Fun with inductance

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  #1  
Old 03-28-17, 06:08 PM
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Fun with inductance

Not really a question which is why I put this here.

I was working on some building wall packs in our smaller bucket truck the other day. I normally wear light Nitrile foam gloves for both grip and minor protection from cuts and scrapes to my hands. Along the back of the building are some big power lines. Based on the size and height I would guess 161 - 345 KV. While working on a wall pack facing the lines I kept feeling what seemed to be little shocks on the back of my hands where there is no Nitrile. Then I noticed when getting close to the light with my screw gun I could hear arcing.

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So I got out my meter, put one probe on the screwdriver metal shank which I was holding and the other probe to the light. You can see in the picture below the voltage that is being induced by the high voltage power lines. Not enough to kill you or really hurt, it was just a little annoying. (BTW that is 61.5v)

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Old 03-28-17, 06:30 PM
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That picture looks like one of the buildings I worked at. A row of self-owned business condos.

I've worked on several jobs almost directly under high voltage lines. Some on a ladder and some in a bucket truck. You have to be very careful what you are doing. We ran a conduit line around the parapet of a building. The run had to be kept grounded while we were working. It was really interesting pulling wire in that pipe.

Any metal parallel to the power lines becomes highly charged.... especially at 15' above the ground.
 
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Old 03-29-17, 07:14 AM
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Many years ago, IEEE spectrum magazine ran an article about a firm that uses helicopters to work on (live!!) high tension power lines, mostly replacing spacers.

The pilot and the tech have to wear chain mail type suits to equalize the potential across their bodies, otherwise the induced potential difference across their bodies would be enough to electrocute them. The tech is strapped out on the skid of the chopper and as the chopper approaches the line, the first thing he does is bond the chopper to the line with a flexible leader so there is no arc to the chopper. Then he does the job, unbonds the chopper and they are on to the next spacer.

There is no way in $#@@% I would do that job! Just the pictures in the article made me queasy.

Title of the article is "Working hot: life at 765 kV", but I think you have to be an IEEE member to access it.
 
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Old 03-29-17, 08:53 AM
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CarbideTipped. Is this what you're talking about?

https://www.youtube.com/watch?v=x94BH9TUiHM

Scary stuff!
 
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Old 03-29-17, 09:35 AM
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Yes! That's it, thanks for that link!

I suppose like anything else, you get used to the work and it becomes routine, but not for me
 
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Old 03-29-17, 10:39 AM
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Apocryphal story, but family in up-state New York mention that as kids, watching trucks or rail cars leaving the GE / Sylvania fluorescent bulb plant was fun, big bright flash as they
went under the Niagara generating plant's high-tension wires...

Hadn't really thought of it before, but looking at the parallel freight rail in the photo,
and what appears to be a perpendicular freight rail and bridge in the background,
what sort of inductance do you get from a 25 ton steel box car doing 75 mph?
 
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Old 03-29-17, 07:36 PM
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I would doubt there is any induction to the tracks and probably very little to the cars.
The tracks are metal and grounded. The cars are pretty much metal and sitting on grounded tracks with metal wheels.
 
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Old 03-29-17, 07:54 PM
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That's a job where the pilot needs to be dead on accurate.

I was looking at the reports that they were entering into the computer...
Gunshot occurrences 5 -- bullets embedded in cable.

I'm curious about one thing.... they leave the helicopter work platform and travel along the wires to work on the "grounded" metal tower.
 
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Old 03-29-17, 08:26 PM
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Do they ever get on the tower though? From the video it looks like they're always on the lines.
 
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Old 03-30-17, 05:20 AM
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Originally Posted by PJmax
I would doubt there is any induction to the tracks and probably very little to the cars.
The tracks are metal and grounded. The cars are pretty much metal and sitting on grounded tracks with metal wheels.
Agreed, but was thinking about anecdotes of freight rails under power lines wearing out faster.

Electrified passenger rails have those problems with stray current-
Urban Decay Arriving on Track 11 | DiscoverMagazine.com
"One amp going through for one year will corrode about 20 pounds of steel."



Seems like 25 tons of steel (per car) moving through the electromagnetic field under the towers would push quite a bit of current through the system...
 
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Old 03-31-17, 06:14 PM
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While the voltage induced into the (grounded) lights in my story, I do not think there was enough current available to do anything.
 
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