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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?
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    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)

Reply
  • 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)

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