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Author Topic: Grouding issues and large pools of water  (Read 11042 times)

Mark Smith

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Re: Grouding issues and large pools of water
« Reply #20 on: October 31, 2005, 01:30:20 pm »

I don't know about the lab, but I think here we really are trying to understand what happened and hopefully learning how to make sure this tragedy doesn't happen again.  The important thing for all of us to remember is no matter the cause, someone lost their life and that is truly regretable.
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John Roberts {JR}

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Re: Grouding issues and large pools of water
« Reply #21 on: October 31, 2005, 02:13:37 pm »

Mark Smith wrote on Mon, 31 October 2005 12:30

I don't know about the lab, but I think here we really are trying to understand what happened and hopefully learning how to make sure this tragedy doesn't happen again.  The important thing for all of us to remember is no matter the cause, someone lost their life and that is truly regretable.


Agreed... we are all guessing at this point. The instructive point is despite all the safety systems and practices available, another person just died, RIP.

The follow up report should be informative.

JR
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Lee Patzius

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Re: Phantom power and large pools of water
« Reply #22 on: October 31, 2005, 02:37:32 pm »

Mark Smith wrote on Mon, 31 October 2005 13:27

Since the different grounding systems have a maximum impedance of about 15 ohms (making assumption both are "grounded")and the body has a minimum wet resistance of about 1000 ohms, only 1.5% of the current goes through the body.


I won't challenge your 1000 Ohm "wet" reference too much. But in my opinion, I totally disagree. The body resistance figures in the reference has mostly nothing to do with a human submersed in water. I suspect it must be referring to point-to-point series resistance, or more likely, probe tip, to probe tip, with wet fingers? And, that figure was probably obtained with a standard high dollar, low volt ohmmeter, like the one sitting next to me.

DO NOT TRY THIS... But a "meg-ohmmeter" on a human body submersed in water would probably be more accurate, but then it would probably kill them, if it put out 50 milliamps. The meg-ohmmeter puts out high voltage. We use meg-ohmmeters to inject high voltage to test and measure insulation integrity. Regular meters show infinite Ohms, while meg-ohmmeters display a whole new story. They show the actual resistance as insulation breaks down, when subjected to high voltage.

Also, we're dealing with human submersion and wet skin. Way more surface contact, way less resistance. Way less voltage required to shock... In my honest opinion.

Quote:

Since we are dealing with 2 GROUNDING systems, not a hot, the voltage difference between the two systems would have to be a minimum of 1,319.5 volts. I don't know of a situation where you would see this.


In 30 some-odd years, I've blown sparks between two grounds on a few, but very rare occasions. (I know... Regardless, I can blow sparks on a 12 volt car battery too.) And I've seen hot grounds NOT blow breakers too. Each situation is totally different.

Quote:

Unfortunately I think we saw a main power contact here and it is sad that it happen.


Yes, I absolutely agree. I suspect the same.
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Lee Patzius


Mark Smith

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Re: Phantom power and large pools of water
« Reply #23 on: October 31, 2005, 04:07:44 pm »

very good point... if the water is energized (or is the ground) then there is a lot of surface contact, so I don't know how accurate my 1 kohm number is.
With your insulation statement, My understanding of insulation is that it has an almost infinite impedance up to it's breakdown voltage.  At that point the characteristics change.
I have seen grounds spark over too, but not in a situation where 120/240 is the primary voltage on the system. (saw it on 161 kV and it was a bad day)
Thanks for replying, I consider this one of the ways I will learn more.
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Nathan Lehouillier

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Circuit tester?
« Reply #24 on: October 31, 2005, 04:51:47 pm »

I own 20+ of these little guys and have advised every one that works with me that they must check every thing they plug into every show. http://www.tripplite.com/products/static/ct120.cfm
Safety measure's are not optional.
Nathan KDS&L
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Lee Patzius

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Re: Phantom power and large pools of water
« Reply #25 on: October 31, 2005, 11:24:50 pm »

Mark Smith wrote on Mon, 31 October 2005 16:07

 
With your insulation statement, My understanding of insulation is that it has an almost infinite impedance up to it's breakdown voltage.  At that point the characteristics change.


Generally speaking, insulation has a very high resistance to DC, but much lower impedance to AC. I suspect we humans conduct pretty much the same way.

Quote:

I have seen grounds spark over too, but not in a situation where 120/240 is the primary voltage on the system. (saw it on 161 kV and it was a bad day)



Whoa! Yeah that makes for a really bad day.

One night, out the corner of my eye, I saw a momentary flash of lightning balls coming out of the earth, inside a 34.5kV substation yard. Everything was still running smooth. I reported it, and they laughed about it for a good couple of days, until an underground 3600 Volt (IIR) feeder line blew up. We called in the big rigs for that job.
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Lee Patzius


Steve Olsen

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Re: Grouding issues and large pools of water
« Reply #26 on: November 01, 2005, 10:56:00 am »

Unfortunate accidents like this really bring home the need know and follow all existing safety standards. I have been an electrician for over 35 years, a musician for "a little longer", and an instructor for about 10 years (those that can't do, teach?)

I have assisted in investigations similar to the accident discussed here. There are literally 100's of possible scenario's that can result in death. As techs we have a duty to be as educated and conscientious as possible.

Personally, I have always had an issue with using ground lifts and with using nonprofessional (2 wire) AC connections (CD players, tape decks, vintage amplifiers, ...). In larger installations, it is common to have more than one point of entry and more than one ground potential. On their own they can be perfectly safe and be within NEC spec. When combined they become a loaded gun.

Please keep safety in mind even if you're short on setup time. And please think twice before using ground lifts.
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Dave Hudzik

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Re: Grouding issues and large pools of water
« Reply #27 on: November 02, 2005, 01:13:43 am »

not the greatest choice of words by another pastor...  Please no one think I am taking this as lightly, this was someones life.

“We're all in a state of shock,” said the Rev. Joel Gregory, professor of preaching at Baylor University's Truett Seminary. “I have never encountered this (kind of accident) in all my years of ministry.”
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Guest

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Re: Grouding issues and large pools of water
« Reply #28 on: November 02, 2005, 03:42:03 pm »

I will bet that many churches will review their configuration  for baptisary mic's in light of this unfortunate accident. Many will assume that since they aren't using phantom power that all is OK (obviously a misnomer).

But it appears that the only fully safe solution would be the wireless mic on a boom. A second would be a hanging condensor choir mic, well out of reach.

Am I missing something?
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Michael 'Bink' Knowles

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Mic level isolation transformer
« Reply #29 on: November 02, 2005, 03:52:06 pm »

Mike McCloskey wrote on Wed, 02 November 2005 12:42

I will bet that many churches will review their configuration  for baptisary mic's in light of this unfortunate accident. Many will assume that since they aren't using phantom power that all is OK (obviously a misnomer).

But it appears that the only fully safe solution would be the wireless mic on a boom. A second would be a hanging condensor choir mic, well out of reach.

Am I missing something?


At least one more safe solution to the problem would be a wired dynamic mic connected to the mixer through an isolation transformer with pin 1 not connected and its housing not grounded.

-Bink
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Michael 'Bink' Knowles
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Mic level isolation transformer
« Reply #29 on: November 02, 2005, 03:52:06 pm »


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