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Clean Earth System

Oz
Hi everyone,

I received earthing system design for  the high rise building. Earthing system  single line diagram shows main earthing system and  separate earthing network as clean earth . In every IDF rooms (Telecom Room) there are earth bar and earth bars connected each other for every floors every IDF rooms and its connected in the end to the  main clean earth bar in the main telecom room and main clean earth bar connected to the separate earth pits with earth electrodes. There are main earthing system as well but there is no connection between the main earthing system and clean earth. Earth pits ,earth electrodes,earth bars are all separate .The thing is telecommunication rack fed by the industrial socket inside the IDF Room  and this sockets fed by UDB(UPS DB) in electrical room. UDB connected to the main earthing system within the earth bar inside the electrical room .Telecommunication rack cabinet body connected to the clean earth bar inside the IDF room. So after all separations between main earth and clean earth , they are connected through rack cabinet bonding. My question is what is the purpose  of the "clean earth " provided in this design . And what I tried to explain above is correct practice?
  • Former Community Member
    0 Former Community Member
    alanblaby:

    Yes, I'd like to know the theory too.

    We installed a large DB in a hospital suite to supply a single scanner, and its associated auxilaries.

    The DB/Panel was custom made by T. Clarke. It was fed direct from the substation by a 5 core SWA cable, iirc, 185mm, though it may have been 95mm, its 2 years ago now.


    Inside the panel was the usual earth bar and outgoing circuit connections on it, then a separate 'Clean Earth Bar' which had a number of 10mm cables going to almost all machines in the room.

    Being as the earth was supplied direct from the substation to the DB, then how can that 'clean earth' be any different to the normal earth bar?


    The scanner suite is most likely a group 2 medical location, so that clean earth bar may actually have been part of a safety earthing system aimed at limiting microshocks to patients in conjunction with a medical isolation system. In that environment you are really trying to control touch potentials. A bit of ohms law tells us that V = I x R, from which we could say that Vtouch = Ifault x R, where R is the resistance (impedance) between an item and the earth reference bar (ie the point of common coupling akin to a room MET). For a given fault current, keeping R low (ie big fat copper cables) reduces touch voltages.


    Regards


    OMS


     


  • 0
    Inside the panel was the usual earth bar and outgoing circuit connections on it, then a separate 'Clean Earth Bar' which had a number of 10mm cables going to almost all machines in the room.

    A bit like the layout in Fig 710.2 of BS 7671? (page 283) (if without the IT supply).

       - Andy.
  • 0
    A Nuclear Power Station has something similar, though, if I remember correctly the clean earth was connected to the dirty earth at one point (under the control, room?). The point, I was told, is that all the twisted pair cable screens and armours needed to be connected to earth once and once only.

    Every multipair screened cable was glanded with an insulated gland and the glands connected so as to provide this path, so if there were two multipair cables from a junction box to the control room, one had its armour and screen used as the earth connection for all the minor cables and one did not. Keeping all this under control on a project with 50,000 cables was fun.

    In a big Power Station there is a concern that the local earth and the earth bar earth may be very different , especially at the instant of a fault on the nearby 400kV system but someone cleverer than I had done the calculation of the chance of such a fault coinciding with someone touching the clean earth and deemed it sufficiently low.
  • 0
    In this design , clean earth and main earth are completely separated and as I understand Its not complying with the BS EN 50310. The only connections between two earthing is bonding wire connected from body of the data cabinet to the clean earth bar in the IDF room .Because industrial socket earth wire which is coming from the UPS distribution board(main earthing) connected to the body of the data cabinet as well. 


    Is this installation cause risk to the data equipment ? Do you think , do we need to connect both earthing system at least in the point of  the main earth bars.
  • 0
    "Is this installation cause risk to the data equipment "

    Yes. Someone has gone to a lot of trouble and expense designing this system and an extra bond at a cabinet would render the whole design pointless.Check with the designer before doing anything extra.If it does not comply with BS7671 then it is his responsibility to ensure equivalent safety.

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