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Thread: Radio Antenna Grounding:

  1. #1
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    Radio Antenna Grounding:

    The question arises, "On antique radios, is it permissible to connect the antenna ground terminal to the safety ground of a 3-prong AC receptacle? I say no because the safety ground is a poor RF ground, and there is the danger of 120VAC appearing on the safety ground. There is also the possibility of a lightning strike entering the safety ground.

    A copper cold water pipe or a ground rod connected to the central ground node would be the correct method in my opinion.
    Don't mess with B+!
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  2. #2
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    Re: Radio Antenna Grounding:

    I am not sure what you are refering to as the safety ground. The third pin on the plug? An "antique" radio would not have such a plug.

    As to where you should ground the antenna, it should be outside so that lightning has a path to ground outside rather than having to come into your home. if the antenna is inside where it won't get hit by lightning, its not as big a deal.

    I don't know what you are referring to as an "RF" ground, or why this would matter for lightning protection.

    I don't know why you would think hooking up an antenna to the ground pin of a common receptacle would put 120V on it. The antenna is not electrified unless it is struck by lightning.

    [ November 24, 2004, 12:40 PM: Message edited by: petersonra ]
    Bob

  3. #3
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    Re: Radio Antenna Grounding:

    We need to know what the "antenna ground terminal" is- where it is and what it is connected to.

    As I remember it is the other end of the antenna input coil within the receiver. The external long wire AM antenna connects to one end of the winding and the other end is grounded. This functions kind of like a counterpoise or ground plane to the wire antenna.

    Customarily this has been connected to the nearest water pipe, radiator or a ground rod. 810.21(F) would apply here so your suggestion to use a ground rod bonded to the service ground is correct.

    -Hal

  4. #4
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    Re: Radio Antenna Grounding:

    Pete, if I may call you that,

    Antique radios typically have a pair of terminals for an external antenna, one for the leadin and one for antenna (RF) ground. One member of the Antique Radio Forum asked if it would be permissible to tie the antenna ground to the safety ground, or perhaps I should call it the fault ground of a 3-prong AC receptacle. I contend that this would likely be a violation of the code and poor practice at best.

    As for 120V on the fault ground, it is possible that the fault ground is open. Then a short or even leakage to the open fault ground in an applicance or tool would result in line voltage on the fault ground and any appliance connected to the faulty fault ground would be at line potential.

    [ November 24, 2004, 01:15 PM: Message edited by: rattus ]
    Don't mess with B+!
    (Signal Corps. Motto)

  5. #5
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    Re: Radio Antenna Grounding:

    Originally posted by rattus:
    As for 120V on the fault ground, it is possible that the fault ground is open. Then a short or even leakage to the open fault ground in an applicance or tool would result in line voltage on the fault ground and any appliance connected to the faulty fault ground would be at line potential.
    presumably if you had some kind of fault between hot and ground inside the radio, the branch circuit protector would trip and shut it off, thats what it is there to do.
    Bob

  6. #6
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    Re: Radio Antenna Grounding:

    Pete, my point is that if the fault ground is open, a short between the fault wire and the hot wire in any appliance on the line would not trip the breaker because the fault wire is open. Then, you would have line voltage on the open fault wire. It is similar to an open neutral wire where a path through the load makes the neutral wire hot.
    Don't mess with B+!
    (Signal Corps. Motto)

  7. #7
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    Re: Radio Antenna Grounding:

    Your scenario would apply to just about anything, not just an antique radio. I'm not sure i would worry all that much about such an unusual situation.
    Bob

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    Re: Radio Antenna Grounding:

    Yes Pete, that would be unusual although open fault grounds are not that rare, and the exact scenario must have played out a few times. The shock hazard is slight, but you could get line voltage on the antenna, which is not good. And, these old radio buffs love their radios more than they love their wives and would mourn if their radio got fried.

    The gist of my question though is, "Does the NEC permit the antenna ground of a radio to be connected to the fault ground at a 3-prong outlet?"
    Don't mess with B+!
    (Signal Corps. Motto)

  9. #9
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    Re: Radio Antenna Grounding:

    I am still somewhat unsure as to what you are refering to when you talk about a fault ground, or an open fault ground. I'm also not all that sure what hazard you think exists from the ground being open, or how you plan to do that. Connecting your antenna ground to the system ground rod will not adequately protect you since the connection may have enough impedence in it that your OCPD won't trip in the event of a ground fault.

    Neither the ground rod, nor the earth connection to your electrical system (your neutral) protects the user from electrical shock. What protects the user is the connection at the service between the neutral and the grounding point. This connection allwos for a low impednace connection that will reliably trip the OCPD in the event a hot wire touches something conductive that is also bonded to the grounding point at your service.

    As far as I can tell, the NEC does not prohibit you from conencting your antenna ground to the ground pin on on your outlet as long as it is properly earthed when it enters your building (assuming its an outside antenna).
    Bob

  10. #10
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    Re: Radio Antenna Grounding:

    Pete, not being a licensed electrician, perhaps I am using the wrong term. I mean the bare wire or sometimes green wire that ties to the neutral wire at the distribution center. If this bare wire is open between the outlet and the breaker box, protection is lost, and you could see line voltage on this bare wire if a defective appliance is connected to that circuit. Capacitive leakage in a good appliance might be enough to cause a problem. The wire could be open for any number of reasons which I am sure you have seen.

    I understand perfectly how this system is supposed to work, but stuff happens, especially in older houses where remodeling and rewiring have been done. Imagine a three wire outlet wired into a two wire circuit. There would be no ground protection at all, although the users think so.

    Now I am not talking about grounding an antenna mast. I am talking of grounding one end of the antenna transformer in the radio which is wound with fine wire and could be zapped very easily.

    I am thinking that connecting a ground road directly the the third wire at the outlet would not be permitted.
    Don't mess with B+!
    (Signal Corps. Motto)

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