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Supplementary bonding conductor in a medical location

Julien Leblanc
Hello,


After reading from many sources about the medical locations' supplementary equipotential bonding, everything is mixed in my head...


Suppose we have a current using device inside a group 1 medical location (TN-S supplied).

Then the protective conductor of that device must be connected to the EBB installed in this location by means of a supplementary equipotential bonding conductor.

Could someone clarify this:  that bonding conductor, should it comply with art. 544.2.1 or with art. 544.2.2 from BS7671, that is, the EBB should be considered an extraneous-conductive-part or an exposed-conductive-part ?


Thank you.
  • 0
    Neither.
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    Then the conductor which connects the protective conductor of the circuit with the EBB should be considered an extension of the CPC and be sized as a CPC?

    Please detail a little.
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    544.2 gives rules for sizing the bonding conductor(s) between exposed- and extraneous-conductive-parts.


    To make sense of that I don't think I'd consider the conductor between one item and the EBB in isolation - rather consider the EBB as just part of the bonding conductors that join the various items and consider the overall connections.


    So say we had two items with exposed-conductive-parts (A and B) and one extraneous item (C), all connected together via a single EBB.


    So:

    1. The entire path between A and B (between exposed conductive parts) would have to meet 544.2.1 (minimum = size of the smaller c.p.c.)


    2. The entire path between A and C (between an exposed-conductive-part and an extraneous-conductive-part) would have to meet 544.2.2 (minimum = half of A's c.p.c. size)


    3. The entire path between B and C would likewise need a conductor of at least half B's c.p.c. size.


    So A's supplementary bonding conductor would have to meet both 1. and 2. above, B's would have to meet 1 and 3 and C's 2 and 3.


    All subject to the usual rule of not less than 2.5mm² if protected or 4mm² otherwise.


       - Andy.
  • 0
    Thank you.

    Your explanation makes lots of sense to me.


    I've found an ABB guide on the internet:
    https://library.e.abb.com/public/8641f847b0d4483f9d959245c8dd05a0/Hospital Line practical guide_web.pdf


    At page 23, it says:

    "The conductors that connect the extraneous conductive parts to the equipotential bonding bus bar are defined as equipotential conductors and must have a cross-section of no less than 6 mm2".

    It looks like they are sizing these conductors as the main protective bonding conductors? But the EBB is not the main earthing terminal...


    Also "The conductors that connect the conductive parts to the equipotential node are protection conductors (PE) and their cross-section must be established using the criteria specified by the general standard; in other words, it must be at least equal to that of the phase conductors."

    There is also an image on the same page with a sizing example.


    I don't understand their approach. In this book, https://shop.theiet.org/guide-to-electrical-installations-in-medical-locations, figure 14.7 clearly shows that we are talking about supplementary bonding conductors, not main bonding conductors or PE.

    Am I misunderstanding something? The IEC standards are very similar to BS standards, so I don't know where does the different approach come from.


    Thank you.

  • 0
    The EBB is neither an extraneous-conductive-part nor an exposed-conductive-part. It is a protective bonding terminal.


    In the UK, the conductor connecting it to the protective earthing system is sized in accordance with Regulation 710.415.2.3:
    The equipotential bonding busbar shall be connected to the system earthing using a protective conductor having a cross-sectional area greater than or equal to the largest cross-sectional area of any conductor connected to the equipotential bonding busbar.



    Andy is quite right, that the sizing of individual bonding conductors for supplementary local equipotential bonding is based on Regulation Group 544.2, but in the case of a medical location, a higher level of performance is required:
    • maximum simultaneous touch voltage not to exceed 25 V AC (Regulation 710.411.3.2.5), whereas in the general case for supplementary local equipotential bonding (Regulation 419.3) the touch voltage is limited to 50 V AC; and

    • Regulation 710.415.2.2 requires that the maximum resistance between earth terminals of socket-outlets or fixed equipment, and exposed-conductive-parts [within the location] and/or extraneous-conductive-parts [of the location] should not exceed 0.2 Ω.



    Whilst BS 7671 is based on HD 60364-series, which is aligned with IEC 60364-series, national variations exist.
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    Also "The conductors that connect the conductive parts to the equipotential node are protection conductors (PE) and their cross-section must be established using the criteria specified by the general standard; in other words, it must be at least equal to that of the phase conductors."

    That sounds like it's come from an "international" (or at least other national) source - the UK is fairly unusual in allowing reduced c.p.c.s (at least on smaller circuits). Most other countries take the, perhaps not unjustified view, that since TN disconnection times (e.g. 0.4s) are based on a touch voltage of half the line voltage then you shouldn't be picking c.p.c.s with higher impedances that might result in a higher touch voltage.


       - Andy.
  • 0
    Thank you.
  • 0
    The equipotential bonding busbar shall be connected to the system earthing using a protective conductor having a cross-sectional area greater than or equal to the largest cross-sectional area of any conductor connected to the equipotential bonding busbar.


    I suppose it will be a separate conductor, or it can be via the largest cpc of all circuits in that location?

    Figures 710.2 from BS7671 and figs 14.4 and 14.7 from https://shop.theiet.org/guide-to-electrical-installations-in-medical-locations seem to indicate a separate conductor.


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