Radio Antenna Grounding:

[rattus]

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.

 

[petersonra]

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 ]

 

[hbiss]

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

 

[rattus]

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 ]

 

[petersonra]

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.

 

[rattus]

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.

 

[petersonra]

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.

 

[rattus]

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?"

 

[petersonra]

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).

 

[rattus]

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.

 

[hbiss]

Re: Radio Antenna Grounding:

Lets look at it this way. Suppose the "antenna ground" is connected to chassis ground (I believe many were in transformer powered sets). You then change the line cord to a 3 wire cord and connect the green to the chassis. No violation there.

-Hal

 

[rattus]

Re: Radio Antenna Grounding:

Hal, perhaps you are right technically, but I still think it violates the spirit of the code. That is, I think the code writers would not have allowed an external antenna (not the mast) ground to be connected to the third wire. And, the old time designers had no idea that the chassis would ever be grounded through the AC outlet.

This question arose because the house had plastic plumbing and the guy wanted an easy way to connect the antenna ground terminal.

 

[hurk27]

Re: Radio Antenna Grounding:

Rattus The transformer the antenna connects to is a balance isolation transformer. connection one side of this transformer to the building grounding conductor would not or could not inpose any voltage into the curcuitry pass this transformer and fry the radio, It would cause a 120 volt shock hazard if someone were to touch the radio or the antenna and Earth . The chassie of the radio should be grounded but if we are concerned about lightning then making this connection will not much of a help if lightning should ever hit the antenna. as the lightning would travel down the lead in wires to get to this grounded source and most likely take out the radio anyway. Lightning must be stoped before it enters a building not after as the radio would be the least valuble item that might be dystroyed as once the lightning enters a building it can and probley will jump to the electricle circuits of the building which could cause allot more damage or even dystroy the building by fire.
To prevent most of the current of a direct lightning strike it will take a good amount of planning and design. you must ground the incomming lead in wires to the building grounding electrode system on the outside of the building. and keep the lead in wires very close to where the buildings service is as lightning is an AC event and distance in the grounding wire can make this grounding usless. You dont need to have a ground at the radio for lightning as per what I said above. grounding at the radio would only be for the safty of the operator if the chassie were to become live and a shock hazard. That is why the ground terminal of the receptacle is sometimes called a safety ground. We call it a equipment grounding conductor or (EGC). This ground wire is required in all new circuits and should be reliable to fault the current if a short was to occur back to the source which would be the service neutral and not Earth. There is a site that will give you some good info on what it takes to protect from lightning but nothing has a garentee to totaly protect from lightning but can be kept to a minium.

Click here: NATIONAL LIGHTNING SAFETY INSTITUTE

Look around here as there is some good info.

 

[hbiss]

Re: Radio Antenna Grounding:

... I think the code writers would not have allowed an external antenna (not the mast) ground to be connected to the third wire.

If that's the case then there must be billions of pieces of equipment with coax inputs that are not code compliant.

... these old radio buffs love their radios more than they love their wives and would mourn if their radio got fried.

The only way to protect something that valuable is to disconnect it when not in use. I wouldn't even worry about the situation you are suggesting, if anything is going to wipe out the input coil it's going to be lightning just like always.


-Hal

 

[hbiss]

Re: Radio Antenna Grounding:

Wayne, you beat me to it!

-Hal

 

[rattus]

Re: Radio Antenna Grounding:

Thanks guys,

The scenario that would fry the antenna coil is a live antenna ground wire and an antenna short to ground. If it can happen, it will.

Had not thought of a coaxial input, but would the outer conductor be connected to chassis ground, or would it be isolated? It seems that the coax leadin would be grounded outside and then again through the chassis to the safety ground. Does that not create a ground loop?

[ November 25, 2004, 08:44 AM: Message edited by: rattus ]

 

[oceanaradio]

Re: Radio Antenna Grounding:

Originally posted by rattus:
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.

[ November 26, 2004, 08:43 PM: Message edited by: oceanaradio ]

 

[oceanaradio]

Re: Radio Antenna Grounding:

Hi guys, first sorry about the quote-without-material posting above, slipped there...

Most of the questions raised were answered in some part, but I can help with the ground requirements for radios...

All modern radio equipment must have their metal cases bonded individually to a single point ground for the radio room. This single point ground is of course bonded to the main structure AC entrance ground as well as antenna mast grounding systems.

An antique radio (assume non-metallic case and 2-prong plug, the worst possibilities) of which the poster speaks is a rather dangerous piece of equipment. They should never, ever be connected to external antennas when there is any chance of approaching thunderstorms!

One member raised the question "doesn't it make a ground loop when coax is grounded at the radio (thru the AC fault-ground), and the antenna is grounded outside"? Yes it does make an unavoidable loop. But the entire grounding system is bonded so that's not a loop that causes any harm. All antenna coax feedlines must be shield-grounded at several locations, including the station entrance and of course again at the radio by coax connector design.

Never, ever, may a separate ground connection (such as unbonded water pipe or unbonded separate gronding electrode) be used for a radio connected to external antennas.

I have never heard any person concerned with the possiblity that an electrical fault somewhere else might also harm their radio IF the neutral or ground was ALSO open somewhere.

Jack Painter
Virginia Beach

 

[dereckbc]

Re: Radio Antenna Grounding:

Originally posted by rattus:
It seems that the coax leadin would be grounded outside and then again through the chassis to the safety ground. Does that not create a ground loop?

Rattus, I assume your radio has 2-wire plug on it with a terminal marked ground. There are a couple of places you can terminate too.
1. Cold water pipe assuming it is metallic and bonded to the electrical service.
2. A driven rod immediately outside the wall of the ham shack. However you would have to dig a trench to run a # 6 AWG or larger bonding jumper back to the AC service ground to make it compliant with NEC.

As far as the loop goes, it is a good loop when you are talking about coax. I design a lot of cell sites for a living, and we bond the coax sheild at least 4 times before bringing it in the building. Doesn't affect signal quality at all. It's a good thing.

What I have built for several Hams is an isolation transformer with an RF ground. 240 in 120/60 out (No neutral conductor) with a ground terminal or what we call a Xo lead. In the Ham shack we either build a build a ground ring or counter poise, bring it in the building, to a small RF ground bar. Bond the transformer XO to the RF ground bar, and any ground terminal to radio equipment to the RF ground bar.

Dereck W5FE

 

[rattus]

Re: Radio Antenna Grounding:

Thanks gents for your erudite explanations. Makes total sense to me.

As for the open fault ground causing a problem, it could be open right at the central ground bus for example, then a fault to case in any grounded equipment on that circuit would make the cases of all other equipment on that circuit live. If a long wire antenna ground terminal is grounded to the open fault ground, the antenna itself would be live. It is indeed a rare occurence, and most people would not think about it, but I am paranoid and think up scenarios such as this and have encountered a few in residential wiring over the years.