where do i connect the shield/drain cable

Discussion in 'General Electronics Chat' started by GARYN, Oct 14, 2013.

  1. GARYN

    Thread Starter Member

    Sep 4, 2009
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    As a mechanically biased engineer with a reasonable knowledge of electrics i would be grateful if you could explain a concept i am a little confused on in simple terms.
    When using sheilded cable to protect from capactive coupling interference do you have to connect the sheild (i know at source end only) to earth, to the system reference point (0v) or does the 0v need connecing to earth as well in order for it to be effective.
    i have read some instructions that say connect to circuit reference point only and not chassis ground. i,e floating shield, and others that say make sure reference point is grounded to earth.
    What is the correct method or is there times when you would use one and not the other.
    Does the method depend on weather your attemping to stop noise escaping from the sheild or stop noise passing through the shield from external devices onto your cables.
    i have asked some electricians i work with and i get the impression there not sure why they are doing what there doing and looking it up on the net can be confusing due to the use of the word ground i.e some people use it to refer to the system reference point and others refer to it as earth.
    What happens to the sheilding when the sheild is used as signal return.
    If anyone can clarify the above in simple terms and explain when to use each application it would be very appreciated, Thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Don't worry about being confused. EMI shielding is a subject that can be difficult to understand.

    The usual purpose of a shield is to prevent external electromagnetic fields from corrupting the signal(s) inside the shield. In general the shield should go the chassis if the chassis is also connected to circuit common. Otherwise it should probably be connected to circuit common. You may have to try both to see which works best if you have an EMI problem.

    For RF protection the shield is connected at both ends. For protection from 50/60Hz powerline interference the shield is usually connected only at one end to minimize ground loop interference (that's only if the shield is not used as the signal return).

    If the shield is used as a signal return then it becomes a coax type connection and the shield must go directly to the circuit common at both ends.

    Hope that helps.
     
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  3. MaxHeadRoom

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    Jul 18, 2013
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  4. t06afre

    AAC Fanatic!

    May 11, 2009
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  5. GARYN

    Thread Starter Member

    Sep 4, 2009
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    Thanks everyone for your responses, this is very helpful.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    This was a pretty fair summary for your issue.

    One other point was raised by others.

    Uk regulations require that exposed conductors should be connected to mains earth (if mains powered) unless special insulation methods are employed.

    Many other regs are similar.

    Conductor refers to anything that can conduct electricity, whether they are part of the circuitry or not, so includes metal casing, control shafts, etc. It also refers to any conductive material not just copper, so includes conductive plastic, steel etc.

    Exposed means that they can be physically touched by a finger. So you would have to put a protective non conductive caging around an non earthed part so comply.

    This earthing arrangement is completerly separate requirement from signal earthing arrangements, although they may or may not be combined.

    You should be aware that many electronic instruments have their input terminals isolated from mains earth. This includes the signal return or earth or shield. The exposed metal parts of such connectors (eg bnc) are included in the special measures already mentioned.
    Such instruments are then described as 'floating'.

    This arrangement allows equipment to be connected to live circuitry, sometimes at high voltage.

    Care should be taken to ensure that exposed conductive parts of battery powered equipments cannot inadvertantly be live, through the shield or other connection, if connected to mains powered equipment.
     
    Last edited: Oct 15, 2013
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  7. MaxHeadRoom

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  8. ian field

    Distinguished Member

    Oct 27, 2012
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    There is a potential gotcha if the equipment at both ends of the cable have a ground connection of their own - this can create a hum-loop. If you're using the shield as a signal return path, you need to ensure no metal casing part of the bit on the end comes in contact with anything earthed - if this is likely; only connect the shield at one end, and give the signal return its own wire inside the shield.
     
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  9. GARYN

    Thread Starter Member

    Sep 4, 2009
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    Hello Max i found the video very good as it explained things in simple terms and went back to basics. It cut out as it was getting good do you know where i can see the rest, i could not find it. Thanks
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you google Dr Bruce Archambeault you should get all his videos.
    I don't agree always 100% with what he says, but on the whole, makes sense.
    I f you don't find it I may have it on record.
    Max.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Is this the one?
    Max.
     
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