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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 128182 times)

Mike Sokol

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Re: freak accident? help!
« Reply #180 on: September 08, 2013, 10:35:53 am »

The buzzing of the Amprobe is nice, but it reacts the same to 120v and to whatever power is on a USB cable. It even reacts the same to the face of an iPad 1. It also doesn't necessarily react the same way to the same stimulus each time.

FYI: I'm going to be presenting a 90-minute No~Shock~Zone seminar for the student AES chapter at Shenandoah University in Winchester, VA on Wednesday, Sept 18 starting at 6 PM. It will include a bunch of different NCVT demonstrations on various electrified objects such as guitars and microphones, as well as my Ground Loop hum diagnostics clamp procedure, and an introduction of my latest theory on GLID (Ground Loop Inter-modulation Distortion) in sound systems and how it affects bass clarity. We'll take some pictures and post them here after the seminar. It's free and open to the pro-sound public, but you should contact me directly in advance if you want to attend so I can put your name on "the list". 
« Last Edit: September 08, 2013, 06:28:11 pm by Mike Sokol »
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Mike Sokol
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Dan Mortensen

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Re: freak accident? help!
« Reply #181 on: September 08, 2013, 02:32:27 pm »

Fluke alone sells 5 or more models. Which one did you get?

Sorry, didn't realize that.

This one:

http://www.amazon.com/gp/product/B000EJ332O/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1

It was a little cheaper, and came from a different Amazon source.
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Mike Sokol

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Re: freak accident? help!
« Reply #182 on: September 08, 2013, 06:23:15 pm »

This one:

http://www.amazon.com/gp/product/B000EJ332O/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1


That's exactly the one I've used in hundreds of NO~Shock~Zone hot-chassis experiments and seminars. Get the standard 90-1000 volt version, which will find a hot microphone or guitar down as low as 40 volts.

I think that spending $25 to potentially save your life is a pretty good payback.  ;)
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Mike Sokol
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Dan Mortensen

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Re: freak accident? help!
« Reply #183 on: September 08, 2013, 07:28:04 pm »

That's exactly the one I've used in hundreds of NO~Shock~Zone hot-chassis experiments and seminars. Get the standard 90-1000 volt version, which will find a hot microphone or guitar down as low as 40 volts.

I think that spending $25 to potentially save your life is a pretty good payback.  ;)

Good, thanks, and I agree on the return on investment, which is why I'm giving them to my crew as a thanks for doing such great work this summer. There's a 5 pack offer on Amazon for $22.80 each, which along with the neon light checkers will give them tools to be electrically safe at work and at home.

Thanks again for your work. I wish I was closer to VA to see your talk. Do you ever come West? I'm confident my AES Section would be happy to host you sometime.
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Mike Sokol

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Re: freak accident? help!
« Reply #184 on: September 08, 2013, 10:18:26 pm »


I wish I was closer to VA to see your talk. Do you ever come West? I'm confident my AES Section would be happy to host you sometime.

I'm not coming out to the west coast until spring 2014, but I will be in eastern Texas Oct 12 thru 17 next month. I've taught seminars in the past at Austin Community College, and since I'll have downtime on Monday-Wednesday that week, I've already contacted the staff there to see if I could hookup with the student AES chapter to do a No~Shock~Zone presentation. Is anyone on this forum with the Austin student AES chapter, or any other pro-audio organizations in the area? If so, contact me directly at mike@noshockzone.org. 
« Last Edit: September 08, 2013, 10:23:31 pm by Mike Sokol »
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Mike Sokol
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Jonathan Johnson

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Re: freak accident? help!
« Reply #185 on: September 09, 2013, 02:09:43 am »

To summarize this thread:
  • 2-wire ungrounded 120V circuits (hot and neutral only) may be present in older construction. The insulation on very old wiring can be discolored making it difficult to distinguish hot and neutral -- if the wires even were color-coded in the first place.
  • When a receptacle is replaced or added to an ungrounded 2-wire circuit, the hot and neutral may be inadvertently reversed.
  • If a grounding-type receptacle is used on this circuit, the installer may "bootleg" the ground by jumpering between the ground and neutral connections.
  • When both of the above conditions are present, the "ground" wire will be hot: 120V relative to true earth ground. This condition has been termed by Mike Sokol (who has performed extensive research into the issue) to be a "Reverse Polarity Bootleg Ground" (RPBG).
  • RPBG cannot be detected by most traditional testing methods. Using a neon test lamp, a three-light receptacle tester, or a voltmeter will not reveal a problem with the wiring unless referenced to a second known, tested & trusted grounding reference.
  • A non-contact voltage tester (NCVT) can be used fairly reliably to identify RPBG conditions, when used in conjunction with the above tools. However, certain conditions may still fool the NCVT. For example, double-insulated devices such as in iPod may cause the NCVT to indicate a false positive, while being inside of an RV connected to an RPBG receptacle may cause the NCVT to indicate a false negative.
  • A person who comes in contact with a metal device connected to an RPBG receptacle and another grounded object will be subjected to a serious electrical shock.
  • GFI breakers and circuit breakers may not protect personnel against bootleg ground faults.
  • Churches may be the most likely structures to experience wiring problems due to the prevalence of old construction and potentially unqualified volunteer labor. Wiring problems can occur in new construction as well.
  • Surveys and research indicate RPBG may be a widespread problem.
  • Audio equipment can be damaged if a device attached to a RPBG receptacle is connected via an audio interconnect cable to another device connected to a properly wired receptacle. This is because there is a 120V potential between the chassis of each device, causing a high-amperage fault current to flow through the interconnect.
  • All bootleg grounds -- including correct polarity bootleg grounds (CPBG) -- are a violation of the National Electrical Code (in the United States). A CPBG can be particularly dangerous in the event of an upstream failure of the neutral conductor: in this event, both the neutral and the ground will become electrically charged relative to true earth ground.
  • If this condition (or any abnormal electrical condition) is observed, the faulty circuit must immediately be locked and tagged out-of-service until a qualified electrician can rectify the problem.

If you've ever been shocked by touching an appliance or audio device or know someone who has, suspect an RPBG issue. It's not a minor issue; someone could get killed. Take it seriously and don't let anyone poo-poo the problem.

Further reading:

(P.S. -- I posted a similar summary in another thread that refers to this thread. I apologize in advance for making you read it twice.)
« Last Edit: September 09, 2013, 05:09:51 pm by Jonathan Johnson »
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Stop confusing the issue with facts and logic!

Mike Sokol

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Re: freak accident? help!
« Reply #186 on: September 09, 2013, 06:57:27 am »

To summarize this thread:

That's an excellent summary. Thanks for putting it together.

The only addition I might note is that ALL bootleg grounds (reverse or correct polarity) are a violation of National electrical code and should be corrected. Also, while a CPBG (Correct Polarity Bootleg Ground) receptacle doesn't appear to be immediately dangerous since its ground isn't hot, if the neutral wire is lost somehow (loose screw or corrosion) then anything with a ground plug connected to it will become hot-chassis energized to 120 volts.

I'm also working on a theory describing how double-bonded G-N bus bars in electrical sub panels as well as CPBG mis-wired outlets can cause an audio distortion I've termed "GLID" for Ground Loop Inter-modulation Distortion. Could explain why some sound systems have undefined or "fuzzy" bass. Stay tuned.... 

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Mike Sokol
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Dan Mortensen

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Re: freak accident? help!
« Reply #187 on: September 09, 2013, 01:35:40 pm »

Thanks for this summary. I thought the part about indicating voltage presence on double insulated and computer parts was important, too, so the user can know what's a problem and what's likely not a problem.

To summarize this thread:
  • 2-wire ungrounded 120V circuits (hot and neutral only) may be present in older construction. The insulation on very old wiring can be discolored making it difficult to distinguish hot and neutral -- if the wires even were color-coded in the first place.
  • When a receptacle is replaced or added to an ungrounded 2-wire circuit, the hot and neutral may be inadvertently reversed.
  • If a grounding-type receptacle is used on this circuit, the installer may "bootleg" the ground by jumpering between the ground and neutral connections.
  • When both of the above conditions are present, the "ground" wire will be hot: 120V relative to true earth ground. This condition has been termed by Mike Sokol (who has performed extensive research into the issue) to be a "Reverse Polarity Bootleg Ground" (RPBG).
  • RPBG cannot be detected by most traditional testing methods. Using a neon test lamp, a three-light receptacle tester, or a voltmeter will not reveal a problem with the wiring unless referenced to a second known, tested & trusted grounding reference.
  • The use of a non-contact voltage tester (NCVT) can be used fairly reliably to identify RPBG conditions, when used in conjunction with the above tools. However, certain conditions may still fool the NCVT.
  • A person who comes in contact with a metal device connected to an RPBG receptacle and another grounded object will be subjected to a serious electrical shock.
  • GFI breakers and circuit breakers may not protect against RPBG faults.
  • Churches may be the most likely structures to experience wiring problems due to the prevalence of old construction and potentially unqualified volunteer labor. Wiring problems can occur in new construction as well.
  • Surveys and research indicate RPBG may be a widespread problem.
  • If this condition (or any abnormal electrical condition) is observed, the faulty circuit must immediately be locked and tagged out-of-service until a qualified electrician can rectify the problem.

If you've ever been shocked by touching an appliance or audio device or know someone who has, suspect an RPBG issue. It's not a minor issue; someone could get killed. Take it seriously and don't let anyone poo-poo the problem.

Further reading:

(P.S. -- I posted a similar summary in another thread that refers to this thread. I apologize in advance for making you read it twice.)
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Mike Sokol

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Re: freak accident? help!
« Reply #188 on: September 09, 2013, 01:49:49 pm »

Thanks for this summary. I thought the part about indicating voltage presence on double insulated and computer parts was important, too, so the user can know what's a problem and what's likely not a problem.

Yes, that's a good point. I'll note here that a NCVT is an accurate test on anything with a GROUND PLUG. There are many appliances that use double-insulation to protect you from shock (such as iPhone chargers and computer power supplies), and these will cause a NCVT to beep even though the appliance is safe to touch. That's because the computer or iPhone or even your wife's slow cooker in the kitchen actually DOES have 60 volts or so on the chassis. But the current is limited to below 1 mA, so you're not electrocuted. Makes me feel a bit uncomfortable when I measure it though....

However, I believe that ALL stage and studio gear should be grounded somehow since there's way too many grounding paths for you to get shocked by if something goes wrong in the double insulation of the power supply. And no matter how well you think you have everything grounded, it's still foolhardy to wade into a baptismal pool playing an electric guitar plugged into a stage amp (I've personally seen this) or holding a hard-wired microphone which provides the ground for water that's been electrified by a broken electric heater element (I've read the obits and written articles about it).

I'm so glad this forum has embraced the idea of electrical safety and spread it around the world. Thank you all for your time and attention to this serous issue. Getting shocked is no laughing matter.   
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Mike Sokol
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Jerome Malsack

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Re: freak accident? help!
« Reply #189 on: September 09, 2013, 02:39:43 pm »

Besides the audio sound system this problem could present itself with a TV/video network, Or Computer network. 

Not as likely with the phones but more phones are starting to have a power wall plug. 
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