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Author Topic: brain storm an optimal human safety system for back line  (Read 97438 times)

frank kayser

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Re: brain storm an optimal human safety system for back line
« Reply #80 on: December 28, 2014, 11:37:26 pm »

While there may be some debate, in my judgement shunting the two safety grounds together will protect against all of the sundry hazards I enumerated, while it counts on the mains power circuit breakers to trip from over current.
Actually a practical way is to add a switch so the two grounds are not shunted until the switch is closed completing the circuit between the two grounds. Of course the cable jockey needs to be careful about touching the mic cable ground and guitar cable ground at the same time. Of course the downside to the shunt having an off switch is that operator may open the switch to reduce hum, if the grounds are more than the several volts apart that the diodes account for, and hum happens. 
Think about it, current is not the problem as much as power dissipation. 20A x 30V is 600W. So any diac will quickly turn into a puddle of melted or vaporized silicon. I'll stay with my diode bridges, I have tested them with 120V  across them and they survive taking out a typical fuse/breaker. no... that would be government over reach... while I wouldn't mind GFCI in bar outlets.

I may be imagining a problem that isn't really serious. If more musicians were killed, we might see more electrical building inspectors that know what they are doing to prevent problems. The last one we heard about was in Argentina and not to stereotype, but who knows what kind of electrical inspections they have.

A possible market for a high end box is for the money channel... Big name talent can afford the insurance for not getting zapped while using their favorite gear... While how many of them are still on wired mics?

So chicken egg... the guys who can afford it don't need it, and the guys who need it can't (won't) pay for it.

Still an interesting mental exercise.

JR


Bear with me please - something isn't clicking.
Am I correct that JR is connecting two separate grounds through diodes, the theory that once the safety ground differential goes >2v, the grounds connect either to bleed off voltage to the side with lower potential, or if the difference is great enough, trip some other safety device upstream?
This "device" would be on the AC line, correct?
Here is my confusion - the "poor man's distro". http://www.triktags.com/power.htm Two separate circuits (hopefully) with grounds bonded to reduce the possibility of ground loops.
Are you suggesting something like a "poor man's distro" except instead of direct bonding, some type of diode arrangement would be used?
Am I missing something?
As a side question, what would happen if the "poor man's distro" were plugged into a correctly wired outlet and the second being RPBG?  Dead short right then and there, correct?  What about arc flash? 


Am I hopelessly wrapped around the axle here?
frank


ps.  I do appreciate all the mental horsepower being spent - I believe more than a mental exercise.  I would like to be able to say no one will be electrocuted on any of my stages. 
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Scott Holtzman

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Re: brain storm an optimal human safety system for back line
« Reply #81 on: December 29, 2014, 12:02:18 am »


 I would like to be able to say no one will be electrocuted on any of my stages.

Actually I can think of a number of folks I would pay to see electrocuted on stage.  However as I don't think I would do well in prison I will leave that to the guy upstairs (not the one in the spot loft).

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Lyle Williams

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Re: brain storm an optimal human safety system for back line
« Reply #82 on: December 29, 2014, 12:31:57 am »

Arc flash depends on the capacity of the feed.  If we are talking about RPBG, then hopefully we are well below that.

If the fault would have been cleared by tripping a breaker, save for the lack of a common ground between FeedA and FeedB, then adding a should-really-be-redundant ground connection would seem like a simple answer.

Grounding the racks to each other should be done before energising either feed obviously.  Otherwise we're just killing the tech instead of the muso.

A distro that ties the grounds together also seems to fix the problem.
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Steve M Smith

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Re: brain storm an optimal human safety system for back line
« Reply #83 on: December 29, 2014, 03:41:01 am »

Actually I can think of a number of folks I would pay to see electrocuted on stage.


We could have a button to press at FOH for when they do something wrong - like in that episode of The Simpsons!


Steve.
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #84 on: December 29, 2014, 10:08:34 am »


Bear with me please - something isn't clicking.
Am I correct that JR is connecting two separate grounds through diodes, the theory that once the safety ground differential goes >2v, the grounds connect either to bleed off voltage to the side with lower potential, or if the difference is great enough, trip some other safety device upstream?
exactly
Quote
This "device" would be on the AC line, correct?
Actually between mic pin 1 and guitar cord sleeve... One a console ground and the other the guitar amp chassis. Both should be connected to their respective mains grounds, but aren't always.
Quote
Here is my confusion - the "poor man's distro". http://www.triktags.com/power.htm Two separate circuits (hopefully) with grounds bonded to reduce the possibility of ground loops.
Not to veer off topic but bonding the distro grounds together can actually create a loop, but loops are not the real problem, ground potential voltage differences are, and bonding the grounds together manages that.  BTW maybe teach them how to spell "bus" also, unless they use buss fuses inside... :P
Quote
Are you suggesting something like a "poor man's distro" except instead of direct bonding, some type of diode arrangement would be used?
Yes and no... I am not directly messing with the FOH power drop ground or the back-line power drop ground. One step removed and loosely bonding  (through diodes) the mic ground to the guitar ground. These are the points of contact for the musicians to electrical faults. A poor man's disto will not protect against a rouge guitar amp, but a poor mans distro "and" a GFCI should.
Quote
Am I missing something?
nah
Quote
As a side question, what would happen if the "poor man's distro" were plugged into a correctly wired outlet and the second being RPBG?  Dead short right then and there, correct?  What about arc flash? 
yup... but as long as your fingers aren't touching the blades of the plug, it should just take out the breaker for the RPBG branch.
Quote

Am I hopelessly wrapped around the axle here?
frank


ps.  I do appreciate all the mental horsepower being spent - I believe more than a mental exercise.  I would like to be able to say no one will be electrocuted on any of my stages.
I know that modern guitar amp design is much safer, has been for decades. Wiring practices in many venues is still suspect as evidenced by the singer/guitar player in Argentina recently. The fact that we are not reading about musician deaths every weekend here suggests that this is mostly under control, but not completely. Statistically playing and singing on stage is probably safer than driving your car to the gig.

JR
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Mike Sokol

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Re: brain storm an optimal human safety system for back line
« Reply #85 on: December 29, 2014, 10:19:49 am »

I was pulling the 4 diode drops (roughly 2.4V peak) out of thin air... do you think I need to go to 3x or 4x bridges (4.8V) to be safe? I don't want to conduct current between safety grounds unless there is a hazardous fault, to keep the audio clean.

For human safety 5V is nothing, and as soon as the diodes heat up and melt they fail as dead shorts anyhow...

JR

JR, some 35 years ago I was able to solve ground-loop hum problems in my band's sound system by using back-to-back 5 volt Zener diodes (BIG ones) to isolate the ground path of the bass guitar amp. So 20+ years after I did that, Ebtech came out with their Hum-X product which uses a pair of reversed/paralleled silicon diodes that begin conducting around 0.5 volts. I've found that the Hum-X product will stop ground loop hum in some cases of small ground loop voltage differentials (GLVD's?), but doesn't help with a lot of larger voltages.

As you've noted earlier, if everything was wired correctly we wouldn't have to worry about this topic, but we all know that's a pipe dream. I was as a church in Dover, DE yesterday conducting a Tune-Up session on a sound system and found one Crown power amp for the monitors humming with a ground loop like crazy. Others amps in the rack weren't humming, so I tried adding a WW ISO box to the XLR feed which stopped the hum perfectly. I didn't have time to do an end-to-end voltage check, but a quick clamp-ammeter test showed around 1 ampere of current in the shield without an iso transformer. Previous tests on my bench suggest around 1 amp of ground loop current per volt of GLVD (Ground Loop Voltage Differential), so I'm thinking there was at least 1 volt difference in the grounds. I also suspect this was isolated to a single outlet which could easily have been caused by swapped neutral and ground wires. Now, I wasn't there to evaluate their power system and had a stage full of musicians to train, so I didn't have time for a full evaluation of their power distro. But I'm pretty sure that something like that was the cause. The key point is that whatever you do that cross-connects the grounds of a stage amp and the mixing console, which will almost certainly be powered by different receptacles, needs to operate with at least several volts of GLVD without producing ground loop currents and potential hum. My WAG is up to 5 volts GLVD blocking would be required before creating hum or a fail/tripping condition.
« Last Edit: December 29, 2014, 10:23:28 am by Mike Sokol »
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frank kayser

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Re: brain storm an optimal human safety system for back line
« Reply #86 on: December 29, 2014, 10:57:16 am »

Ding! Thanks, JR, for walking me through your thought process.  Makes much more sense now. 
frank
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #87 on: December 29, 2014, 11:03:46 am »

JR, some 35 years ago I was able to solve ground-loop hum problems in my band's sound system by using back-to-back 5 volt Zener diodes (BIG ones) to isolate the ground path of the bass guitar amp.
Yup me too... back on the '70s I used some diode bridges between chassis and circuit grounds in a rack mount studio product to prevent corruption from the sundry stray ground currents coming from other rack mount products in the same rack.

Power zeners might work, there needs to be enough mass to them that they don't vaporize before the main breaker trips. Small diodes will not hang when exposed to full mains voltage. 
Quote
So 20+ years after I did that, Ebtech came out with their Hum-X product which uses a pair of reversed/paralleled silicon diodes that begin conducting around 0.5 volts. I've found that the Hum-X product will stop ground loop hum in some cases of small ground loop voltage differentials (GLVD's?), but doesn't help with a lot of larger voltages.
I contacted them privately a few years ago and suggested that they use more robust diodes (and/or get UL involved, who would test for reliability.) I never heard back from them, but in the '80s,  I had the Peavey agency guy ask UL about this and they were very receptive, but I was not willing to invest the ten's of $k to open a UL file and prove the approach with formal testing. Ground noise can be managed using proper input and output circuit designs while safely bonding the chassis to mains ground, so no need to fix a non-problem. 
Quote
As you've noted earlier, if everything was wired correctly we wouldn't have to worry about this topic, but we all know that's a pipe dream. I was as a church in Dover, DE yesterday conducting a Tune-Up session on a sound system and found one Crown power amp for the monitors humming with a ground loop like crazy. Others amps in the rack weren't humming, so I tried adding a WW ISO box to the XLR feed which stopped the hum perfectly. I didn't have time to do an end-to-end voltage check, but a quick clamp-ammeter test showed around 1 ampere of current in the shield without an iso transformer. Previous tests on my bench suggest around 1 amp of ground loop current per volt of GLVD (Ground Loop Voltage Differential), so I'm thinking there was at least 1 volt difference in the grounds. I also suspect this was isolated to a single outlet which could easily have been caused by swapped neutral and ground wires. Now, I wasn't there to evaluate their power system and had a stage full of musicians to train, so I didn't have time for a full evaluation of their power distro. But I'm pretty sure that something like that was the cause. The key point is that whatever you do that cross-connects the grounds of a stage amp and the mixing console, which will almost certainly be powered by different receptacles, needs to operate with at least several volts of GLVD without producing ground loop currents and potential hum. My WAG is up to 5 volts GLVD blocking would be required before creating hum or a fail/tripping condition.

Thanks, I'll take that under advisement.

I have continued to think about the premium (smart outlet strip) solution and this is not cheap or trivial. Even with a microprocessor it isn't trivial to detect all the possible errors. My current thinking is to test for any bootleg ground (not simple) and warn if that is detected. If significant voltage or current is detected between the mains and equipment grounds the relay opens or never closes. I suspect this is too expensive, and market to small for this to ever happen, but fun to think about.

JR
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Stephen Swaffer

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Re: brain storm an optimal human safety system for back line
« Reply #88 on: December 29, 2014, 09:25:29 pm »


 no... that would be government over reach... while I wouldn't mind GFCI in bar outlets.
 
So chicken egg... the guys who can afford it don't need it, and the guys who need it can't (won't) pay for it.

Still an interesting mental exercise.


Unfortunately people who can't (won't) take responsibility for their own  well being are too often the justification for government over reach.

I agree about the mental exercise-and hopefully the discussion will raise awareness and understanding among others.

As for the GFCIs for backline-I am curious how many of these venues are required to have periodic safety inspections?  It would be a minimal additional check to use a tester like my local/state inspectors use on every new recept in new construction.  I don't have as much problem with government "overreach" when they are mandating the landlord/venue meet certain standards to protect the tenant/customer.  It might help to pursue raising awareness of this hazard among the inspectors through the  Association of Electrical Inspectors.
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Lyle Williams

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Re: brain storm an optimal human safety system for back line
« Reply #89 on: December 30, 2014, 01:38:14 am »

If you like GFCI/RCD, bring your own.
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