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Cable connection between the equipotential earth bonding bar (EBB) and the and distribution board

Simon Bourke

This follows on from a prior question and discussion: Cable size between equipotential earth bonding bar and distribution board in a Group 1 medical location

Regulation 710.415.2.3 states: The EBB shall be connected to the system earthing using a protective conductors having a cross-sectional area greater than or equal to the largest cross-sectional area of any conductor connected to the EBB.

Note HTM 06-01 provides no further definition of the point of connection of the EBB and the connection to the systems protective earth.

The clause is silent on the location; however, the consensus was:  the local Distribution Board  rather than the origin suffices.

Question 1 : The following statement has provided by a hospital engineer questioning this. Is this considered a user preference rather than regulatory compliance:

''Where practicable, medical equipotential bonding should be connected directly to the main earthing terminal to ensure integrity and clarity of the earthing system. 

 Risks of wiring EBB to a DB;

  •  Increased dependence on downstream connections
  • Higher chance of unnoticed disconnection
  • Harder inspection and fault tracing
  • Greater lifecycle risk
  • You are relying on the DB & MET connection
  • Any future alteration, loose termination, or undocumented change can ;Increase impedance/break the bonding path
  • The EBB is a safety reference, not just a CPC
  • Increased impedance & higher touch voltages under fault conditions
  • DBs are: Opened/modified/extended & re-terminated
  • During future works: Earth Bars get disturbed/conductors get moved or resized/Temporary disconnections occur
  • The medical equipotential system can be compromised without anyone realising
  • Parallel earth paths and circulating currents cause issues with testing

 Direct MET connection provides a solution that is as follows;

  • Lowest risk
  • Clearest compliance
  • Preferred by healthcare AEs on a new project 

Question 2 : From the statement above is this statement correct:  The EBB is a safety reference and not just a CPC

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  • 0
    Simon Bourke said:
    The clause is silent on the location; however, the consensus was:  the local Distribution Board  rather than the origin suffices.

    I agree that the clause is not entirely clear, but "protective conductor" is in the singular, which might be read as a single cable without any junctions.

    The requirement appeared in BS 7671:2018+A2:2022, so there must be existing installations which do not comply in respect of the CSA. So why was the requirement introduced?

    gkenyon said:
    Are the diagrams in Fig 710.2 and 710.3 not illustrative of the requirements?

    They are, but although Fig 710.2 shows the "connection to protective earth in distribution system" (to the left of the EBB), it does not specify where the other end is attached to the system earth. It merely shows TN distribution system, which implies that a local DB would suffice.

    If a local DB is used, then presumably the CPC of the distribution circuit which supplies it must also meet the minimum CSA requirement for the bonding connexion.

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  • 0
    Simon Bourke said:
    The clause is silent on the location; however, the consensus was:  the local Distribution Board  rather than the origin suffices.

    I agree that the clause is not entirely clear, but "protective conductor" is in the singular, which might be read as a single cable without any junctions.

    The requirement appeared in BS 7671:2018+A2:2022, so there must be existing installations which do not comply in respect of the CSA. So why was the requirement introduced?

    gkenyon said:
    Are the diagrams in Fig 710.2 and 710.3 not illustrative of the requirements?

    They are, but although Fig 710.2 shows the "connection to protective earth in distribution system" (to the left of the EBB), it does not specify where the other end is attached to the system earth. It merely shows TN distribution system, which implies that a local DB would suffice.

    If a local DB is used, then presumably the CPC of the distribution circuit which supplies it must also meet the minimum CSA requirement for the bonding connexion.

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  • 0 in reply to Chris Pearson
    Chris Pearson said:
    They are, but although Fig 710.2 shows the "connection to protective earth in distribution system" (to the left of the EBB), it does not specify where the other end is attached to the system earth. It merely shows TN distribution system, which implies that a local DB would suffice

    I think this depends on particular situation, although the 'source earth' in the TN system may well be common. Another example, from the IET Guide to Medical Locations:

    Surely it's the 'most efficient' place to provide the earthing, and somewhere hopefully that's common to the TN supplies feeding the primary of the IT system, and any 'TN' equipment in the location? Hopefully all these places are rather near the location itself?

    BS 7671 isn't, and can't be, a 'manual' or 'handbook' that covers all eventualities. There are good reasons to keep things more flexible ... to permit a multitude of conformant designs.

    But in doing that, there is a reliance on designers having knowledge of the pro's and con's of doing something one way vs another.

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