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Author Topic: Temporary grounding  (Read 5215 times)

John Roberts {JR}

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Re: Temporary grounding
« Reply #10 on: April 03, 2016, 05:23:29 pm »

Another thing to consider when pounding 8 or 10 ft. ground rods into every city park, corporate parking lot, or back yard where you have a gig is what it might hit under ground. Maybe you just bust the sprinkler system or maybe you puncture a gas line and take out a city block. (Gas can flow along the outside of a damaged pipe and into confined spaces.) So, I agree, once it's in leave it there for next year's event, but a shallow grounding solution sounds very attractive to avoid this risk.

Best,

--Frank
Funny a few years ago when I had my home's water mains pipe replaced, the plumber used manual labor to dig the trench, because to use a machine digger he would have to file for permission with the utilities and perhaps weeks of delay before my water was restored.
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Fast forward to about a month ago a contractor (working for the power company) that was dropping in some new light poles to add lights around a town jogging/walking track, placed one new pole smack in the middle of the town water main.. We had dirty water for a few days.

Seems they should have known better. I busted the power company worker I knew (from playing basketball with him), and he blamed it on the sub contractor.  :o

JR
   
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Ike Zimbel

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Re: Temporary grounding
« Reply #11 on: April 03, 2016, 07:28:10 pm »

The plan would be to use this as a temp generator ground for a few days at most. And the dirt around it could be kept wet by dumping a gallon of water on it every day. I get a little worried when I see someone driving an 8-ft ground rod with possible underground pipes and wires.


True story: I did an outdoor festival once, in Montreal, which was set in the parking lot of a race track. Due to a scheduling "thing" we didn't get the PA until the morning of the event, and when we finally got that flown and powered up, not long before our noon start time, it was humming and buzzing like mad. Long story short, when we finally tracked down where the generator was grounded, we found that the providers had just jammed the ground rod in a pile of gravel back stage. I had someone go and pour a bottle of water on the ground rod and you could actually hear the gurgling and splashing of the water as it modulated the ground noise in the PA. Problem was solved when the ground was tied to an existing ground rod at the base of a lamp standard...which was right next to the pile of gravel!
iz
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Dennis Wiggins

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Re: Temporary grounding
« Reply #12 on: April 03, 2016, 08:00:29 pm »

...An 8-ft ground rod will typically measure around 25 ohms impedance if it's in decent shape in conductive soil, and up to 100 ohms if the soil is really dry. It will be interesting to see exactly what impedance a dozen 12" spikes in wet ground will have.   

25-100 Ohms between the ground rod and what?  I know that sounds like a dumb question, but is it to a known good POCO ground?

-Dennis
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Frank DeWitt

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Re: Temporary grounding
« Reply #13 on: April 03, 2016, 08:37:25 pm »

I went to an accident where a fire truck had rolled off the side of a country road into a creek (it was on it's side in the water.)  When the dozer turned up I thought it was going to make a mess of the truck pulling it back up the hill.

The dozer driver proceeded to shift the level of the roadway down to the fire truck, and then flip the truck neatly back on it's wheels.

Farmers think in ways mere mortals can't.  Farmers can also do things that mortal's can't.

It was a farmer, not an engineer who figured out how to make a hay baler tie a knot.  Think about it.  How would you make a machine tie a knot.   I also have a video of two red necks (related to farmers) working together to lift a big antique generator onto a low boy truck.  One was driving a tree picker used for logging and the other a oversized tow truck.  Each one lifted one side and that generator went straight up with not so much as a wiggle. Backed the low boy trailer under it and they set it down just as nice.
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Mike Sokol

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Re: Temporary grounding
« Reply #14 on: April 03, 2016, 08:38:30 pm »

25-100 Ohms between the ground rod and what?  I know that sounds like a dumb question, but is it to a known good POCO ground?

-Dennis

Great question. There's two ways to measure this. The most common way is something called a fall of potential test. Basically, you drive a second short ground rod some 100 ft away from the ground rod under test. Then a known current is induced between the test rod and the ground rod. A third test rod is put in the earth between the other two rods. This one measures the voltage drop as its moved between the two other rods at 10 ft intervals. Sounds more complicated than it is, but it does take some time to do properly, and the ground rod under test needs to be disconnected from any building grounds. Basically you need to take voltage measurements at measured intervals to make sure it graphis as a proper S-Curve. Then you can calculate the ground rod impedance. See first diagram below. A "Fall of Potential" tester costs around $500 or so. I've got a B&K one on my desk.

The second type of tester uses a high frequency current in a special type of clamp-meter to inject current into the ground rod under test while it's tied into the POCO's grounding grid. So it compares the impedance of your single ground rod to all the millions of other ground rods on the bottom of power poles. These testers cost around $1,500 or so and I've been begging for one, but still no go. That's what I want to use to test my Bed-O-Nails design since it doesn't require all the graphing work.

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Lee Buckalew

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Re: Temporary grounding
« Reply #15 on: April 03, 2016, 08:40:47 pm »

Another thing to consider when pounding 8 or 10 ft. ground rods into every city park, corporate parking lot, or back yard where you have a gig is what it might hit under ground. Maybe you just bust the sprinkler system or maybe you puncture a gas line and take out a city block. (Gas can flow along the outside of a damaged pipe and into confined spaces.) So, I agree, once it's in leave it there for next year's event, but a shallow grounding solution sounds very attractive to avoid this risk.

Best,

--Frank

Yes, we always had the stage site set up for utilities locate and also confirmed if we could leave a rod in place.

Lee
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Mike Sokol

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Re: Temporary grounding
« Reply #16 on: April 03, 2016, 08:44:18 pm »

Yes, we always had the stage site set up for utilities locate and also confirmed if we could leave a rod in place.

Lee

Yup, you really don't want to be driving in 8 ft ground rods without the Utilities giving you the all clear.
« Last Edit: April 03, 2016, 10:36:45 pm by Mike Sokol »
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Mike Sokol
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Re: Temporary grounding
« Reply #16 on: April 03, 2016, 08:44:18 pm »


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