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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 114184 times)

John Roberts {JR}

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Re: freak accident? help!
« Reply #30 on: May 03, 2012, 09:46:45 pm »

If we're done scaring each other, to offer some practical advice, you need a reliable external ground reference to determine which outlets are trying to kill you, BUT you can safely make it through 99.9% of your gigs, plugging into random outlets that are available to you, just by confirming that the outlets all agree with each other.  i.e. if there is no significant voltage between the sundry grounds, they are probably all OK, and if they are all bad the same way it's pretty much relative so less of a threat.

A voltmeter and a very long extension cord can generally allow you to confirm that you don't have any killer outlets lurking around. If you can plug in close to the electrical panel, that is likely to be properly grounded..

JR
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Mike Sokol

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Re: freak accident? help!
« Reply #31 on: May 03, 2012, 10:30:58 pm »

If we're done scaring each other, to offer some practical advice, you need a reliable external ground reference to determine which outlets are trying to kill you, BUT you can safely make it through 99.9% of your gigs, plugging into random outlets that are available to you, just by confirming that the outlets all agree with each other.  i.e. if there is no significant voltage between the sundry grounds, they are probably all OK, and if they are all bad the same way it's pretty much relative so less of a threat.

A voltmeter and a very long extension cord can generally allow you to confirm that you don't have any killer outlets lurking around. If you can plug in close to the electrical panel, that is likely to be properly grounded..

JR
I've actually built that exact rig you describe, but its best use is detecting ground loop differential voltages. Be aware that your test could in fact protect the gear, but kill the artist or engineer who touches a mic or guitar and a real ground such as a metal rail at the same time.

I've used a Fluke VoltAlert on hundreds of ground testing situations both real-world and test-bed, and find that it's close to 100% accurate at finding elevated voltages on sound gear. Even though a Fluke VoltAlert is rated for 90 to 1,000 volts, it will reliably trigger on as little as 40 volts AC. Since any "hot" ground is likely to be at a full 120 volts, you can detect a dangerous ground voltage from inches to a few feet away from electrified gear. It's as simple as pointing a non-contact AC tester in front of the outlet to know if the ground is energized, and using a standard voltmeter to make sure it's really 120 volts. I simply walk around the stage an point a VoltAlert at every mic and guitar amp on stage, and in a few minutes KNOW that the stage is safe from shock. Using a GLIT (Ground Loop Impedance Tester) such as the Amprobe INSP-3 is the final test that will confirm if the ground will actually carry sufficient current to trip a 20 amp circuit breaker if things short out to the chassis.

Since I built a test rig that will let me create these hot-ground and ground-loop situations at will, both on the bench and in the field, I've done a lot of experimenting with the best ways to find and eliminate ground loop hums and detect hot chassis situations. I would like the opportunity to demonstrate how this works to the pro-audio industry, and will turn over my design data and field notes to any manufacturer who wants to replicate my experiments for peer review.

Another thing I've been promoting in my ASSIST sound installer classes is that venues that will be installing new sound systems should have an assay done on their entire electrical system to determine if there's any hot or open grounds as well as limit ground loop differential voltage due to double-bonded G-N connections in sub panels. Of course, a standard electrician or inspector doesn't know enough about grounds to detect a RPBG or Ground Loop differential, so they need to be educated on sound system grounding. But once I show them how it works, they understand completely.

Mike Sokol
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John Hyun

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Re: freak accident? help!
« Reply #32 on: May 03, 2012, 10:39:17 pm »

thanks Mike for the info.  they do help.

but I still have some unanswered questions a few posts back.  if anyone can chime in, please advise!

thanks.
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Samuel Rees

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Re: freak accident? help!
« Reply #33 on: May 03, 2012, 10:41:43 pm »

The obviously logical and responsible solution is to get a magnetic implant in your hand so you can "sense" live gear. Haha.
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Mike Sokol

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Re: freak accident? help!
« Reply #34 on: May 03, 2012, 10:58:04 pm »

thanks Mike for the info.  they do help.

but I still have some unanswered questions a few posts back.  if anyone can chime in, please advise!

thanks.

To answer your questions:

Quote
1) I definitely feel obligated to inform the reception hall.  Our liaison came in when she saw all the smoke and asked if everything was okay.  She didn't seem to think it was serious since nobody spoke to me afterwards.  If I'm the first to get this problem, I won't be the last.  How should I go about this?  I have a feeling it might just get brushed by.

Yes, I would ask the hall to contact an electrician to verify grounds. And show him my video on RPBGs and have them run a test wire from a real ground such as the main box. Not to do so could cause some future sound tech to face a similar or worse situation. 

Quote
2) I didn't get to test my gear yet (the EQ which the heated cable was connected to).  Is there any damage I should inspect?  My snake seemed to work fine, but would the heat cause any damage (other channels, the box, etc.)

You need to open up everything that was connected to another piece of gear and visually inspect the ground wires for signs of melting. Note that you could blow the ground wires off of gear with audio transformer outputs which many not show up until you hook it up to something else.

Quote
3) So seeing that the channel with the EQ was the only one frying, it was probably the outlet the rack was connected to?  If that's the case, how come none of the other stuff connected heated? the mixer and rack was connected to same outlet.

If you're lucky the damage will be confined to a single current path as you describe.

Quote
4) the rack has a power conditioner.  guess that doesnt help in this situation does it?

Nope, they don't do squat to protect you from RPBG outlets. There's no way for them to know the ground is hot if the neutral is also hot.

Quote
5) what exactly do I need to test?  seem's like people are posting different things.
I would open up and visually inspect everything you can for blown traces and wires, and try out each piece of gear separately. You can certainly damage transistors and IC's as well, so the damage may not be confined to just burnt grounds.

Hope this helps.... Mike
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John Roberts {JR}

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Re: freak accident? help!
« Reply #35 on: May 03, 2012, 11:34:00 pm »

I've actually built that exact rig you describe, but its best use is detecting ground loop differential voltages. Be aware that your test could in fact protect the gear, but kill the artist or engineer who touches a mic or guitar and a real ground such as a metal rail at the same time.

I've used a Fluke VoltAlert on hundreds of ground testing situations both real-world and test-bed, and find that it's close to 100% accurate at finding elevated voltages on sound gear. Even though a Fluke VoltAlert is rated for 90 to 1,000 volts, it will reliably trigger on as little as 40 volts AC. Since any "hot" ground is likely to be at a full 120 volts, you can detect a dangerous ground voltage from inches to a few feet away from electrified gear. It's as simple as pointing a non-contact AC tester in front of the outlet to know if the ground is energized, and using a standard voltmeter to make sure it's really 120 volts. I simply walk around the stage an point a VoltAlert at every mic and guitar amp on stage, and in a few minutes KNOW that the stage is safe from shock. Using a GLIT (Ground Loop Impedance Tester) such as the Amprobe INSP-3 is the final test that will confirm if the ground will actually carry sufficient current to trip a 20 amp circuit breaker if things short out to the chassis.

Mike Sokol

Going into my way-back machine I vaguely recall a device based on old neon lamps that IIRC took 90V to light. But the one I recall had two metal probes so required direct conduction.

That Fluke Volt alert(tm) sounds like a useful tool.

If you have a metal superstructure... measure it...  This isn't rocket science.  I fear the trusty plug in receptacle tester is giving people a false sense of security.

JR


 

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Rob Spence

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Re: freak accident? help!
« Reply #36 on: May 04, 2012, 12:00:42 am »

I don't have anything to add to the safety discussion but I do have a question for the OP on his configuration.

Why is there an EQ wired into the output of the DSP?
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Mike Sokol

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Re: freak accident? help!
« Reply #37 on: May 04, 2012, 07:46:30 am »

Going into my way-back machine I vaguely recall a device based on old neon lamps that IIRC took 90V to light. But the one I recall had two metal probes so required direct conduction.

That Fluke Volt alert(tm) sounds like a useful tool.

If you have a metal superstructure... measure it...  This isn't rocket science.  I fear the trusty plug in receptacle tester is giving people a false sense of security.

JR
You are correct.... a 3-light receptacle tester misses a lot of faults, and can indicate the outlet is wired correctly, when in fact it could be wired with a hot ground which can cause equipment damage and electrocution. If your mixing console or stage amps are plugged into a hot-ground, a non-contact tester will beep at you if you point it at a mic, amp, or metal surface. It will even beep if you point it at the outside of any XLR cable connected to the gear. The larger the charged surface, the further away the VoltAlert will detect. So a microphone at 120 volts will detect from 4 inches away, while a mixing console at 120 volts with a few square feet of area will detect at 12 inches away. An RV or tour bus at 120 volts will detect at perhaps 2 to 3 feet away.   

See http://www.youtube.com/watch?v=jF3Ntoa8ab8 for my video on how to use a VoltAlert to test outlet polarity.

See http://www.youtube.com/watch?v=obeh9m4OMv4 for a Q&A on the VoltAlert itself.

If you have any questions on how this works, my email is mike@fitsandstarts.com

Mike Sokol
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Jay Barracato

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Re: freak accident? help!
« Reply #38 on: May 04, 2012, 07:58:19 am »

You are correct.... a 3-light receptacle tester misses a lot of faults, and can indicate the outlet is wired correctly, when in fact it could be wired with a hot ground which can cause equipment damage and electrocution. If your mixing console or stage amps are plugged into a hot-ground, a non-contact tester will beep at you if you point it at a mic, amp, or metal surface. It will even beep if you point it at the outside of any XLR cable connected to the gear. The larger the charged surface, the further away the VoltAlert will detect. So a microphone at 120 volts will detect from 4 inches away, while a mixing console at 120 volts with a few square feet of area will detect at 12 inches away. An RV or tour bus at 120 volts will detect at perhaps 2 to 3 feet away.   

See http://www.youtube.com/watch?v=jF3Ntoa8ab8 for my video on how to use a VoltAlert to test outlet polarity.

See http://www.youtube.com/watch?v=obeh9m4OMv4 for a Q&A on the VoltAlert itself.

If you have any questions on how this works, my email is mike@fitsandstarts.com

Mike Sokol

Mike,

Just to confirm, you are talking about the $25 gadget shaped like a pen? At that price, why wouldn't everyone carry one? I will be adding it to the tool box this weekend.
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Mike Sokol

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Re: freak accident? help!
« Reply #39 on: May 04, 2012, 08:27:11 am »

Mike,

Just to confirm, you are talking about the $25 gadget shaped like a pen? At that price, why wouldn't everyone carry one? I will be adding it to the tool box this weekend.
Yup, here's the model I use in all my tests and demonstrations:

Fluke 1AC A1 II   ACV Detector 90-1000V, ENGL/LASPN/CFR Flat-tip

You can get one at Sears for $27 http://www.sears.com/shc/s/p_10153_12605_03470489000P and it's also available on Amazon for a few bucks less (but you pay shipping). http://www.amazon.com/dp/B000EJ332O/ref=asc_df_B000EJ332O2002345?smid=A1CTV5F7AC4RR1&tag=dealtmp3938-20&linkCode=asn&creative=395105&creativeASIN=B000EJ332O

Make sure you get the standard 90 to 1,000 volt model (Fluke 1AC A1 II) which will detect hot grounded chassis down to 40 volts. Fluke also makes a 24-volt model for alarm and machine control testing, but it's too sensitive to differentiate between hot and neutral contacts on an outlet. Basically, the low-voltage version will beep on ANYTHING, so get the standard 90-1,000 volt model.

And yes, I think that everyone who plugs their expensive sound gear into unknown outlets every week should keep one in their tool bag. It takes literally a few seconds to confirm that the grounds are not electrified and safe to plug in. It will also let you know if a guitar player has broken off the ground pin on his power cord which will electrify his stage amp and the strings on his guitar as well. Hey, I don't want musicians telling me they got shocked from one of my mics, when they really got shocked from their own guitar....

Mike
« Last Edit: May 04, 2012, 08:38:33 am by Mike Sokol »
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