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Author Topic: AC Power for small outdoor event  (Read 13835 times)

Mike Sokol

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Re: AC Power for small outdoor event
« Reply #60 on: September 22, 2014, 10:34:33 pm »

No one wants to take a stab at what the fault current will be in the scenario I described above?

Assuming the human body has approximately 1,200 to 1,500 ohms hand-to-hand resistance (with moist hands) and 120 volts AC exists between the current source (nicked wire) and sink (grounded chain link fence) then there will be around 80 mA to 100 mA fault current through the victim in such a scenario. Just 10 mA of current is a very painful shock. 20 mA is enough to overcome your hand release reflex so you can't let go of an energized wire. And 30 mA for more a few seconds is considered to be enough to cause ventricular fibrillation in a large portion of the population with old people and young kids being most at risk. Without intervention with a defibrillator, you could be dead in minutes depending on your heart condition to begin with. An 80 to 100 mA hand-to-hand or hand-to-foot shock for more than a few seconds is considered lethal in most cases. A lot of the danger depends on if the fault current travels through the chest cavity or not, so a finger-to-finger shock on one hand is not dangerous to the heart, but if sustained long enough will cause long-term nerve damage in the hand.

So no, a human's internal body resistance won't produce enough fault current to trip even a 15 amp circuit breaker. 

The over-current circuit breakers in a generator, or any other service panel, are only there to protect the downstream wiring from overloading and heating up enough to cause a fire. They are NOT there to protect your gear from over-current damage (that's the job of the internal fuse) and certainly not there to protect a human from being electrocuted.

On the other hand, The 6 mA trip point of a standard domestic GFCI has been designed to stop any high-resistance shock that approaches danger levels, even for sustained contact. There are commercial/industrial GFCIs that have adjustable threshold points up to 30 mA, which is right at the level of lethality for sustained contact. These were originally designed for mines and such that had a lot of moisture in the motors that would trip a 6 mA breaker too often. Of course, a low-impedance fault of an ohm or so (typical stage wiring) will produce up to 100 amps of fault current which should clear any stage breakers. Of course, once you get to camlock current levels of 100 or more amps, then arc flash danger becomes the main concern. I believe that all personnel handling potentially live camlocks should be wearing PPE gear because the resultant arc flash of a 200 amp camlock connecting to a cable that was shorted to the same fence could produce a significant fireball that would be very dangerous even from several feet away. So for low-voltage hookups (under 600 to 1,000 volts as defined by the NFPA) the arc flash energy available must be closely monitored.
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Mike Sokol
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Stephen Swaffer

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Re: AC Power for small outdoor event
« Reply #61 on: September 23, 2014, 11:03:19 am »

Of course the "if....s" can be endless-but another aspect of this is that if the cord is pulled pver/through the fence it would be possible for a damaged cord to energize the fence, unless it is bonded it would be questionable if a 15A breaker would trip.  To me from an electrical standpoint, especially at outdoor events fences pose a quandry-with metal poles they will be well grounded, but its hard to really argue that they need to be bonded.  I would use extra caution with power wring around them.
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Steve Swaffer

Jerome Malsack

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Re: AC Power for small outdoor event
« Reply #62 on: September 23, 2014, 11:48:08 am »

A metal pole in the ground is not always considered a good ground and could be encased in concrete reducing the grounding ability. 

That is why antenna masts are not considered grounded by the mast poles and the foundations. 
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Stephen Swaffer

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Re: AC Power for small outdoor event
« Reply #63 on: September 23, 2014, 12:55:17 pm »

I suspect that has more to with RF than power grounding.  All new construction in Iowa with a concrete footing is required to use a "Ufer" ground-formally know as a "concrete encased electrode"-because it is a superior ground to a grounding rod-at least that is what the experts claim-I just do what the AHJ tells me I have to to pass!  It would not surpise me to be told that a Ufer ground does not work as well with RF.
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Steve Swaffer

Justin Schack

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Re: AC Power for small outdoor event
« Reply #64 on: September 23, 2014, 02:17:37 pm »

So in case anyone cares about how this actually went, the block party was Saturday and all went off swimmingly. The only little hiccup I had was that the various power cords on some of my equipment were not long enough to reach my two Furman power strips without using — ugh — extension cords. Exactly what I wanted to avoid in the first place, but it was just three fairly short extension cords running from powered speakers and a bass amp to the bricks, which were connected to the 120V outlets on the Honda inverter (instead of hundreds of feet of extension cord from a wall outlet in someone's home). Next time I'll invest in some longer AC cords so we won't have to use the extension cords.

I did notice I got more 60-cycle hum out of my guitar amplifier when using my single-coil Fenders than when I do at home, but it was nothing out of the ordinary. And other than that there were no noise, power or other issues. The generator was quiet enough that we had it stationed about 10 feet away from the drummer without it being too loud for us to hear what we needed to hear. We did not ground it, however. It was out on the street behind where we set up. I know later in the thread there was some talk about grounding the genny, but I also remember back to early in the thread when someone — I think it was Mike — said that tons of events have gone off just fine with an ungrounded genny as the power source.

Thanks again to everyone for their thoughts and advice. Much appreciated.
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Stephen Swaffer

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Re: AC Power for small outdoor event
« Reply #65 on: September 24, 2014, 06:58:30 pm »

It was out on the street behind where we set up. I know later in the thread there was some talk about grounding the genny, but I also remember back to early in the thread when someone — I think it was Mike — said that tons of events have gone off just fine with an ungrounded genny as the power source.

Thanks again to everyone for their thoughts and advice. Much appreciated.


Glad to hear that things went well!

Not to beat you up-but let me play devils advocate for a minute.  While it may be true that tons of events (including yours) have gone off just fine with an ungrounded power source-the same could be said for ice bucket challenges-but I have heard about several lately and the ones we (and the press gives free advertising to) hear about are the ones gone wrong.

Who made the decision not to ground the genny?  Doesn't matter much to anyone now-but it would be a very important question HAD something gone wrong.  If your name was the answer to that question, would you want to explain to a courtroom-or to a friends family, "so and so said online that lots of events have gone off fine with an ungrounded genny."

Just trying to get the wheels turning-I certainly can't look you in the eye and say I have always done things the right way myself.

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Steve Swaffer

ProSoundWeb Community

Re: AC Power for small outdoor event
ยซ Reply #65 on: September 24, 2014, 06:58:30 pm ยป


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