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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?

Parents
  • 0

    I have a particular scenario in mind in which there is a complete failure of the Protective Measure- Automatic Disconnection of Supply (ADS) due to circuit protective devices both MCBs and RCDs, as well as fuses and RCBOs, being completely ineffective, because of a failure of the earthing system, which results in there being a recommendation to install supplementary bonding to provide additional protection.

    Edit- I have just checked and as I thought, Graham is the lead author of the book I am looking at and I’m sure he knows where this is going.

Reply
  • 0

    I have a particular scenario in mind in which there is a complete failure of the Protective Measure- Automatic Disconnection of Supply (ADS) due to circuit protective devices both MCBs and RCDs, as well as fuses and RCBOs, being completely ineffective, because of a failure of the earthing system, which results in there being a recommendation to install supplementary bonding to provide additional protection.

    Edit- I have just checked and as I thought, Graham is the lead author of the book I am looking at and I’m sure he knows where this is going.

Children
  • 0 in reply to Sparkingchip
    Sparkingchip said:
    because of a failure of the earthing system,

    Such an arrangement would not meet the requirements of Section 411, and, if a workplace, possibly not the Electricity at Work Regulations (Regulation 8) either?

    It would be better to use an alternative means of protection against electric shock, such as Class II equipment or SELV.

    To be clear, a broken PEN conductor is not necessarily a broken earthing system ... although it does involve EPR due to a broken neutral. However, I can see cases where it can be ineffective earthing in which case see Regulation 114.1 of BS 7671 and the ESQCR.

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