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Author Topic: Portable Generator Grounding  (Read 12315 times)

Mike Sokol

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Re: Portable Generator Grounding
« Reply #10 on: March 15, 2014, 01:37:52 pm »

The OSHA reference specifically applies to construction sites-which of course would be a major market for genny manufacturers. And a 2 wire floated neutral would be safe, in that you really don't have a neutral per se.

If you floated the neutral on a 3 wire 240 volt system, a hot to ground fault could result in parts of the system being 240 volts to ground-obviously a much more hazardous situation.

That's correct. But the OSHA reference is what drives the generator manufacturers to float the neutrals on their gennys under 5KW, even though I don't consider that safe for RV power or music stage power. And I do believe that's why there's so much confusion on the subject of floated neutrals and no ground rods for portable generators. Because ground rods are not required for construction generators, and are too much trouble for RV camper sites, it's assumed that they're not required for anything else. And because generators under 5KW can eliminate GFCI protection if their float the neutral, then that's the design default. Again, I don't consider floated neutrals without an earth ground on a portable generator to be safe for distributed power systems used for a music stage. There's just too many ways it can go bad in a hurry.   
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Stephen Swaffer

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Re: Portable Generator Grounding
« Reply #11 on: March 16, 2014, 05:58:59 pm »

Somehow my post was incomplete.

It seems that the intent of OSHA and the NEC is to allow cord and plug connected equipment to be run on a 2 wire non-bonded genny without a ground rod.  Cord and plug connected being what you can go down to your favorite store buy, and literally plug and play with virtually no interconnections.  This scenario might fit a very small gig-2 speakers on a pole etc.

IMO a bonded neutral is always safer for A/V work and a ground rod is a good idea especially in wet or damp conditions.

To tread on questionable ground here, I wonder what the best grounding scheme is for portable A/V.  For installs we drive 2 8 foot ground rods, an 8 foot rod is a pain to pull out  (OK 7 foot 6 inches is a pain-when you hit an old foundation!)  I wonder, especially when wet if multiple 4 or 6 footers might be easier and just as effective?  Just thinking in print here-so don't get too upset with me!
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Mike Sokol

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Re: Portable Generator Grounding
« Reply #12 on: March 16, 2014, 06:54:07 pm »

Somehow my post was incomplete.

It seems that the intent of OSHA and the NEC is to allow cord and plug connected equipment to be run on a 2 wire non-bonded genny without a ground rod.  Cord and plug connected being what you can go down to your favorite store buy, and literally plug and play with virtually no interconnections.  This scenario might fit a very small gig-2 speakers on a pole etc.

IMO a bonded neutral is always safer for A/V work and a ground rod is a good idea especially in wet or damp conditions.

To tread on questionable ground here, I wonder what the best grounding scheme is for portable A/V.  For installs we drive 2 8 foot ground rods, an 8 foot rod is a pain to pull out  (OK 7 foot 6 inches is a pain-when you hit an old foundation!)  I wonder, especially when wet if multiple 4 or 6 footers might be easier and just as effective?  Just thinking in print here-so don't get too upset with me!

Last year I came up with a design for a temporary ground that might be quite good for portable A/V work. This was a plate about half a meter square (19" x 19" for the yanks) with 100 metal spikes about 2" (50 mm) long pointing down into the ground. The idea was to place it in front of your vehicle tire and drive over it, thus spiking it into the ground. This was made out of perforated (expanded metal) so you could dump a jug of water over it to wet the soil, and had a lug to attach the generator ground. It also had a place to insert a handle to lever it up out of the ground once the gig was done. I never built one, but it would be an interesting experiment to see if it would pass the NEC 100 ohm max impedance listed for 8-ft ground rods before you needed a second rod. I have a fall-of-potential ground rod tester so if I did build one I could get a reading in my back yard to see if it works at all. Of course, this would drive inspectors crazy unless it had official NEC sanction, but you get the idea. 
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TJ (Tom) Cornish

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Re: Portable Generator Grounding
« Reply #13 on: March 16, 2014, 10:34:10 pm »

Thinking out loud, I suspect that a multiple shallow spike solution like your "bed of nails" would work really well - sometimes.  If the ground is wet, you've likely got a nice low-impedance path.  However, a couple dry days and the top layer of soil dries out quickly.  When I dig on my property, I'm always amazed at how much moisture there is 12" down - even when it has been really dry.  I suspect a deep ground rod would have more reliable contact with the earth since it taps soil that is relatively unaffected by day-to-day weather conditions.
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Jonathan Johnson

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Re: Portable Generator Grounding
« Reply #14 on: March 17, 2014, 01:08:28 am »

It seems that the intent of OSHA and the NEC is to allow cord and plug connected equipment to be run on a 2 wire non-bonded genny without a ground rod.  Cord and plug connected being what you can go down to your favorite store buy, and literally plug and play with virtually no interconnections.  This scenario might fit a very small gig-2 speakers on a pole etc.

IMO a bonded neutral is always safer for A/V work and a ground rod is a good idea especially in wet or damp conditions.

Thing to remember is that nearly all handheld power tools marketed in the U.S. -- except for some very old ones -- are double insulated and have two wire cords with no ground conductor. Many pieces of A/V equipment are NOT double insulated, and should be grounded.

With two-wire equipment, having ground and neutral non-bonded may be irrelevant -- especially if only one tool is in use at a time. A two-wire power tool used in wet conditions could have a ground fault, and if the ground and neutral are not bonded there will be no complete circuit and the user should (theoretically) not receive a shock.

However, most genny's have at least a duplex receptacle, so one must assume that two pieces of equipment could be used at a time. Under certain conditions -- such as the example of two drills in the water -- the possibility of a ground fault shock hazard exists even without bonded neutral and ground.
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Mike Sokol

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Re: Portable Generator Grounding
« Reply #15 on: March 17, 2014, 01:47:06 am »

Thing to remember is that nearly all handheld power tools marketed in the U.S. -- except for some very old ones -- are double insulated and have two wire cords with no ground conductor. Many pieces of A/V equipment are NOT double insulated, and should be grounded.


And that's the primary reason I believe that ALL generators (portable or otherwise) powering outside stages with A/V equipment should be both Neutral-Ground bonded as well as earthed via a ground rod of some sort. Plus the stage structure and any stairs should be connected to this same G-N-E bonding point. I don't see why a small (2 or 3 KW) generator should be treated any differently than a 50 or 100 KW generator doing a show. The amount of current required to kill somebody is extremely small (20 or 30 mA) compared to the load currents available from ANY generator. Certainly a 2KW generator can kill you just as dead as a 100KW genny. I think we professionals in the music industry are so used to getting shocked, that we sometimes forget just how dangerous it can be.

Now note that I'm not trying to make extra trouble for anyone doing a show, nor do I want restrictive legislation put in place by suits in some office who have no idea how this all really works. But I can only report what the data points out, and I just don't like this floaty-ground generator thing. There's too many ways for it to go wrong in a hurry.
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Jonathan Johnson

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Re: Portable Generator Grounding
« Reply #16 on: March 17, 2014, 03:00:06 pm »

But I can only report what the data points out, and I just don't like this floaty-ground generator thing. There's too many ways for it to go wrong in a hurry.

Going back to my previous post (in which I was kind of thinking "out loud" and not sure where I was going with it), I suspect one reason for the "floaty-ground generator thing" is to avoid nuisance tripping of breakers and GFIs on construction sites. Especially here in the Pacific Northwest, builders often end up working out in the rain. Having to go reset the GFI every time you pull the trigger on your skilsaw because there's a little moisture in the motor gets old REAL QUICK. This leads to people bypassing GFI -- which in a bonded G-N-E and damp environment situation makes things dangerous. If you can eliminate the need for a GFI on a single power tool without significantly decreasing safety** ("I know! Instead of a GFI, let's float the neutral! Then there's no complete circuit and nobody gets shocked!") there will be a market for that, and market pressure will drive public policy to allow these small gensets to have floated neutrals.

I'm not advocating for floating neutrals; but I can see where they're coming from.


** In somebody's mind.
« Last Edit: March 17, 2014, 03:25:09 pm by Jonathan Johnson »
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frank kayser

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Re: Portable Generator Grounding
« Reply #17 on: March 17, 2014, 04:02:21 pm »

Ok.  Floating neutral solves a PITA.  Except in R/V where floating neutral is a PITA.


Let's look at it from a different angle:  Is there ANY situation where a floating (un-bonded neutral) is SAFER than a bonded neutral?
Is a genset with a floating neutral SAFER or less safe with a GFI?
and the opposite

Is a genset with a floating bonded SAFER or less safe with a GFI?


frank

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TJ (Tom) Cornish

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Re: Portable Generator Grounding
« Reply #18 on: March 17, 2014, 04:07:23 pm »

Ok.  Floating neutral solves a PITA.  Except in R/V where floating neutral is a PITA.


Let's look at it from a different angle:  Is there ANY situation where a floating (un-bonded neutral) is SAFER than a bonded neutral?
Is a genset with a floating neutral SAFER or less safe with a GFI?
and the opposite

Is a genset with a floating bonded SAFER or less safe with a GFI?


frank
G->N bonds are better in every situation where there isn't a second G->N bond.  In other words, as long as the generator is not feeding a structure with an existing bond, the generator should be bonded.  The trouble comes in home backup power situations - the panel already has a G-N bond, and if you create a second one, you now have made your ground between the service panel and the generator a current carrying conductor.  This could potentially become energized, and will goof up GFCI operation for generator receptacles.
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Mike Sokol

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Re: Portable Generator Grounding
« Reply #19 on: March 17, 2014, 04:21:50 pm »

G->N bonds are better in every situation where there isn't a second G->N bond.  In other words, as long as the generator is not feeding a structure with an existing bond, the generator should be bonded.  The trouble comes in home backup power situations - the panel already has a G-N bond, and if you create a second one, you now have made your ground between the service panel and the generator a current carrying conductor.  This could potentially become energized, and will goof up GFCI operation for generator receptacles.

I agree 100%. And because any power distro you might have for your sound system WILL, by NEC code, have a separated Ground and Neutral, then all generators used for outside stage power SHOULD provide the G-N bond. Since generators under 5KW, by OSHA specs and practical GFCI reasons, DON'T have a G-N bond to begin with, adding a simple bonding kludge plug is perhaps the easiest way to do this, especially for rental situations where they don't want you messing around inside the generator wiring.

Now, we just need a simple way to provide an earth ground rod (or bed of nails) and tie the generator G-N bond point to a ground lug on the metal stage. I can't think of a single outdoor situation where that would be more dangerous than floating everything, simply because there are too many everythings to float for an outdoor live show. Whew!  :o
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Mike Sokol
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