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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?
  • 0
    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?
  • 0
    To have two separate and unconnected earthing systems in the same building is not really permitted for safety according to BS 7671 in the UK (and other CENELEC national standards harmonized to HD 60364-series, and globally to IEC 60364-series). Whilst a separate functional earth system is permitted, it should be connected in at least one place (MET to MFET) - it cannot be otherwise, as the functional earthing system with its own electrodes is an extraneous-conductive-part.


    However, usually in buildings with ICT systems, multiple bonding is provided between the protective and functional earthing systems. In general, the systems should follow BS EN 50310 (IEC 30129). This standard shows the multiple bonding between protective and functional earthing systems extending to power distribution units within racks themselves.


  • 0
    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?


    Multiple connections (what used to sometimes be called "cross-bonds") reduce inductance. This is what's shown in Fig A444.2, Fig A444.3 and A444.4 in BS 7671 - and is also recommended by BS EN 50310.


  • 0
    What Alanbalby describes, is that not the least effective means of reducing EMI?
  • 0
    I am curious to know why these additional earthing systems use such large cables and earth bars - much bigger than a CPC.
  • 0
    It is sometimes done to create separate circuits to form segregated "clean" and "dirty" supplies to things that need protection, and possible sources of interference respectively  As part of that the earthing may be split as near to the supply origin as possible.

    Even so, for ADS to work safely and well, there should always be some connection between the clean and dirty earths, though that connection  may be arranged to be a very low impedance at 50 Hz and still a significant impedance to fast pulses / high frequencies. This may take the form of a ferrite cored inductor or just a long route, usually via the main earth terminal.

    Such systems need sympathetic maintenance with understanding, as it is all to easy to defeat the intention by accidentally cross coupling the clean and dirty sides in the mistaken belief that it will improve things.


    More generally segregated systems are often a sign of troubles in the face of EMC issues, and it is generally  preferable to fit suppression and filtering local  to the 'dirty' equipment if at all possible, so the whole system is just "clean" and then there can be a single low impedance ground plane
  • 0
    Chris Pearson:

    I am curious to know why these additional earthing systems use such large cables and earth bars - much bigger than a CPC.


    Less inductance - the frequencies we are talking about are far above 50 Hz and because the effect of inductance is proportional to frequency, this is the driver.


  • 0
    gkenyon:
    Chris Pearson:

    I am curious to know why these additional earthing systems use such large cables and earth bars - much bigger than a CPC.


    Less inductance - the frequencies we are talking about are far above 50 Hz and because the effect of inductance is proportional to frequency, this is the driver.




    Graham, thank you. A bit over my head!


  • 0
    Chris Pearson:
    gkenyon:
    Chris Pearson:

    I am curious to know why these additional earthing systems use such large cables and earth bars - much bigger than a CPC.


    Less inductance - the frequencies we are talking about are far above 50 Hz and because the effect of inductance is proportional to frequency, this is the driver.




    Graham, thank you. A bit over my head!




    Well, I can explain it another way.


    The impedance of an inductor Z is given by the formula


    Z=2πfL where



    • π=3.141759

    • f is the frequency of the AC waveform we are looking at (50 Hz for mains, but might be measured in kHz or MHz, or perhaps GHz, with modern computer equipment)

    • L is the inductance of the cable

    For example, if a length of cable has an inductance of 1μH, it's impedance will be:


    • 0.3 mΩ at 50 Hz (mains frequency)

    • 0.3 Ω for signals at frequencies of 50 kHz (a typical switching frequency in a switched-mode power supply)

    • 31 Ω for signals at frequencies of 5 MHz (this is actually 2.5 times faster then the clock speed of a BBC Micro from the 1980s)

    • 10 kΩ for signals at frequencies of 1.6 GHz (not an unusual clock speed for a Core i3 processor - an i7 might run at up to twice that).


    The following techniques can help reduce inductance in an earthing/bonding system for EMC, and hence keep the impedances down:

    • Larger csa conductors

    • Keep connections as short as possible

    • Use rectangular section copper tape or braid, especially where cross-sectional area > 25 mm2.

    • Make multiple connections where short connections are not practicable.

    • Avoid conductors with ferrous metals (or provide a large csa copper conductor in parallel with it)

  • 0
    This is why it pays to be a member of this forum, thankyou Graham and Mike for your replies, which as usual, are very helpful.

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