere in circulation a document that describes the cross sections better - if no one else posts it in a few minutes, I'll try and link a copy

    Mike

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  • 0

    What is the impedance of the earth rod you already have ? It may be enough on its own. Also the wire to the electrodes can be quite a lot thinner than the typical TN-x woud require - the rod resistance to ground means that the current is limited and even 4mm would be ok for those connections. 

    It is perfectly OK to have one large TT system with interconnected CPCs and several electrodes in many places, but equally islands each with an RCD and electrode fed by 2 core supply is also OK 

    In general the armour of a 10mm SWA can be taken as comparable to the copper cross section you will need in most cases. 

    There is somewhere in circulation a document that describes the cross sections better - if no one else posts it in a few minutes, I'll try and link a copy

    Mike

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