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Author Topic: Grounding vs Bonding video  (Read 2652 times)

Mike Sokol

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Grounding vs Bonding video
« on: February 04, 2015, 05:31:10 PM »

Here's a pretty good video from Mike Holt explaining grounding vs bonding, and how electrocution can occur when you get between different potentials. In our typical stage scenario this could be the electrified strings of a guitar and a properly grounded microphone, or visa-versa. https://www.youtube.com/watch?v=3vvvv5QVZoA
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Jonathan Johnson

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Re: Grounding vs Bonding video
« Reply #1 on: February 05, 2015, 03:44:00 PM »

Here's a pretty good video from Mike Holt explaining grounding vs bonding, and how electrocution can occur when you get between different potentials. In our typical stage scenario this could be the electrified strings of a guitar and a properly grounded microphone, or visa-versa. https://www.youtube.com/watch?v=3vvvv5QVZoA

OK, Mike, here's a question for you. Let's say you've got a stage that is properly wired, grounded, bonded, etc. to the electrical system, and all powered gear on the stage is in good repair.

The FOH position (mixer) also has been set up correctly, with good wiring practices and all of the equipment is in good repair.

So far, so good. But the stage and FOH are derived from different circuits connected to the same electrical service, so there is a common ground bond (low impedance connection) between the circuits.

Due to the length of the run, there is an induced voltage in the equipment grounding conductor supplying the FOH position. If we measure with a high-impedance voltmeter, we find a several volt differential between the strings of the guitar and the shell of the microphone.

Is there a shock hazard? Will the impedance/resistance of the human body cause the voltage differential between the guitar and the microphone to drop to near zero, resulting in negligible current through the body?

OK, let's add another wrinkle. Everything is as previously described, EXCEPT that the stage power and FOH power are from separately derived sources: two utility transformers (with separate ground rods); utility power and a generator (each with a separate ground rod); or two generators (with separate ground rods). There is no grounding bond between the power sources beyond the dirt itself (high impedance connection). What is the shock hazard?

(Much of this has been discussed before, but it seems like this might be a good place to bring it up again.)
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John Roberts {JR}

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Re: Grounding vs Bonding video
« Reply #2 on: February 05, 2015, 04:42:17 PM »

OK, Mike, here's a question for you. Let's say you've got a stage that is properly wired, grounded, bonded, etc. to the electrical system, and all powered gear on the stage is in good repair.

The FOH position (mixer) also has been set up correctly, with good wiring practices and all of the equipment is in good repair.

So far, so good. But the stage and FOH are derived from different circuits connected to the same electrical service, so there is a common ground bond (low impedance connection) between the circuits.

Due to the length of the run, there is an induced voltage in the equipment grounding conductor supplying the FOH position. If we measure with a high-impedance voltmeter, we find a several volt differential between the strings of the guitar and the shell of the microphone.

Is there a shock hazard? Will the impedance/resistance of the human body cause the voltage differential between the guitar and the microphone to drop to near zero, resulting in negligible current through the body?
Even if the several volts is low impedance the current developed through the resistance of the human will be too low to worry about. You may feel a 9V battery if you lick it with your tongue, but you won't feel 9V if you touch it with your fingers. 
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OK, let's add another wrinkle. Everything is as previously described, EXCEPT that the stage power and FOH power are from separately derived sources: two utility transformers (with separate ground rods); utility power and a generator (each with a separate ground rod); or two generators (with separate ground rods). There is no grounding bond between the power sources beyond the dirt itself (high impedance connection). What is the shock hazard?
What's the voltage? If tens of volts what's the impedance behind that voltage? If you connect a VOM between the two grounds first in volts scale, then in current, what do you read..? If significant volts that blow the VOM fuse in current mode, maybe worry about it.

The answer to that hypothetical is "It depends" but unless you are near some leaky electrical train track or subway I wouldn't expect serious current flowing in the ground between power drops. When in doubt measure.
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(Much of this has been discussed before, but it seems like this might be a good place to bring it up again.)
JR
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Mike Sokol

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Re: Grounding vs Bonding video
« Reply #3 on: February 05, 2015, 05:33:35 PM »

OK, Mike, here's a question for you. Let's say you've got a stage that is properly wired, grounded, bonded, etc. to the electrical system, and all powered gear on the stage is in good repair.

The FOH position (mixer) also has been set up correctly, with good wiring practices and all of the equipment is in good repair.

So far, so good. But the stage and FOH are derived from different circuits connected to the same electrical service, so there is a common ground bond (low impedance connection) between the circuits.

Due to the length of the run, there is an induced voltage in the equipment grounding conductor supplying the FOH position. If we measure with a high-impedance voltmeter, we find a several volt differential between the strings of the guitar and the shell of the microphone.

Is there a shock hazard? Will the impedance/resistance of the human body cause the voltage differential between the guitar and the microphone to drop to near zero, resulting in negligible current through the body?


In this case, there would be a very low impedance fault current source, probably a fraction of an ohm. And this could easily supply dozens or even hundreds of amperes of fault current between the stage and the mixer. So you don't need a high-z meter at all to measure this differential voltage. And because of the low-z nature of this circuit there could only be a few volts difference. This can be dangerous to things like the shield of an XLR line, which a typical run being around 1 ohm. I've personally measured 2 or 3 volts in this situation which translates into 2 or 3 amperes of fault current. And that could certainly make a lot of ground-loop hum.

However, the human body is a non-linear 1,000 ohms or so which suggests perhaps 1 mA per volt of differential hand-to-hand. You probably won't get that much current per volt human shock in that low volt range since the skin needs to have its epidermis layer penetrated before reaching body fluid. As JR noted, even though there's a lot of current available, there too little voltage to cause a human shock.

Quote
   
OK, let's add another wrinkle. Everything is as previously described, EXCEPT that the stage power and FOH power are from separately derived sources: two utility transformers (with separate ground rods); utility power and a generator (each with a separate ground rod); or two generators (with separate ground rods). There is no grounding bond between the power sources beyond the dirt itself (high impedance connection). What is the shock hazard?

In this case there still should be a very low voltage differential between the stage and mixer, and if they're interconnected with standard XLR cables it will be only a few volts. If there was no shield connection between the stage and mixer, say by using audio isolation transformers in the line with ground lifts) then you'll likely measure more voltage, but not a lot more. Again, I've personally measured 5 volts in that sort of situation. Not dangerous for human contact. However, a lightning strike in the area would induce a HUGE differential between the two system which would be deadly. And there's the possibility that something like a open neutral on the one transformer could induce all sorts of return current that would try to equalize through an interconnecting systems. I think there would be more of a noise injection problem rather than a dangerous voltage that could cause human shock. But I'll re-read this later tonight and see if I still make sense in a few hours.
« Last Edit: February 05, 2015, 06:02:05 PM by Mike Sokol »
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Mike Sokol
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Re: Grounding vs Bonding video
« Reply #3 on: February 05, 2015, 05:33:35 PM »


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