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Sub Boards or Incomer??

tonydon

Hi

We have a large commercial site fed by 2 onsite substations…

The substations feed panel boards in our side of the subs  i.e. Incomer

The panel boards then feed sub boards within the site, some of which will feed other downstream boards

Our questions are (for condition reporting)

  1. section i… Is the supply characteristic always referring to the incoming supply
  2. when testing a board is the supply protective device the supply feeding this particular board i.e. potentially an mcb in another upstream board that supply's this board … or  is the supply protective device the device on the incoming supply to the site  i.e this will then be the device listed on each board (the same throughout the site)
  3. Is the supply characteristic the supply to the whole site or the supply to just this board
  4. the ZE cannot be reasonably ascertained by test,  but, we also cannot get our supplier to give us this reading by inquiry  What should we do?

 

  • 0

    Don't be too hung up on the details of the model forms - they're really intended for smaller installations (e.g. up to 100A/phase) with a single source of (grid) supply. For larger or more complicated sites you may well need to amend them to suit the circumstances.

    I'm not sure from your description whether the two transformers are DNO owned or part of the consumer's installation (supply provided at HV). Nor whether the two transformers supply separate distinct areas of the site or can have their outputs combined or switched into a common supply - either normally or say if one transformer/HV supply fails. The latter can affect Ze and PSSC values significantly of course.

    Calculation is always an option if you can't measure (effectively enquiring of yourself rather than the DNO) - usually backed up with with some means (e.g. inspection or test) to ensure that there's no lurking fault that invalidates the calculated result.

       - Andy.

  • 0

    Hi Andy

    Thanks for the reply

    We have 2 x DNO subs each supplying different areas of our large building (not cross bonded)

    The subs feed 2 large switch rooms that the feed all the boards on site (probably about 100)

    I have tried to get the ZE from our supplier (Scottish power) but they seem to go out of their way to make this info unavailable

  • 0

    If distances are short, then you may be able to use the rating plate on the transformer, and maybe add a small correction for the length of cable to it .

    You can assume for PSSC that for example a 5% 1MW transformer droops 100% at 20MW, and that that associated current per phase would define the Zs at the terminals.

    In a complex nested system and this sounds like it is, you may be better reporting Zdb at each board, so Zdb 1 is the Zs reading taken at the main switch of board 1. 

    mike

  • 0

     

    I concur with AJ that the forms in Appendix 6 are likely unsuitable for the larger installation. I often deal with large rambling sites such as hospitals and industrial locations with many buildings served from some remote common source to which access is often restricted. If I cant get the relevant supply characteristics I simply declare this (I use my own template forms and declare as a limitation). Not once in all the years I have been involved in IandT has anyone ever questioned this. It would be great if they did as it might indicate that someone has at least taken the trouble to read the damn things!

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