Voltage on a Metal Roof

[Dennis Alwon]

A member of our local Association called me and said he had 40 volts from roof to ground rod. He said the gutter guys were there to install the gutters and quit because they were getting zapped.

So the member pulled the meter and disconnected the load side neutral from the meter base. Still power on the roof. I told him that maybe something was coming in on the water lines since it was city water.

He added that there was high tension wires near the home. I didn't think that could be an issue especially enough to zap someone but we have all seen fluorescent light bulbs light under the wires.

Anyway, the next day another member of our group volunteered to take a look especially since he is an electrical engineer. The voltage on the roof that day was 250 volts. I assumed stray voltage but you can't get zapped from stray voltage- I thought.

Anyway, they took a wire connected it to the roof and then to a ground rod and problem was solved.

That is all the info I know but I was curious if anyone has had that experience. Can you really get 240v from high tension wires? The power company stated that at certain times of the day the voltage would go higher and lower.

If this is true why would they allow a home to be built by those wires.

 

[LarryFine]

I wonder if the house has a hip roof with the gutters in a continuous loop?

 

[Dennis Alwon]

I wonder if the house has a hip roof with the gutters in a continuous loop?

What would that make a difference? I suspect most roofs are continuous.

 

[tortuga]

A coworker ran into this before, same exact situation hi-tension wires + energized metal roof / gutter.
I wonder if the homeowner could use the power for anything?

 

[MCosentino87]

A member of our local Association called me and said he had 40 volts from roof to ground rod. He said the gutter guys were there to install the gutters and quit because they were getting zapped.

So the member pulled the meter and disconnected the load side neutral from the meter base. Still power on the roof. I told him that maybe something was coming in on the water lines since it was city water.

He added that there was high tension wires near the home. I didn't think that could be an issue especially enough to zap someone but we have all seen fluorescent light bulbs light under the wires.

Anyway, the next day another member of our group volunteered to take a look especially since he is an electrical engineer. The voltage on the roof that day was 250 volts. I assumed stray voltage but you can't get zapped from stray voltage- I thought.

Anyway, they took a wire connected it to the roof and then to a ground rod and problem was solved.

That is all the info I know but I was curious if anyone has had that experience. Can you really get 240v from high tension wires? The power company stated that at certain times of the day the voltage would go higher and lower.

If this is true why would they allow a home to be built by those wires.

Sooo you're saying if I live close to a right of way and put a metal roof on my shed, I can just connect my sub panel to my roof?
No seriously, I saw a video on youtube not long ago of a farmer connecting a pretty large capacitor to a metal fence running underneath some high tension lines. He was able to charge it enough to produce a decent arc! Here's the link if you're interested.

 

[LarryFine]

What would that make a difference? I suspect most roofs are continuous.

I'm picturing a closed loop behaving as a transformer secondary.

 

[Dennis Alwon]

I forgot that he also measured only .6ma

 

[ptonsparky]

I've had a shielded cable laid out under primary, prior to installing, that would wake you right up.

 

[tortuga]

Sooo you're saying if I live close to a right of way and put a metal roof on my shed, I can just connect my sub panel to my roof?
No seriously, I saw a video on youtube not long ago of a farmer connecting a pretty large capacitor to a metal fence running underneath some high tension lines. He was able to charge it enough to produce a decent arc! Here's the link if you're interested.

Interesting well 0.6ma at 250 volts 24/7 is only 1.3 kwh per year basically nothing,
but perhaps if they got copper gutters and could get it up to .6A that would be 131 kwh / year

 

[Little Bill]

Similar story, only in that involved voltage on a metal roof.
I was working at a property a few years back and the GC came and told me the painters were getting shocked when they placed their aluminum ladders against the metal roof. I figured maybe because the roof was hot and they thought is was a shock. They said they even saw sparks.
So I connected a wire from the ground bar in an outdoor subpanel and ran it up to the roof. I connected one lead of my meter to the wire and the other to the roof. I measured 120V!
I fixed a clamp on the wire and had the painter clamp it to the roof. I saw sparks then for sure.

I fixed the wire and meter so I could read it from the ground. I then started turning breakers off in the subpanel to see where the voltage might be coming from. I turned off every breaker and the voltage was still there. So I went to the panel feeding the sub and turned off the feeder breaker. The voltage went away after that. Since the sub was powering pool equipment, I left the sub off until I could check further.

The property owners had previously asked me to replace the two pumps at the pool equipment. So as I was going to remove the old pumps, I decided to see what they would do when turning on the subpanel and breakers to each. When one of the breakers to one of the pumps was turned on, the breaker tripped. So I thought I would replace the pumps and then check to see if the voltage went away on the roof. I saw, when removing the pumps, that the EGC connection was corroded, and the equipotential bond wire was not connected to the pumps. So I made sure to connect them to the new pumps. After completing the pump install, I checked for voltage on the roof and it was gone.

The only thing I could think of that caused the voltage was the gas line for a pool heater was picking up voltage from the bad pump. There was a stove in the upper room of the building with a metal flu going through the roof and energizing the metal roof. Replacing the pumps and connecting the bonding wire made the voltage disappear. so I don't know if it was just the pump, or combination of bad pump and missing bond wire.

 

[Fred B]

Sooo you're saying if I live close to a right of way and put a metal roof on my shed, I can just connect my sub panel to my roof?
No seriously, I saw a video on youtube not long ago of a farmer connecting a pretty large capacitor to a metal fence running underneath some high tension lines. He was able to charge it enough to produce a decent arc! Here's the link if you're interested.

Seen too that you can cause a fluorescent tube light to light up holding one end and pointing the other end up toward the overhead high voltage lines. Also seen someone use the same principle to energize their electric fence.

But unlikely to be that much to cause the issue with the OP roof. Most likely a fault issue somewhere, and might even be at a neighboring house.

Had similar situation with a gas stove with no electric, fully energized, non contact tester ringing at between 2-3 ft from the appliance and a switch strap that had 240V to remote earth. Traced to a j-box that had exhaust fan and another circuit running through it. Removed the exhaust fan wire from in the box (it wasn't working anyway) Now the voltage was limited to the switch strap, Found a damaged wire shorted to ground and open neutral on the shorted nm. Found out that a close lightning strike had occurred shortly before the HO was getting shocked off the light switch. Had Volt readings over different testing times of between 60V to as high as 320v on the strap to remote grounded source (not sure how getting that high of a reading). First though was an open neutral to the utility but No such voltage at the main panel. Did find a second j-box with 2 more circuits that did have a common connection to the first j-box. Never got a chance beyond getting the voltage to go away by disconnecting the shorted wires, but never "fixed it as the HO didn't want me to come back as he had no money to pay.

 

[kwired]

I'm assuming the "high tension" mentioned is like over 100kV?

Around here lines with that high of voltage just never run that close to buildings. For most part even lower transmission voltages would be a little rare to be that close to a building as well but maybe not totally unheard of either. Local distribution lines less than 40 kV are seen right next to or even over buildings at times though.