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3200A Bonding Conductors

Tom OB

Hi All,

Currently working on an early stage design for a project with a 3200A main service from a 2MVA substation. Main cable is 8x 4x 630mm2. Earthing is TNS.

Following the rule of thumb, the earthing conductor (half line conductor) would be 4x 4x 630mm2 and the main bonding (half earthing conductor) would be 2x 4x 630mm2.

We have multiple earth bars in switchrooms, plant rooms and comms rooms across the project. It would be unrealistic to run this much copper to each item bonded to each earth bar, but BS7671 doesn't explain the correct method to size bonding in a more complex installation.

IET GN8 table 5.1 states main bonding for a 70mm2 - 400mm2 conductor can be 25mm2 for a TNS system. Does this rule continue to apply as the line conductor increases? Is a blanket size of 25mm2 across the project sufficient?

All submains have their own correctly sized CPCs, this is purely for bonding to structure, mechanical services, lightning protection, comms equipment etc.

Any help much appreciated.

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    Indeed - but seeing the use of the phrase "It would be unrealistic to run this much copper to each item bonded to each earth bar, " is not at all clear how much current each of those items connected to the bar may be carrying under a fault or diverted current condition.  A supplementary bond to a ladder frame on a masonry wall with a light on it that has a circuit fused of 5A, presumably not a lot, and less still to any random pole in the ground with no electrics, - and that (correctly) is your thinking. It rather depends what is being bonded though - bonding to something metallic  supporting  a 1kA sub main, I'd be less confident.
    Only a few things will be like this, and the rest can have much thinner bonding - perhaps I could have been clearer. And in some peoples minds in places with supplies from two transformers the link between them is also a bond, not a CPC. (though some of us prefer one to be TT.)


    As an aside,  15mm copper pipe is about 35mm2 copper wire equivalent in terms of resistance and a lot better cooled, but again yes, for things like thin oil pipes, the pipe itself may be the fusible element.

    M.

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  • 0

    Indeed - but seeing the use of the phrase "It would be unrealistic to run this much copper to each item bonded to each earth bar, " is not at all clear how much current each of those items connected to the bar may be carrying under a fault or diverted current condition.  A supplementary bond to a ladder frame on a masonry wall with a light on it that has a circuit fused of 5A, presumably not a lot, and less still to any random pole in the ground with no electrics, - and that (correctly) is your thinking. It rather depends what is being bonded though - bonding to something metallic  supporting  a 1kA sub main, I'd be less confident.
    Only a few things will be like this, and the rest can have much thinner bonding - perhaps I could have been clearer. And in some peoples minds in places with supplies from two transformers the link between them is also a bond, not a CPC. (though some of us prefer one to be TT.)


    As an aside,  15mm copper pipe is about 35mm2 copper wire equivalent in terms of resistance and a lot better cooled, but again yes, for things like thin oil pipes, the pipe itself may be the fusible element.

    M.

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