How to protect a low voltage intercom audio line against surges

Discussion in 'General Electronics Chat' started by JoeMJr, Apr 19, 2016.

  1. JoeMJr

    Thread Starter New Member

    Apr 19, 2016
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    I have an audio-video wired intercom system for communication between front gate and house..
    The internal and external units are connected by a 4 wire (2 balanced twisted pairs) cable. One for audio and another for video.
    To avoid it to be burned out again by nearby lightning discharges, I have already protected the power line with Power SPDs.
    However I want to protect also both low signal lines.

    So I browsed the internet and found an AAC thread which was not conclusive. (link)
    Further Google browsing Ied me to find a recommendation to use two unexpensive surge protection modules for low voltage lines or telecom equipment. See picture and site below.

    [​IMG]

    http://www.citel.fr/en/produit/tele...ange/citel-range-plug-in-surge-protector.html

    I wonder whether you guys know any other unexpensive solution, better than this one.
    Thank you in advance.
     
    Last edited: Apr 19, 2016
  2. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Yes -- Implement them as two unbalanced (Spec. coaxial) transmission lines having the shields grounded at each end.

    Best regards
    HP
     
  3. JoeMJr

    Thread Starter New Member

    Apr 19, 2016
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    HP, thank you for answering, but your answer was not exactly the one I expected to hear because the system demands the 2 lines to be balanced for noise zeroing. This is mandatory. Surely I can use a shielded cable keeping the 2 twisted pairs balanced inside, like for instance a CAT6 network cable and grounding the shield at each end.

    Even though, additionally to that, I wonder whether you guys know any other inexpensive solution to protect the 2 balanced pairs against nearby lightning discharge voltage peaks , with better devices than this one I have presented above.
    Thank you !
     
  4. marcf

    Member

    Dec 29, 2014
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    19
    Looks like there are lots of cat5/cat6 cable lighting arrestors out there as well as ones for telco lines. Could you not adapt one of these for your use? After all, a telephone line pair is the epitome of a balanced line. Back in the day when people actually used telephones connected to the CO, lighting arrestors were widely used. They consisted of a gas filled module with 3 leads, one for ring one for tip and a common grounding lead.
     
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  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Fair enough!:) -- Run the pairs through a grounded metallic conduit.

    The principal difficulty with the 'surge absorber' approach lies in the broadband nature of induced transient energy (specifically, arbitrary standing-wave length) -- moreover, surge absorbers applicable to low EMF applications exhibit significant capacitance - not a problem, perhaps, with analog audio but generally inconsistent with analog video (and all manner of digital) signals...

    Best regards and good luck!
    HP:)
     
    Last edited: Apr 20, 2016
  6. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    @JoeMJr

    FWIW, I concur with the above -- provided all of following conditions are met:

    1)
    Your peak signal EMF is commensurate with that of the absorber's intended application.
    2) The absorber presents a 'worst case' impedance and overall 'shape' (i.e. frequency response) amenable to your signals' bandwidth.
    3) The absorbers are located as close to the terminations as possible!

    Best regards - and, again good luck!
    HP:)
     
    Last edited: Apr 20, 2016
  7. JoeMJr

    Thread Starter New Member

    Apr 19, 2016
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    marcf, the use of cat5/cat6 network cable lighting arrestors is a good suggestion.
    Just I checked a spec and saw that they are made for 5v signal lines. So I will first call the intercom maker to ask about the signal levels. The intercom power supply is 12V, so the signal levels could be greater than 5 volts. I think this should be no problem, because the arrestor cutoff voltage should be far greater than 12v. As they are prepared for at least 100Mhz traffic, they should be adequate for video as well. Thanks!
     
  8. JoeMJr

    Thread Starter New Member

    Apr 19, 2016
    4
    0
    Yes, that would have been a great idea, but sorrily the plastic conduit is already installed inside the walls there... But I can use shielded CAT6 cable instead.

    I have thought of using the telephone lines surge absorber MPH-160. But exactly because of this spurious capacitance I have searched some other kind of absorber adequate for low voltage signal lines or telecom equipment. And I found the DP80-TW from Citel that I described above in my first post. Like the MPH160 they are a combination of varistors + gas bulb. But I believe the DP80-TW is not a low cost part. At least I could not find its price anywhere. So the idea of using a RJ45 surge absorber seemed to me as being feasible.
    Thank you for all.
     
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