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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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    BS7671 114.1 states that for a supply given in accordance with ESQCR it shall be deemed that the connection with Earth of the neutral of the supply is permanent.

    BS7671 Section 722 and Annex A722 does seem to contradict that statement, and the EVSE COP guides us to taking an Open-PEN fault into consideration with supplementary bonding being additional protection that we may consider installing in some circumstances.

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    BS7671 114.1 states that for a supply given in accordance with ESQCR it shall be deemed that the connection with Earth of the neutral of the supply is permanent.

    At the transformer. Possibly elsewhere too. ESQCR forbids the DNO to disconnect neutral from earth.

  • 0 in reply to Sparkingchip
    Sparkingchip said:

    S7671 114.1 states that for a supply given in accordance with ESQCR it shall be deemed that the connection with Earth of the neutral of the supply is permanent.

    BS7671 Section 722 and Annex A722 does seem to contradict that statement, and the EVSE COP guides us to taking an Open-PEN fault into consideration with supplementary bonding being additional protection that we may consider installing in some circumstances.

    That's one way of looking at things.

    The other way is that a PEN conductor is also a neutral conductor, and it's the broken neutral element that causes the EPR, not the lack of connection with Earth of the neutral at the premises. There may well still be multiple earths in place between the intake and the point of fault.

  • 0 in reply to Chris Pearson
    Chris Pearson said:
    At the transformer. Possibly elsewhere too. ESQCR forbids the DNO to disconnect neutral from earth.

    And I believe also to maintain the distribution network to prevent, so far as reasonably practicable, danger?

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