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Author Topic: Testing Generator Voltage  (Read 11798 times)

Mike Sokol

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Re: Testing Generator Voltage
« Reply #40 on: June 09, 2014, 11:37:33 pm »

I agree that getting a good ground in dry soil is very difficult.  Code is happy with 2 8' rods regardless of effectiveness-and I think for audio/lighting gennys that should be fine-especially considering my earlier examples.  If 2 8' rods doesn't provide an adequate ground, it is doubtful anyone will be grounded well enough just through the earth to get hurt.  A conductive stage/structure is a different story.

It's been discussed before on this forum, but for those who missed it, here's the Ufer Ground: http://en.wikipedia.org/wiki/Ufer_ground

It should be noted that besides lightning safety, an earth ground helps to keep your local "ground plane" close to earth potential. With a totally floating ground plane, your stage can float to pretty high voltage from leakage currents or inductive voltage from overhead power lines. Yup, if your metal stage is set up anywhere near high-tension lines and not properly "grounded" then you can easily get up to 10,000 volts above earth potential. It's supposed to be less than 5 mA current and not lethal (according to a PoCo engineer who I talked to who designed the test) but that's equivalent to grabbing a spark plug on a running lawnmower engine. I did that ONCE as a kid and don't think it would make your clients happy to be on the receiving end of such a shock. Of course, any kind of shock on an elevated stage can be life threatening simply from the possibility of a startle reflex causing a fall down the steps or off the stage. 

Jonathan Johnson

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Re: Testing Generator Voltage
« Reply #41 on: June 10, 2014, 12:26:10 am »

Now what if your generator is positioned several hundred feet away from the stage on dry, sandy soil and your stage is on a lawn that is watered weekly? You might drive a ground rod near the generator, but because of soil conditions and distance you may have a low impedance/resistance between the stage and the earth near the generator. But your personnel experiences low resistance between equipment and earth. Bonding the stage structure (and any other structure where powered equipment is used) to the generator grounding system is wise. I also wonder if maybe driving a ground rod near the stage would be a good idea?

Remember, there are two "grounding" systems involved:
  • the EGC, or Equipment Grounding Conductor, that is supplied from the power source (generator, permanent power, or other tie in) and the GEC, or Grounding Electrode Conductor, which is the green wire in your distro and power cords and is bonded to the chassis of equipment
  • the GEC, or Grounding Electrode Conductor, which bonds the power source to earth via a ground rod plus other systems and structural components such as building framing, stage structure, gas piping, water piping, etc.

There should only be one intentional bonding point between the EGC and the GEC: at the power source.
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Mike Sokol

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Re: Testing Generator Voltage
« Reply #42 on: June 10, 2014, 12:38:24 am »

Now what if your generator is positioned several hundred feet away from the stage on dry, sandy soil and your stage is on a lawn that is watered weekly? You might drive a ground rod near the generator, but because of soil conditions and distance you may have a low impedance/resistance between the stage and the earth near the generator. But your personnel experiences low resistance between equipment and earth. Bonding the stage structure (and any other structure where powered equipment is used) to the generator grounding system is wise. I also wonder if maybe driving a ground rod near the stage would be a good idea?

That's why I believe that bonding the generator ground to the stage metal is more important than the actual ground rod impedance in most cases. And perhaps a secondary ground rod at the stage could be a good idea just for the scenario you mention. For instance, I do a gig nearly every year where the big generator is parked along the access road in the gravel, while the metal stage is 100 feet away sitting on the wet grass. I certainly want a ground rod at the generator, but see the advantage of adding one to the stage as well. More to consider.

Stephen Swaffer

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Re: Testing Generator Voltage
« Reply #43 on: June 10, 2014, 10:54:23 am »

Jonathan raises a very good point.  Working in a rural area, supplying outbuildings is common.  If there is a panel in the outbuilding, code requires that we install a grounding electrode system for that building.  The end result is 4 wires from supplying building-2 hots, neutral and a GEC, then a couple of ground rods at each building with a GEC to the panel.  When the buildings are close-say less than 30 or 40 feet, I have gotten away with using the same ground rods for both buildings, and just running a GEC to each.

In existing installs, I can get away without an EGC, if there are no metallic paths between buildings-but good luck convincing the AHJ that your genny on wheels is an existing install!  IMO, the EGC is mandatory, no matter how many ground rods you have the ambition to drive.

Practically, I personally would make sure the stage is bonded.  I would drive a ground rod at the genny to make sure the genny frame is at the same potential as the earth, then if it is "soupy" wet I'd be o with it.  If a situation like Jonathan mentioned were encountered, I would lean towards adding one at the stage.
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Mike Sokol

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Re: Testing Generator Voltage
« Reply #44 on: June 10, 2014, 12:12:42 pm »

Practically, I personally would make sure the stage is bonded.  I would drive a ground rod at the genny to make sure the genny frame is at the same potential as the earth, then if it is "soupy" wet I'd be ok with it.  If a situation like Jonathan mentioned were encountered, I would lean towards adding one at the stage.

I agree 100%

Jonathan Johnson

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Re: Testing Generator Voltage
« Reply #45 on: June 10, 2014, 03:40:43 pm »

That's why I believe that bonding the generator ground to the stage metal is more important than the actual ground rod impedance in most cases. And perhaps a secondary ground rod at the stage could be a good idea just for the scenario you mention. For instance, I do a gig nearly every year where the big generator is parked along the access road in the gravel, while the metal stage is 100 feet away sitting on the wet grass. I certainly want a ground rod at the generator, but see the advantage of adding one to the stage as well. More to consider.
Thinking about this some more...

If the stage is bonded, that can protect those who are on the stage. FOH, on the other hand, often sits out in the grass -- bonded stage doesn't do much good. Having a ground rod at the stage MAY protect personnel at the FOH position, assuming good ground conductivity between FOH and the stage, in the event ground rod at the genny doesn't. If you really wanted to get paranoid, I suppose you could drive ground rods at every location hosting powered equipment, but considering that many people will just drive the rod below the surface rather than pull it out when they're done, the city parks department might get a bit peeved at having a bazillion ground rods driven into their lawn. And it will drive the metal detector folks crazy when they come through after the show. ;-)
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Mike Sokol

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Re: Testing Generator Voltage
« Reply #46 on: June 10, 2014, 05:05:23 pm »

Thinking about this some more...

If the stage is bonded, that can protect those who are on the stage. FOH, on the other hand, often sits out in the grass -- bonded stage doesn't do much good. Having a ground rod at the stage MAY protect personnel at the FOH position, assuming good ground conductivity between FOH and the stage, in the event ground rod at the genny doesn't. If you really wanted to get paranoid, I suppose you could drive ground rods at every location hosting powered equipment, but considering that many people will just drive the rod below the surface rather than pull it out when they're done, the city parks department might get a bit peeved at having a bazillion ground rods driven into their lawn. And it will drive the metal detector folks crazy when they come through after the show. ;-)

Let's not be too paranoid. Since the mixing console and all its connected racks will be bonded to the incoming power line EGC, then it will eventually be bonded to the ground rod(s) at the stage and genny. That's good enough for me, and I suspect any inspector or code guy looking at it.

Stephen Swaffer

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Re: Testing Generator Voltage
« Reply #47 on: June 10, 2014, 09:11:06 pm »

I thought FOH personnel were expendable anyway?

Seriously, if there is enough moisture in the ground to be a hazard, 99% of the time the ground rod at the genny will be practically sufficient-and in any case driving a 2nd rod at that location makes you code compliant regardless of the ground impedance.
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