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Cross sectional area of a protective conductors

MS01
Can someone give me a some advice on if we have a earth electrode system made up of the structural rebar when using the adiabatic equation to size of the bonding conductor comes out at 300mm. MY questions is does each bond need to be this size or can it be made up via a series of smaller bonds that are equal or exceed the 300mm requirement? My opinion is each bond needs to be this size to deal with the fault current as the direction it will flow is unknown. What are peoples thoughts? I cannot see anything in the regs on this.

  • Former Community Member
    0 Former Community Member
    The 11kV side will likely have a Neutral Earthing Resistance of something like 6.4 Ohms so the HV fault current under EF is limited to around 1000A - of course it could also be a 4 ohm unit and they may be in parallel on the HV feeders so 1.7kA seems credible. It may be an earthing transformer that replaces the NER's. In any event, the HV electrode (or the steelwork earth) is simply to limit earth potential rise and operate earth fault protection. The OP notes that this is a fully cabled system with earthed cable screens (ie the reference to "global" earthing)


    It is simply not credible to put 100kA into the electrode - expect concrete to be spalling off in all directions if that was true due to rebar heating.


    As suggested, draw out the fault loop - that should show that the rebar and bonding conductors are in parallel with a thumping great chunk of copper which is the neutral plus the earthed screens of this conductor and each phase conductor - the relative resistances should be showing you that the division of current will dominate in the neutral not the electrode or the bonding.


    Depending on the type of facility, the design might consider the possible fault scenario where the phases are intact, the system neutral is severed and under earth fault, the only route would be via bonding and rebar back to the transformer - but that also assumes a catastrophic loss of system protection etc. I might consider that scenario for certain specific clients where a LOOP event would really be quite dangerous, but if so, we would also be designing deep earthing electrodes under the building, surface earthing electrodes around the building and so many divergent paths of the mesh back to the transformer slabs as to still make multiple 300mm2 conductors unlikely. Keep in mind that even at lightning levels of fault current, most standards still only mandate between 50mm2 and 70mm2 of copper (typical copper tape is 3mm x 25mm to give 75mm2 CSA)


    Regards


    OMS
  • 0
    Ah...  Feeling suitably foolish now for not spotting the fact it is HV cables and concentric HV earth.   A NER at the other end it is then, and of course that will make transferred rise of potential at the load end quite 'cold'.  I really should be more careful commenting on HV, my experience is quite skewed towards the non-standard.



  • Former Community Member
    0 Former Community Member
    Never foolish, Mike - ? you were almost spot on - looking at the numbers I think it's a 4 ohm unit (I think you estimated 3.8 Ohms) just that it's the "other end", so a bit colder the transformer end


    Key thing is the OP's 100kA earth fault into the rebar isn't really credible


    Regards


    OMS


  • 0
    OMS, and Mapj1, where do you learn such in-depth stuff?

    I've havent ever worked on HV, or LV transformers, I have done a few connections to the LV side of the DNO transformer, always supervised by a DNO guy.

    It would be nice to learn more about this side of engineering.

    Thanks.
  • Former Community Member
    0 Former Community Member
    alanblaby:

    OMS, and Mapj1, where do you learn such in-depth stuff?

    I've havent ever worked on HV, or LV transformers, I have done a few connections to the LV side of the DNO transformer, always supervised by a DNO guy.

    It would be nice to learn more about this side of engineering.

    Thanks.


    For me, it was the usual combination of education, training and experience - apprentice trained in a German company that had big involvement in mining and heavy industry so we were always pushing lots of power about the place - I went on and did a first degree in power engineering before moving into Building Services and doing a degree in mechanical engineering. I've been lucky enough to work in sectors where the size of the electrical infrastructure makes it similar to DNO and transmission networks anyway - for example I'm looking at a data park that soaks up the last 60MVA of 33kV  - and will be looking at another phase that brings in 300MVA at 132kV - other examples would be 80MVA intakes into airfields in hot and dusty places or 50MVA intakes in very remote areas to serve critical desalination facilities. Lucky enough to be in the right places I guess is the short answer


    Not sure if courses are still running, and the qualification is more relevant to DNO staff, than those at the end of the network so to speak, but Aston University used to run a distance learning course over 6 or so modules that would get you a "Certificate of Competence" in Electricity Distribution - there were also single modules focused on substation design and earthing and a bit more detail on Power Transformers

    try here


    Regards


    OMS


  • 0
    alanblaby:

    OMS, and Mapj1, where do you learn such in-depth stuff?

     


    If there was a single source that contained all the information that anyone ever wanted to know and anyone could have access to it, you would likely find that some would be better than others in taking it onboard and disseminating it. Maybe it’s just a gift and I suspect both OMS and Mapj1 have it in spades!

    For most folk, not being able to kick a ball like David Beckham ought not to put them off enjoying football, self included. 


  • Former Community Member
    0 Former Community Member
    Not me Lyle - I actually find it difficult explaining technical aspects generally


    I have never had a natural grasp of breaking down the ideas into simpler aspects to explain to people - which is a bit of a failing for engineers, actually - it's something I've had to learn to do, as well as conquering my fear of actually presenting stuff to non technical audiences.


    As I like to mutter, Consulting engineering is a great career for me, as long as I don't have to talk to the bloody managers, accountants and clients


    Mike's yer man for explanations in my opinion


    Regards


    OMS




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