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Cable size between equipotential earth bonding bar and distribution board in a Group 1 medical location

Simon Bourke

The IET regulations require that the resistance of the conductors, including the resistance of the connections, between the terminals for the protective conductor of socket-outlets and of fixed equipment or any extraneous-conductive-parts and the equipotential bonding busbar (EBB) shall not exceed 0.2 Ω.

However the cable connection between the EBB and the Main Distribution board, is not defined, (identified in red in the image below) - either in terms of:

 1 - maximum resistance 

2 - minimum cable size 

3 - if the cable needs to connect to the distribution board that serves the room or should go back  to the Main distribution board.

4 - if there are number of EBB's can they be connected by a single cable in a daisy chain arrangement back to the distribution board.

Is any able to provide guidance on the four questions above?

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    The 0.2 Ω limit applies to all protective conductor connections to the EBB. Between the EBB and the main board, the cable should be sized for fault current (typically ≥6 mm² copper) and can either connect directly to the main board or via local distribution, as long as total resistance stays ≤0.2 Ω. Daisy-chaining multiple EBBs is acceptable if this criterion is met. Always verify with post-installation testing (BS 7671).

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  • 0

    The 0.2 Ω limit applies to all protective conductor connections to the EBB. Between the EBB and the main board, the cable should be sized for fault current (typically ≥6 mm² copper) and can either connect directly to the main board or via local distribution, as long as total resistance stays ≤0.2 Ω. Daisy-chaining multiple EBBs is acceptable if this criterion is met. Always verify with post-installation testing (BS 7671).

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