Over Head Grounding Grid at Home Depot

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tom baker

tom baker

First Chief Moderator & NEC Expert
Staff member
Location
Bremerton, Washington
Occupation
Master Electrician
In Tumwater Washington State, a Home Depot has part of its parking lot under high voltage (perhaps 300 KV) lines. There is a overhead "grounding" grid under the lines. Charlie (moderator) said its to reduce the induced voltage in a shopping cart under
the lines. Take a look:
http://i42.photobucket.com/albums/e334/TomBakerPhotos/GroundingGridfromStore.jpg
http://i42.photobucket.com/albums/e334/TomBakerPhotos/Luminaireoverheadground.jpg
http://i42.photobucket.com/albums/e334/TomBakerPhotos/CarsunderOHP.jpg
http://i42.photobucket.com/albums/e334/TomBakerPhotos/OHPandParkingLot.jpg
 
Re: Over Head Grounding Grid at Home Depot

The overhead ground grid will also make sure that any "falling line" will have a return path for fault clearing.
 
Re: Over Head Grounding Grid at Home Depot

Originally posted by tom baker:
...its to reduce the induced voltage...
Click to expand...
Having been hit by an induced 25kv line, I can understand...BUT...

Why the heck was the lot graded so high?

I can just see some new store manager in 5 years telling some new hire to "...go hang some banners/signs from that stuff..."
 
Re: Over Head Grounding Grid at Home Depot

Jim, thats what I thought orginally. I sent the pictures to Charlie E (Moderator-POCO engineer) and his coworkers had done an installation similar and it was to reduce induced voltage underneath.
 
Re: Over Head Grounding Grid at Home Depot

Man, I have never seen anything like this before. To be honest, I would be a little afraid to get out of my vehicle! :eek:
 
Re: Over Head Grounding Grid at Home Depot

The two things that strike me is that the arrangement of the wires doesn't look very uniform, I would expect something that more closely resemembled an actual grid. The supporting columns also look massively over-built.

Neat, though. I've never seen anything like that. Have heard stories about people getting nipped while doing ballast changes on poles under HV lines.

-John
 
Re: Over Head Grounding Grid at Home Depot

Tom,

They are the experts. I was just postulating another use.
 
Re: Over Head Grounding Grid at Home Depot

Most don't even see it, they are pulling into the store to get that 2x4.

I may go back with a floresent lamp and walk around to see if it will light up from induction in different locations under the HV lines.
 
Re: Over Head Grounding Grid at Home Depot

Interesting, makes me wonder what math was involved on the grid spacing. I have designed a few grids in my days, and we design them on 1/4 wave lengths. Care to guess what the deminsions of a 1/4 wavelength is at 60Hz. Hint, not measured in inches, feet, or yards, more like miles. :eek:
 
Re: Over Head Grounding Grid at Home Depot

IN THEORY - The Numbers Are :
Meters = 300 Divided by Frequency in MHz
Feet = 984 Divided by Frequency in MHz
Inches = 11,811 Divided by Frequency in MHz.

In Theory:
984 / .000060
= 16400000 feet / 5280
= 3106.06 miles x .25
= 776.5 miles = ? wave lenght

IN PRACTICE - {The-Real-World} - The Numbers Are :
Meters = 285 Divided by Frequency in MHz
Feet = 936 Divided by Frequency in MHz
Inches = 11,235 Divided by Frequency in MHz.


In the real world:
936 / frequancy (.000060)
Wave lenght @ 60hz
= 15,600,000 feet / 5280
= 2954.55 miles x .25
= 738.6 miles = ? wave lenght

Must be one big parking lot, or very big loading coils :eek:
 
Re: Over Head Grounding Grid at Home Depot

The sad thing is there are many schools across the country that have been given property to use for play grounds and athletic fields from power company's that the same problem exist. I looked at a job last summer for a high school athletic field that had 300KV+ lines running right through the middle of it, "It was donated by the POCO" :roll:

Also many parks are also done this way.
 
Re: Over Head Grounding Grid at Home Depot

Originally posted by bphgravity:
Man, I have never seen anything like this before. To be honest, I would be a little afraid to get out of my vehicle! :eek:
Click to expand...
If you look at the picture, I only see one brave soul with a car under those lines!!

I think these lines are really there to zap anyone who parks in a handicap spot without a sticker.

Steve
 
Re: Over Head Grounding Grid at Home Depot

Some brilliant guy took a well known quote and made it more precise and to the point:
Perception manifests itself as though it were reality.
Click to expand...
The perception a customer will have, while parking or walking to the car under that grid, is that something dangerous is going on overhead. The truth may be that this grid is there to provide a level of safety. But that truth is absolutely going to take a back seat to the perception.

That was a bad marketing idea, a bad aesthetic choice, and (IMHO) a bad technical solution as well. If there were a real, measurable, hazard from the overhead lines, then other solutions should have been given priority.

OK. I'll admit it. The brilliant guy that made up that quote was me. :cool:
 
Re: Over Head Grounding Grid at Home Depot

That quote is why I like graphic warning signs. It gives the impression that things can get real bad if you don't follow the warning.

is



is
 
Re: Over Head Grounding Grid at Home Depot

In case you didn't realize it, Tumwater is next to the State Capital, City of Olympia.
And the main office for our state electrical is just across I-5 from this location.

Tumwater was orginally famous for Olympia Brewing and to its artesian water.

The utility here - most likely Bonneville Power - had an easement for the primary, the overhead ground was probably a condition of use.
 
Re: Over Head Grounding Grid at Home Depot

I bet it helps keep the birds away too.

At marinas, we often pull sting from one piling to another so the seagulls won't get on the docks. When they come in for a landing, they catch the site of the string, but can't seem to focus and they fly away. I would have never believed it until I saw it.
 
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