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TN ring mains in hospital theatres

MS01
Is there any issue with installing a TN ring circuit in hospital operating theatre?


I appreciate Medical IT circuits are in a radial wiring pattern and ultimately any medical equipment is plugged into these circuits, but i'm talking about the 2  /3 TSSO's in the space that are for PC use?  


Within BS 7671 clause 710.415.2.3 which talks about earthing there is a note which states:- It is recommended that radial wiring patterns are used to avoid ‘earth loops’ that may exacerbate electromagnetic disturbances.


My question is do these TN circuits need to follow this radial pattern or can ring mains be installed? 


Matt
  • Former Community Member
    0 Former Community Member
    Why do you need a ring final circuit for a slack handful of sockets with almost no load on them - wouldn't a 20A radial be far better suited


    Regards


    OMS
  • 0
    Ok so it's about 8 /10 sockets in total, split across 2 areas.
  • 0
    Are the TSSO's offices actually in the operating theatres?
  • 0
    I must say, as one who worries about EMC and earth loops a lot, I find that advice a bit surprising. Only a defective ring final is going to generate or respond to magnetic fields more than a radial.

    The thing that really throws the earth currents off kilter is providing parallel paths that do not follow the same route as the L and N currents. This occurs everywhere we bond to any already earthed object for example  at a water heater with a metal feed pipe, or a socket box that attaches also to structural steels.

    IF there really is an EMC problem, the advice must be to avoid these cases  too, but in fact they are not only not prohibited, but in effect mandated.


    A radial will be fine, but don't make EMC the reason not to use a ring.  I presume whoever offers the advice  has not actually measured the fields around a ring.
  • 0

    mapj1:

    I must say, as one who worries about EMC and earth loops a lot, I find that advice a bit surprising. Only a defective ring final is going to generate or respond to magnetic fields more than a radial.

    The thing that really throws the earth currents off kilter is providing parallel paths that do not follow the same route as the L and N currents. This occurs everywhere we bond to any already earthed object for example  at a water heater with a metal feed pipe, or a socket box that attaches also to structural steels.




    If it really is in the operating theatre, supplementary local equipotential bonding is required by BS 7671, so such an arrangement would be in place anyway?


    I have to add, though, that whilst on occasion a fortuitous alternate path might form a "loop", these cannot be avoided in most real installations, even where bonding doesn't come into it - for example, simple connections between electronic devices: even unscreened wired Ethernet has a controlled static discharge path to Earth (well should have according to the standard, I know an unplugged laptop breaks this rule, but some laptop PSUs that don't have earthed DC output also have a high resistance discharge path DC to mains Neutral). These discharge paths are also serve to discharge common-mode noise.


    Interestingly, the solution to this problem is to provide more earth paths using multiple bonding networks (CBN and mesh-BN) - see BS EN 50310 and BS IEC 61000-5-2.


    So, how does ring vs radial stack up now? Well, arguably sightly better because it's a lower inductance path overall.






    IF there really is an EMC problem, the advice must be to avoid these cases  too, but in fact they are not only not prohibited, but in effect mandated.


    A radial will be fine, but don't make EMC the reason not to use a ring.  I presume whoever offers the advice  has not actually measured the fields around a ring.




    As above, I'd definitely agree.

    Is there enough computer equipment expected to make it a high protective conductor current circuit in any case ... if so, even if the live conductors aren't in a ring, the solution may well lead to a ring arrangement for the protective conductor?

     

  • 0
    So would it be acceptable to use a ring circuit in an operating theatre for general sockets, not for medical equipment as they would obviously be under a medical IT circuit.
  • 0
    Could it be said this clause It is recommended that radial wiring patterns are used to avoid ‘earth loops’ that may exacerbate electromagnetic disturbances. is stated under equipotential bonding so is specific to the earthing arrangement. Noting that a TN radial circuit may have a bond to the EBB and back to the local DB which in turn has a link to the EBB so would ultimately cause a ring arrangement ? 
  • 0
    There is no technical reason not to use a ring for general use sockets  for computers/office equipment. So long as they cannot be mistaken for part of an IT (in the earth sense) system, and go nowhere near it.   There may be a house standard that means it is not expected, and arguably division of loads over more circuits of lower MCB rating are less likely to all goes off totally and work has to stop dead. Needless to say it needs to be very clear which sockets are supplied from which circuit.

  • Former Community Member
    0 Former Community Member

    Chris Pearson:

    Are the TSSO's offices actually in the operating theatres?




     

    I think the OP is referring to Twin Switched Socket Outlet , Chris


    I bit of a read through the current HTM 06-01 should resolve most of the arguments given this is a Clinical Risk Category 5 environment (or Group 2 location)


    Regards


    OMS
  • 0

    OMS:




    Chris Pearson:

    Are the TSSO's offices actually in the operating theatres?




     

    I think the OP is referring to Twin Switched Socket Outlet , Chris


    Oops! I was thinking of "theatre sterile supplies officer". ?


    (That grocer's apostrophe was misleading.)

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