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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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    Medical locations aren't my area at all, but I imagine what's intended is similar to supplementary bonding in other situations - i.e. to reduce to a minimum the touch voltages between simultaneously accessible exposed- and extraneous-conductive-parts. To that end, connections are normally kept short as possible as longer connections inevitably mean higher resistances and so larger voltage differences for a given fault (or other protective conductor) current. Connections to remote parts of the installation also increase the risk of "importing" voltage differences from elsewhere.

    As for "not just a c.p.c", yes in general bonding conductors are protective conductors,  but not circuit protective conductors.

      - Andy.

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    Medical locations aren't my area at all, but I imagine what's intended is similar to supplementary bonding in other situations - i.e. to reduce to a minimum the touch voltages between simultaneously accessible exposed- and extraneous-conductive-parts. To that end, connections are normally kept short as possible as longer connections inevitably mean higher resistances and so larger voltage differences for a given fault (or other protective conductor) current. Connections to remote parts of the installation also increase the risk of "importing" voltage differences from elsewhere.

    As for "not just a c.p.c", yes in general bonding conductors are protective conductors,  but not circuit protective conductors.

      - Andy.

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