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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 127011 times)

Jonathan Johnson

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Re: freak accident? help!
« Reply #190 on: September 09, 2013, 05:02:45 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.

Thanks for reminding me of that. At the risk of making the summary as long as the whole thread, I modified it with a couple of points:
  • 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.
  • 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.

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. 

Any device with grounded components, whether an audio system, refrigerator, or power tool can be hazardous when connected to an RPBG receptacle, or a CPBG receptacle with a failed neutral wire upstream of the bootleg ground.

Any system with multiple components interconnected by signal lines with a grounded shield can be damaged by these conditions.
« Last Edit: September 09, 2013, 05:09:02 pm by Jonathan Johnson »
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Stop confusing the issue with facts and logic!

Jonathan Johnson

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Re: freak accident? help!
« Reply #191 on: September 09, 2013, 05:19:13 pm »

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).

The underlying lesson is that people using a baptismal pool should ONLY use wireless microphones and interconnects. Wired microphones should not be within reach of the baptismal pool, and the pump and heater for the baptismal pool should be connected to a GFI breaker (required for hot tubs by the National Electrical Code).

It's interesting to note that the National Electrical Code specifies an exclusion zone above bathtubs: any lighting fixture within three feet horizontally or eight feet vertically above the rim of the bathtub must be listed as suitable for damp or wet locations. Any receptacle within six feet of a sink, tub, or other water source must also be GFI protected. I think we can apply these same principles to audio devices and baptismal pools.
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Matt Edmonds

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freak accident? help!
« Reply #192 on: September 11, 2013, 01:02:52 pm »

Glad I found this thread. I will be buying a voltalert ASAP. I found some 277v ac the hard way trying to repair some ceiling tiles in a church. However this was due to my own negligence. Let's just say I'm lucky to only have a scar on my left hand. It could have been way worse. Anyways thanks for this super informative thread. :)
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Tommy Peel

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Re: freak accident? help!
« Reply #193 on: September 11, 2013, 04:17:03 pm »

I've recently done some tests on iPhones plugged into Apple chargers, and found much the same thing. Basically, anything without a ground plug (double insulated) plugged into a wall outlet will bias up to around 50% of the line voltage. So depending on which way you plug in your wall wart, you'll likely measure 40 to 80 volts on chassis of the appliance or gear. Now, this isn't normally dangerous because the double insulation keeps the leakage current through you to less than 1 mA (I think UL limits are 0.5 mA, if memory serves).

However, if your wall wart becomes damaged (dropped too many times) or wet (dunked in the shower), then I do believe it's possible for this double-insulation to fail, which will pass the line current into you. And if you're on something nicely grounded (or standing in the shower like that woman in China last month) and reach out to pick up your iPhone while plugged into a wall charger, then you can die from electrocution.

Again, 2 or 3 mA is a small shock. 10 mA is a big shock. 20 mA you can't let go of the conductors. 30 mA your heart goes into fibrillation in a few seconds. 100 mA nerve damage starts and death is almost certain. Since your body has about 1,000 ohm hand to hand or foot, then it's easy to do the math and see that 30 volts AC can cause 30 mA of current to flow through your heart. Without immediate CPR and AED intervention, you're now dead.

I just won't touch anything directly plugged into an ungrounded wall wart while standing in water. Way too dangerous for me, and I've done some pretty crazy technical things.

Much more on this later, but I'm so glad you brought up the point. Yes, you're iPhone is indeed at 60 volts AC while holding it to your ear while plugged into a wall charger. The current limiting of the insulation is the only thing keeping you alive. Pretty scary when you think about it.

That is pretty scary... Just curious do laptop(including MacBook) chargers that have a ground exhibit this effect? On my MacBook Pro's charger you can use the longer cord which has a ground(and metal lug in on the charger end to make it seem like it does something) or the one that lets you use it as a wall wart style two prong charger.
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Mike Sokol

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Re: freak accident? help!
« Reply #194 on: September 11, 2013, 06:36:29 pm »

That is pretty scary... Just curious do laptop(including MacBook) chargers that have a ground exhibit this effect? On my MacBook Pro's charger you can use the longer cord which has a ground(and metal lug in on the charger end to make it seem like it does something) or the one that lets you use it as a wall wart style two prong charger.

The longer cord on the MacBook chargers does indeed have a ground pin on the plug which will ground the chassis of your laptop. So the short "ungrounded" plug will allow your laptop chassis to float around 60 volt AC or so, while the long "grounded" wire will tie your laptop chassis to the receptacle's safety ground. I never worry about this in the house, but plugging your laptop into an ungrounded "wall wart" while relaxing by the pool would be a bad idea.

Again, ANYTHING with an ungrounded plug is "supposed" to be double-insulated and not allow more than a fraction of a milliamp of shocking current. HOWEVER that double-insulated shock barrier can be defeated by damage to the "wall wart" such as dunking in water or perhaps dropping it too many times and breaking something internally. I've not done any serious tests on this, but I do know that if you use a high-impedance meter on an "ungrounded" appliance, stage amp, mixing console, or whatever, it will generally have around half of line voltage on it. So while 60 volts at 0.5 mA isn't a shock hazard, if the current leakage goes up, then 60 volts could cause up to 60 mA of current through your body. And only 30 mA of shock current is deadly.   

Please keep me posted of any shock condition you find. I'm building a database.
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Tom Bourke

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Re: freak accident? help!
« Reply #195 on: September 19, 2013, 07:31:57 pm »

Please keep me posted of any shock condition you find. I'm building a database.
I have a couple for you.

I have run into MANY VCRs or DVD/VCR combo units, polarized cords, that will put about 30 to 35 volts on the chassis and shell of any coax connectors on the back if plugged into a revers polarity outlet.  I have gotten shocked by this and seen it damage other equipment.

I have a line lump PSU with a grounded IEC plug that has the output power floating about 35 V.  It has a molex connector like an old computer hard drive with 12V and 5V outputs.  Common to each rail is right, just that the common is 35V above ground.  This is on a known good outlet.

Another non shock situation are the laptops that put out hum on the audio connector when running from a grounded outlet. I think it was Dell that was the main offender.  Those would quiet down with a non grounded PSU, normal PSU with a ground lift on the plug, or from battery.  An audio ISO transformer would NOT make them quiet.  They were the only situation I would condone a widow maker ground lift adaptor.
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John Roberts {JR}

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Re: freak accident? help!
« Reply #196 on: September 19, 2013, 09:27:50 pm »

I have a couple for you.

I have run into MANY VCRs or DVD/VCR combo units, polarized cords, that will put about 30 to 35 volts on the chassis and shell of any coax connectors on the back if plugged into a revers polarity outlet.  I have gotten shocked by this and seen it damage other equipment.
I recall seeing something similar to the stinger cap inside some consumer VCRs trying to cop a ground from the neutral. These were fairly small caps but I suspect the voltage you see depends on a divider formed with whatever you measured it with. Still not cool, but if UL blessed it they are probably small (safe) currents.
Quote
I have a line lump PSU with a grounded IEC plug that has the output power floating about 35 V.  It has a molex connector like an old computer hard drive with 12V and 5V outputs.  Common to each rail is right, just that the common is 35V above ground.  This is on a known good outlet.
Again this voltage may depend on how you measure it. Have you tried to ground it to see how much current? A VOM on it's current scale will tell you, If you want to know.
Quote
Another non shock situation are the laptops that put out hum on the audio connector when running from a grounded outlet. I think it was Dell that was the main offender.  Those would quiet down with a non grounded PSU, normal PSU with a ground lift on the plug, or from battery.  An audio ISO transformer would NOT make them quiet.  They were the only situation I would condone a widow maker ground lift adaptor.

Computers suck, but they are better than no computers.

JR
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Tom Bourke

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Re: freak accident? help!
« Reply #197 on: September 21, 2013, 01:44:14 pm »

I recall seeing something similar to the stinger cap inside some consumer VCRs trying to cop a ground from the neutral. These were fairly small caps but I suspect the voltage you see depends on a divider formed with whatever you measured it with. Still not cool, but if UL blessed it they are probably small (safe) currents. Again this voltage may depend on how you measure it. Have you tried to ground it to see how much current? A VOM on it's current scale will tell you, If you want to know.
Computers suck, but they are better than no computers.

JR

I just moved cross country so I am still unpacking all my stuff.  When I can get to the power supplies again I will measure them.  It was enough current to get a decent shock from.
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Brian Jones

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Re: freak accident? help!
« Reply #198 on: October 01, 2013, 08:24:53 pm »

Guys... this is not a freak accident. It was caused by something I call a Reverse Polarity Bootleg Ground (or RPBG). This occurs when an old building has new grounded outlets added by bonding the ground screw to the neutral screw because there was no separate ground wire to begin with. That by itself is electrically safe (however, it's illegal per the NEC), but many older buildings had black power wires for both the hot and neutral, and some were simply wired backwards with the white/neutral wire being hot and the black/power line being actual neutral. See the attached diagram. In that case any piece of gear plugged into a Reverse Polarity Bootleg Grounded outlet will have its chassis energized to 120 volts. If you then connect that piece of audio gear to something else that's plugged into a correctly wired outlet, you can have 20 amps or more of current flow down the shield, which melts wires and destroys gear. The really scary thing is that a 3-light tester will tell you that this reversed outlet is wired correctly, when in fact both the neutral and ground contacts are at 120 volts and the hot side is at earth potential. Please see http://www.youtube.com/watch?v=_pwCY4_LwJo&feature=youtu.be&noredirect=1  for a video I did a few weeks ago that describes how you can use a $20 non-contact AC tester in conjunction with a cheap 3-light tester to qualify grounds in power plugs. That's the only easy way to determine if an outlet will blow up your gear.

After discussing this testing issue with a few meter manufacturers, it seems that the entire industry has missed this problem. In fact, electrical inspectors routinely use a 3-light tester to qualify outlets in renovated buildings, but that's where the hot and neutral wires in the wall are most likely to be reversed.

I'm covering a lot of this on www.noshockzone.org and trying to get Lowes and Home Depot to offer training to consumers and electricians on how to check for this condition. Please contact me with any questions or comments.

Mike Sokol - mike@fitsandstarts.com

Thanks for this. I picked up a non-contact voltage tester today and it will be in my toolkit and used at every gig from here forward. I, like most, already had the three-light tester.
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Gus Housen

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Re: Dangerous AC situation in reception hall - PLEASE READ
« Reply #199 on: January 30, 2014, 01:00:36 pm »

I would get rid of that snake, I dont think I could trust it
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