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Grounding of generators


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#1 JD Hartman

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Posted 22 February 2010 - 04:13 PM

I realize that the answer to this will be different for NY vs. Calif., etc., so this is more of general inquiry. In NYC, NY, it's common practice to ground a generator to the nearest hydrant. You're putting blind faith into the network of iron pipes and unknown soil composition(s) to provide a low resistance path for your generator ground. Without taking a reading with a megger, how can you be certain you are successful? I was on a shoot with a boatload of poorly maintained lights, many of which had housing which had become live. Would we have been safer, removing the hydrant ground, thus isolating the supply from the earth?
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#2 John Sprung

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Posted 23 February 2010 - 02:22 AM

Where there's no other infrastructure available, some guys just drive a 3 ft. piece of rebar into the ground, and find that it helps a bunch with the nuisance shocks. If you try to float, capacitance will give your system some random and shifting reference to ground, and you'll get little shocks sometimes. But the impedance is so high you don't get more than an irritating amount of current usually. The danger in that case is mainly from hurting yourself as you jump back in reaction to the shock and yell or mutter "s--t!"

No matter what kind of ground rod you have, it'll never have low enough impedance to trip an overcurrent breaker. The little stick of rebar in reasonably moist ground is enough to make your GFCI's work. To be really careful, use more than one, one at the generator, and another at the far end of your main feed, where you're working.





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#3 Paul Bruening

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Posted 23 February 2010 - 04:00 AM

Hey John,

This reminds me of a related thing from almost thirty years back. I had a buddy whose father was an electrician for a department store chain. He did the installations for their new stores. If they were in a situation where they didn't want to leave bars in the ground, especially, copper they used something called "whiskers". They burned the insulation off scrap wire to sell to the recyclers. Some of that bare wire they made into something that looked like a long, old fashioned, straw broom: a bunch of bare strands loose on one end and clamped to a short copper bar on the other. They laid it on the dirt with the ends splayed out. Then, they poured a jug of water over the dirt and stomped the bare wires into the mud. At lunch time someone would have to go out and pour some more water on it.

Does that sound stupid or smart?
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#4 John Sprung

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Posted 23 February 2010 - 09:03 PM

Does that sound stupid or smart?


For a temporary, probably OK, though not as good as driving a ground rod. For permanent installation, definitely not to code.





-- J.S.
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#5 Guy Holt

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Posted 24 February 2010 - 06:39 PM

For a temporary, probably OK, though not as good as driving a ground rod. For permanent installation, definitely not to code.


There is a parking garage under one end of Boston Commons that inhibits the driving of ground rods more than a couple of feet. So what we do is to dig a shallow 10ft long trench and bury the ground rod in it. It must meet code because it satisfies the Electrical Inspectors in Boston.

- Guy Holt, Gaffer, ScreenLight & Grip, Boston
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#6 John Sprung

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Posted 24 February 2010 - 07:43 PM

True, if you can't go vertically because you hit ledge or something, code lets you put the rod horizontally, as deep as possible. But it has to be 10 ft. of solid rod or type K copper pipe. Some ordinary wire tied up to look like a broom wouldn't fly.




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#7 timHealy

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Posted 26 February 2010 - 07:33 PM

Any of you guys try and pound a piece of rebar into the ground or dig a ten foot trench in Manhattan lately?
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#8 timHealy

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Posted 26 February 2010 - 07:46 PM

I realize that the answer to this will be different for NY vs. Calif., etc., so this is more of general inquiry. In NYC, NY, it's common practice to ground a generator to the nearest hydrant. You're putting blind faith into the network of iron pipes and unknown soil composition(s) to provide a low resistance path for your generator ground. Without taking a reading with a megger, how can you be certain you are successful? I was on a shoot with a boatload of poorly maintained lights, many of which had housing which had become live. Would we have been safer, removing the hydrant ground, thus isolating the supply from the earth?


Hey JD,

In your situation you probably did the right thing and long as the electricians weren't working in pools of water. But I don't think grounding to a hydrant is a risky thing. Hydrants are attached to the NYC water supply and I see firemen walking around neighborhoods all the time cracking them to make sure they are in working order. As long as you are not in the ass end of Brooklyn or the Bronx where everything is dilapidated. Lose the vendor if you can.
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#9 Paul Bruening

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Posted 26 February 2010 - 08:00 PM

His "Whiskers" were all covered in mud. I'm guessing it was easier than digging and was reusable.
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#10 John Sprung

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Posted 26 February 2010 - 10:07 PM

Actually, the fire hydrant in any condition is likely to be way better than any ground rod you could drive. The pipe it's connected to is way longer than 10 ft., and way bigger than 5/8" diameter.





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#11 Brian McGee

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Posted 05 March 2010 - 04:03 PM

I realize that the answer to this will be different for NY vs. Calif., etc., so this is more of general inquiry. In NYC, NY, it's common practice to ground a generator to the nearest hydrant. You're putting blind faith into the network of iron pipes and unknown soil composition(s) to provide a low resistance path for your generator ground. Without taking a reading with a megger, how can you be certain you are successful? I was on a shoot with a boatload of poorly maintained lights, many of which had housing which had become live. Would we have been safer, removing the hydrant ground, thus isolating the supply from the earth?

Ground at the source only, generator ! Grounding at a distro point can defeat the main breaker on generator.
For verification, put one end of your volt meter on a hot leg and the other to the ground rod, It will show line voltage if it is right.
Also, a good ground will show less than 200 ohms to anouther "known Ground" at 6 feet .
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#12 John Sprung

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Posted 05 March 2010 - 05:12 PM

Ground at the source only, generator ! Grounding at a distro point can defeat the main breaker on generator.
For verification, put one end of your volt meter on a hot leg and the other to the ground rod, It will show line voltage if it is right.
Also, a good ground will show less than 200 ohms to anouther "known Ground" at 6 feet .


Hmmm -- If you mean bonding neutral to ground, then, yes, it should happen exactly once, at the source. You don't want current flowing in the safety ground, or offsetting its voltage even a tiny bit. And you never want to loop around a GFCI, cause it'll never do anything but trip.

I don't see how bonding the safety ground to additional rods in the earth is anything but good -- or how it could affect an overcurrent breaker, as the impedances are way too high. At our previous house, I put in two separate ground rods, at opposite corners of the building, and Building and Safety had no problem, thought it was a good if redundant idea. You're actually required to drive a separate ground rod if you feed a separate building such as a detached garage more than 5 ft. from the main house.

I'm not sure about that 200 Ohm rule of thumb. Code says 25 Ohms or less, or you have to drive a second rod. But that's not done with an Ohmmeter. Official ground rod testing is a bunch more complex, with meggers and fall-of-potential and stuff:

http://ecmweb.com/gr...ing_techniques/

That web site has some new info, or at least stuff from after the last time I had to mess with ground testing.





-- J.S.
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#13 Dan Witrock

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Posted 27 May 2010 - 08:23 PM

What is the best way to attach the generator to a fire hydrant? From a plant to even a 6500w little guy.

Thanks

-Dan
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#14 Phil Rhodes

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Posted 27 May 2010 - 09:08 PM

To add a little international perspective.

Time and again I have been working as a video assist operator, usually doing visual effects previsualisation, and I've ended up drawing inch-long sparks between BNC connector backshells. This usually happens when I have my cart running from generator mains, and a remote monitor on battery power, and it happens because they almost never bother grounding generators.

Yes, you read that right, electricians in London habitually fail to ground generators. At all. Using anything. They don't even try.

Their line usually is that it's impossible to do it to regulation standard, on a so they simply don't bother. Of course as a practical matter, throwing the earth rod down a drain or tying it to a lamp post or even just rolling the generator truck over a stake laid in a puddle is a fairly effective way of bleeding away the leakage from all those rusty old HMI ballasts. Conversely, trying to make me ground the entire lighting rig using a bit of 75-ohm coax and a magliner that happens to be in contact with a wrought-iron fence is an extremely bad way of doing that, and leads to me jumping up and down and shrieking obscenities.

Now, I don't want to give anyone the impression that British film electricians are particularly lazy or self-obsessed or liable to work precisely to rule when it suits them and then lounge around smoking when it doesn't, but in fact they are. Therefore while I appreciate that shoving the earth rod in a fish pond represents valuable time not spent hanging around the truck reading the paper, it does save anyone who touches a monitor quite a lot of pain.

Therefore, feel free to throw some frayed cable in the mud or clamp onto a fire hydrant or do whatever it is you do, but please, do something.
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#15 JD Hartman

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Posted 28 May 2010 - 11:51 AM

What is the best way to attach the generator to a fire hydrant? From a plant to even a 6500w little guy.

Thanks

-Dan


C clamp with the ground wire terminated in a lug and bolted firmly to the body of the clamp. Usually the clamp will fit on one of the hydrant spigots (whatever the proper term is for where you attach the hose).
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