"Hot" roof

augie47

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
New construction. Home is being built within 75ft of a TVA high voltage line (161kv)...construction workers were being shocked by the metal roof. Voltage measurement to ground is around 500v. Would bonding the roof to ground rods eliviate the problem ??
 
I would say insulate the roof. Or figure out a way to eat at the electric field prior to the charge being on the roof.

If the electric field being generated by the wire is enough to induce voltage than making the building a better reference to ground might not help. It could keep the roof at a closer voltage to zero and mitigate the issue but so would putting in random steel beams closer to the line to eat at the electric field.

Do you know what direction the line is in? Like the vector. So would the building's roof be 75' east of the closest conductor and then 100ft up or does that 75' include verticality?

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I look at it like a faraday cage but your walking on the cage.
 
You know what. Maybe try reducing your ground impedance. Thought I would probably try to utilize some type of rod to capture or eat away at the field.

I found this from a old online document.

"An electric field is stopped by ground and can be shielded by enclosing it within a grounded metallic object. Trees, bushes, houses, etc., will also significantly attenuate electric fields.3Two electric fields may tend to add together or cancel each other depending upon whether they are in the same or opposite directions.4Electric fields will induce a charge on ungrounded metallic objects within the field. Thus, electric utilities take precautions to ground fences and metal buildings that are very close to transmission lines"
 
I would try bonding the metal roof to a single standard length ground rod.
Then bring a wire from a shallow ground rod on the side of the house opposite to the high voltage line just for reference purposes. Measure the voltage between this wire and the ground rod that was bonded to the roof. That way you can see what you are up against to determine if more extensive measures are needed. Measuring the current on the wire bonding the roof would be helpful to understand the situation. Start with a standard clamp meter on the most sensitive scale, and then use a leakage type clamp meter if a higher sensitivity is necessary to observe the level of current flowing.
 
synchro said:
I would try bonding the metal roof to a single standard length ground rod.
Then bring a wire from a shallow ground rod on the side of the house opposite to the high voltage line just for reference purposes. Measure the voltage between this wire and the ground rod that was bonded to the roof. That way you can see what you are up against to determine if more extensive measures are needed. Measuring the current on the wire bonding the roof would be helpful to understand the situation. Start with a standard clamp meter on the most sensitive scale, and then use a leakage type clamp meter if a higher sensitivity is necessary to observe the level of current flowing.
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Then when your done power your landsacpe lights off it. :)
 
tortuga said:
Then when your done power your landsacpe lights off it. :)
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I've heard of cases where people were taking power from metal structures near transmission lines. I think something would be needed to limit the voltage though. Maybe a small transformer or reactor that goes into saturation at a low enough voltage, assuming that there's enough current to energize it. Possibly something solid state but it would have to be rugged enough. Of course, I don't approve of this. 😆
 
There is shopping center in Atlanta that has a tv broadcasting antenna in the parking lot. All of the stores are ran in emt, no mc or bx allowed. All of the fittings and boxes are spot welded together. The inspector said a loose locknut could cause the signal to be reflected back to the tower. I would not want to work in that area for a very long time!
 
augie47 said:
New construction. Home is being built within 75ft of a TVA high voltage line (161kv)...construction workers were being shocked by the metal roof. Voltage measurement to ground is around 500v. Would bonding the roof to ground rods eliviate the problem ??
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Are they getting shocked when they work on the roof, or are they getting shocked when they are doing work near the roof and happen to touch it? Is the problem (for example) when they are standing on a metal ladder working on the side of the house?

You mention 500V to ground from the metal roof. Have you measured the current flow when you connect the metal roof to a ground rod?

I suspect that bonding the roof to a ground electrode is likely to help, but you will need to ensure that all relevant bits of metal are bonded together.
 
My understanding is the workers are on metal ladders working near the metal roof and receive shocks when they come in contact wit the roof.
No current measurements have been yet as of now,.
 
hillbilly1 said:
There is shopping center in Atlanta that has a tv broadcasting antenna in the parking lot. All of the stores are ran in emt, no mc or bx allowed. All of the fittings and boxes are spot welded together. The inspector said a loose locknut could cause the signal to be reflected back to the tower. I would not want to work in that area for a very long time!
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That seems strange. Do semi trucks or cars/vans park in the parking lot there? Are they not large reflectors? Seems like TV now is mostly in the UHF band, so it doesn't take much length to get a half wave or full wave reflector.
 
hbiss said:
So, now inspectors are broadcast engineers?

-Hal
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The inspector may be working for the tower owner or their engineers. My understanding is that semiconducting junctions from loose connections near RF sources can cause significant interference problems....
 
Can cause rectification (detection) of the RF which results in harmonics above and below the original carrier. They could be considered illegal emissions outside of the main carrier if it's able to travel far enough from the passive radiator. That would result in fines from the FCC.

Probably a good reason why you put transmitters on top of mountains or in the middle of fields, not in the middle of parking lots in congested areas. 😖

-Hal
 
Not likely they are getting hit with 500 v or there would probably be more repercussions from that high voltage. I would try bonding the roof to the grounding electrode system and see what happens
 
winnie said:
The inspector may be working for the tower owner or their engineers. My understanding is that semiconducting junctions from loose connections near RF sources can cause significant interference problems....
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Inspector was working for Dekalb county. I was walking into a Havertys with a roll of mc on my shoulder to cut in an outlet on a display. He just happened to drive by at that very moment, and asked what I was fixing to do, when I told him, he said no I was not! LOL! He explained why. He was actually there to bust another electrical contractor doing a remodel on another store in the shopping center without a permit. Found out later, his son worked for that contractor previously, and they only used him as a go for, and never taught him anything.
 
hbiss said:
Can cause rectification (detection) of the RF which results in harmonics above and below the original carrier. They could be considered illegal emissions outside of the main carrier if it's able to travel far enough from the passive radiator. That would result in fines from the FCC.

Probably a good reason why you put transmitters on top of mountains or in the middle of fields, not in the middle of parking lots in congested areas. 😖

-Hal
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They probably built the shopping center around the transmitter. It was one of the oldest stations in Atlanta, and the shopping center was probably built in the 60’s.
 
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