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Author Topic: Grouding issues and large pools of water  (Read 10903 times)

glasgowsoundman

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Re: Phantom power and large pools of water
« Reply #10 on: October 31, 2005, 10:53:04 am »

We just had our own baptismal service on Sunday morning. When we do baptisms, all the introduction/interview/testimony part happens before anyone gets in to the pool. No microphones, wired or wireless, get anywhere near the baptism pool.

My thoughts and prayers are with all those affected by this tragedy.

Regards,

Duncan
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Duncan Hutchison
Technical Team Deputy Leader
Sound & Recording Engineer
Destiny Church Glasgow
1120 Pollokshaws Road
Glasgow
G41 3QP
Scotland
UK
www.destiny-church.com

Mark Smith

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Re: Grouding issues and large pools of water
« Reply #11 on: October 31, 2005, 11:01:45 am »

I don't think this could be a "fluke".  I believe it was a mistake by someone either in the installation or a recent repair.  A properly installed grounding system would not allow this to happen.  I imagine another alternative is that the installation could be old enough that it supersedes some of the ground fault codes.  But something had to degrade in the system to cause this even if this is true.  
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Stephen B.

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Re: Grouding issues and large pools of water
« Reply #12 on: October 31, 2005, 11:09:07 am »

Clayton Luckie wrote on Mon, 31 October 2005 09:38


Do you go to this church?

cl



No, I don't, but I did an internship with the pastor a few years ago.  He was a very cool guy and will be missed by his congregation.  Waco really seems to attract some tragic occurences.
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"Badness is only spoiled goodness."

C.S. Lewis

Lee Patzius

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Re: Phantom power and large pools of water
« Reply #13 on: October 31, 2005, 11:13:16 am »

Stephen B. wrote on Mon, 31 October 2005 10:17

This is a very tragic thing that has happened.  I find it really hard to believe that this church would have had a bad wiring job considering their permanent worship leader is one of the giants of contemporary worship music.  How much more serious are the ramifications if the church was wired professionally?

Remember that right now here in Waco, we are primarily concerned with supporting the family and friends of the pastor with prayer and love.


You can actually have a good grounded system, but still get between two grounds separated by distance and source, and experience a shock.

Otherwise we'd never have hum.

A properly installed GFCI helps prevent these kinds of tragedies.
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Lee Patzius


Mark Smith

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Re: Phantom power and large pools of water
« Reply #14 on: October 31, 2005, 12:12:20 pm »

While ground loops can cause hum, I can't think of a situation where you could have enough current capacity to cause a problem...  Maybe with two completely seperate services and grounding systems, but you still usually have no more than a couple of ohms impedance between the two grounds and that is an outside number.  Any thoughts?
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Marty McCann

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Re: Grouding issues and large pools of water
« Reply #15 on: October 31, 2005, 12:43:49 pm »

More than likely a gray cheater gounding adapter was put into the system and not grounded, or perhaps someone cut off the third or ground pin on an AC cord.  It only takes 15 milliampere to kill.

marty
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JimCreegan

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Re: Phantom power and large pools of water
« Reply #16 on: October 31, 2005, 12:43:58 pm »

If a hot is connected to a ground (through a fault) with enough resistance to avoid a blown circuit that current will go to any ground avaliable.  It takes less than you may think to kill.

http://www.smud.org/safety/world/hurt/chart.html

Sacramento Municipal Utility District

A milliamp is 1/1000th of an ampere, a measure of electricity.

   * A person can just feel less than 5 milliamps of electrical current.
   * A person can't let go of an object with 5 to 20 milliamps of electrical current.
   * 20 to 60 milliamps of electric current is possibly fatal.
   * 60 to 8000 milliamps of electric current is probably fatal.

A GFCI is a Ground Fault Circuit Interrupter, a device which protects against serious shock. The trip setting for a GFCI is less than 5 milliamps.

JimC
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Lee Patzius

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Re: Phantom power and large pools of water
« Reply #17 on: October 31, 2005, 12:58:17 pm »

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

While ground loops can cause hum, I can't think of a situation where you could have enough current capacity to cause a problem...  Maybe with two completely seperate services and grounding systems, but you still usually have no more than a couple of ohms impedance between the two grounds and that is an outside number.  Any thoughts?


Yeah, in the church scenario, two grounds with enough current to kill may sound far fetched until you realize 20 to 50 milliamps is all it takes.

Then add water. Body resistance drops extremely low.

Add voltage to a wet human body, conductivity is sure to increase (resistance drops). Much like an arc through air as an analogy, air measures near infinity Ohms until broken down by conduction of an arc.  

Not to mention the capacitance in the body too. It passes AC.

Even 1 volt, at 1 ohm, kills a person 20 times over.

My initial response was aimed at what could've possibly gone wrong on a professionally installed system.

NOTE! Even with GFCI's, if it WERE two grounds, it IS possible that a GFCI would NOT have helped, because the grounds don't get interrupted by the GFCI. Nor do grounds get interrupted by circuit breakers.

Far fetched? Not when a person is touching an electrical object in a pool of water.

This is one situation of the "flip side of the coin" where grounding can actually kill. Had the power system been floating, (such as an ungrounded balanced 120 Volt system) or, had the mic been ungrounded, there would have been zero potential as he charged the mic case.  

But in the above paragraph, unbalanced/ungrounded 120 volt systems do not exist. So we MUST keep things grounded. Or someone else will eventually energize it high, or sink it to ground for you. We dedicate the grounding conductor, and keep it standardized.

The most likely situation is that the pool became energized hotter than the mic ground, and he got killed. The mic was most likely grounded through audio shields, even IF a power ground lift adapter was used somewhere. Case grounds, audio shields,  and metal enclosures always seem to find earth one way or another. A GFCI WOULD have helped in this "likely" situation.


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Lee Patzius


Bennett Prescott

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Re: Grouding issues and large pools of water
« Reply #18 on: October 31, 2005, 01:10:44 pm »

There was just a very heated and lengthy discussion on this in the LAB. It's very sad to see it play out again in this format so soon.

For those of you looking for a quick "here's how grounding works" cheat sheet, I've posted a moderately comprehensive set of guides in the downloads section of my website... the first link is to an excellent presentation by Bill Whitlock that is slowly but surely opening the minds of sound engineers everywhere.

http://www.campuspa.com/downloads/downloads.html

[edit: can't link right to my own website... what am I coming to?]
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-- Bennett Prescott
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Mark Smith

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Re: Phantom power and large pools of water
« Reply #19 on: October 31, 2005, 01:27:30 pm »

my comment was referencing ground loops that cause hum. 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. 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.  Reference on the body resistance:
http://www.osha.gov/SLTC/etools/construction/electrical_inci dents/eleccurrent.html

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

Re: Phantom power and large pools of water
« Reply #19 on: October 31, 2005, 01:27:30 pm »


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