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Author Topic: brain storm an optimal human safety system for back line  (Read 105679 times)

Stephen Swaffer

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Re: brain storm an optimal human safety system for back line
« Reply #290 on: June 24, 2015, 06:02:49 pm »

One comment on the GFCI-stinger.  Someone, on another thread, alluded to doing this in an installed GFCI.  IMO, this is a violation of Code and a bad idea.  Since this is primarily for commercial applications and most commercial uses metallic boxes and given that the conductors entering a given box are not GFCI protected-the device box and mounting strap for the GFCI need to be solidly grounded to trip OCPD in the event of a fault.

A GFCI protected strip is an entirely different situation.
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Steve Swaffer

John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #291 on: June 24, 2015, 06:36:02 pm »

One comment on the GFCI-stinger.  Someone, on another thread, alluded to doing this in an installed GFCI.  IMO, this is a violation of Code and a bad idea.  Since this is primarily for commercial applications and most commercial uses metallic boxes and given that the conductors entering a given box are not GFCI protected-the device box and mounting strap for the GFCI need to be solidly grounded to trip OCPD in the event of a fault.

A GFCI protected strip is an entirely different situation.

OCPD Obsessive Compulsive Personality Disorder? I ASSume you mean blow the branch fuse/breaker?

+1  The legal status of this is still TBD WRT UL ground labeling et all. The 0.15uF stinger cap is sized to exceed the 5mA GFCI fault current threshold, not 15A. If there is an external mains voltage fault... It will not trip any breakers but the path will be current limited to below hazardous levels. 

JR
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Stephen Swaffer

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Re: brain storm an optimal human safety system for back line
« Reply #292 on: June 24, 2015, 10:53:43 pm »

OCPD=Over Current Protective Device so yes a fuse or circuit breaker.

My concern is the difference between an installed GFCI and a portable device.

If you use the stinger cap on an installed GFCI-especially in a a metal box, then there is potential for non-GFCI protected conductors to come in contact with the box(happens more often than you would think).  Since the stinger cap will not trip the OPCD, then the box is energized thus energizing the ground pin on the GFCI-from a conductor that is not GFCI protected creating a hazard.

I think the idea is a good one-but good ideas wrongly applied can be bad.
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Steve Swaffer

John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #293 on: June 24, 2015, 11:03:29 pm »

OCPD=Over Current Protective Device so yes a fuse or circuit breaker.

My concern is the difference between an installed GFCI and a portable device.

If you use the stinger cap on an installed GFCI-especially in a a metal box, then there is potential for non-GFCI protected conductors to come in contact with the box(happens more often than you would think).  Since the stinger cap will not trip the OPCD, then the box is energized thus energizing the ground pin on the GFCI-from a conductor that is not GFCI protected creating a hazard.

I think the idea is a good one-but good ideas wrongly applied can be bad.
The junction box can and probably should be grounded . Just the Ground lead feeding the outlet needs to be cap coupled for stinger GFCI.

But still agreed,,, not intended for permanent install... since by definition a permanent junction box being permanent should be wired correctly. This is most useful IMO when plugging into unknown power drops.

Performance spaces may be different again, but still if all of the power drops in the venue are good there is no problem.  Permanent back-line outlets where the FOH is often on tails or semi-permanant wiring might have some merit.

JR
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Don't tune your drums half-ass. Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Stephen Swaffer

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Re: brain storm an optimal human safety system for back line
« Reply #294 on: June 25, 2015, 11:16:14 am »


The junction box can and probably should be grounded . Just the Ground lead feeding the outlet needs to be cap coupled for stinger GFCI.


That would work-except mounting strap for the receptacle is typically connected to the receptacle ground so the cap would be shorted out and ineffective.  It would almost have to be a custom built GFCI receptacle (an isolated ground GFCI !)-and that would have to be UL listed to meet Code-and as you pointed out the market probably would not justify the investment.

Not trying argue-just trying to make sure people think through possibilities.  In another thread, someone asked about tying the grounds on a couple of Furman's together that were fed by 2 separate power sources.  Fed from 2 separate GFCI/stingers would place 2 caps in parallel thus doubling the potential current.  GFCI's should still protect-but now this does mandate that all potential sources be GFCI protected.

It is usually relatively easy to get things to work correctly as intended-not always so easy to anticipate all the incorrect ways something might be used-which I am sure you understand after a career in design.
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Steve Swaffer

John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #295 on: June 25, 2015, 12:49:40 pm »

That would work-except mounting strap for the receptacle is typically connected to the receptacle ground so the cap would be shorted out and ineffective.
Indeed it is... I remembered that last night after the lights were out but just before my head hit my pillow. I did not get up again last night.

My smart GFCI outlet proto is mounted into a plastic junction box but that's probably not to code either.
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It would almost have to be a custom built GFCI receptacle (an isolated ground GFCI !)-and that would have to be UL listed to meet Code-and as you pointed out the market probably would not justify the investment.
I think there may be exceptions for use with a non-bonded ground as long as the status or non-status of the ground is labelled.
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Not trying argue-just trying to make sure people think through possibilities.  In another thread, someone asked about tying the grounds on a couple of Furman's together that were fed by 2 separate power sources.  Fed from 2 separate GFCI/stingers would place 2 caps in parallel thus doubling the potential current.  GFCI's should still protect-but now this does mandate that all potential sources be GFCI protected.
We can think this to death...  The stinger cap current is just enough to trip the GFCI (around 7mA IIRC) so two or three in parallel are still not deadly. It takes some tens of mA to get stuck to the current.

Another concern is stinger mixed with a non-stinger outlet used on say guitar pedals, where the stinger cap is now shorted out and the musician is at risk again. The good news is that my tester tells me the stinger cap prevents ground loop hum should that dual ground path scenario occur, but the guitar player is not protected from external mains faults (like RPBG at FOH or RPBG on the second outlet).
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It is usually relatively easy to get things to work correctly as intended-not always so easy to anticipate all the incorrect ways something might be used-which I am sure you understand after a career in design.
Yup... while we can debate how easy it is to get a hum free guitar rig, and how much of a risk this (hot mic) is. But I do not debate that people are remarkably creative about operating things wrong.

I believe that the stinger equipped GFCI power strip is about as harmless to human safety as I can imagine.

#1 if the guitar amp goes rouge and dumps current to it's chassis, it will trip the GFCI removing power.
#2 if the FOH has a hot ground, or if the outlet is wired RPBG the hazard will be current limited to 7 mA.

Lifting the guitar amp ground (as many musicians still do) will actually be less dangerous for RPBG situations, while providing zero protection agains internal amp faults. 

Only my smart outlet lifts ground and releases power if it detects excessive current in the ground. This is too complicated and expensive to be commercial. and UL will have an even harder time blessing a safety ground path that goes through a normally open relay.

I am addressing a fairly narrow type of fault. Musicians are more likely to expose themselves to multiple different branch circuit grounds than normal consumers. This is probably rare enough that UL wouldn't want to make special rules.

FWIW I also like the idea of using wireless mics for guitar players (and baptismal clergy, and swimming pool karaoke, and etc). A far cheaper solution to mic shock hazard would be an insulated outer covering for mics. A dynamic mic could even use a stinger cap ground, but phantom powered mics would not be happy.   

Note: I have seen a special line cord/plug set that senses current in a ground shield and open up power if a threshold current is exceeded. A variant on this for a GFCI power strip might work and already be listed. (I'll check that out).

JR

[edit] OK ignore that protected line cord...it senses for current shield to neutral, so ground is either pass through or missing... not much help there.  [/edit]
« Last Edit: June 25, 2015, 01:02:39 pm by John Roberts {JR} »
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Don't tune your drums half-ass. Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Stephen Swaffer

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Re: brain storm an optimal human safety system for back line
« Reply #296 on: June 25, 2015, 03:24:35 pm »

"I believe that the stinger equipped GFCI power strip is about as harmless to human safety as I can imagine."

I agree 100%-and by keeping it as power strip, you avoid a whole can of worms when it comes to building it correctly and Code compliance.  Probably the ideal way to build this is with something like this:

http://www.leviton.com/OA_HTML/ProductDetail.jsp?partnumber=GFM20-3C&section=40493&minisite=10251

with the ground from the cord going to the cap.  This way any hot conductors in the power strip proper are   GFCI protected. 

Making it a portable power strip permits you to control its use-since it is designed for a specific use.  Code has many reference to "engineering supervision" or "under supervision of qualified personnel."  So an argument can be made for exceptions in a controlled supervised manner.  Installed receptacles in public places  are not controlled by any means!
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Steve Swaffer

John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #297 on: June 25, 2015, 04:23:03 pm »

same color as mine...


JR
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Don't tune your drums half-ass. Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Stephen Swaffer

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Re: brain storm an optimal human safety system for back line
« Reply #298 on: June 25, 2015, 06:55:18 pm »

:) I figured it was safe to assume yours was done correctly-just wanted to make sure that copycats understood that some of the nuances really were important-not just happenstance.
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Steve Swaffer

Steve Loewenthal

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Re: brain storm an optimal human safety system for back line
« Reply #299 on: June 25, 2015, 07:18:31 pm »

So I started at the beginning of the thread and read all of page 1. I'll probably read 1 page a day, even though it looks like I can already see how it ends. Looks like a good ending and I am confident I'll learn a few things as my read progresses.
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Steve Loewenthal

"I'm, just the guy in a band that owns the PA and I'm trying to figure out how it works. (Been trying to learn somethin' about it for about 20 years and I hope somethin' learns me soon)"

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Re: brain storm an optimal human safety system for back line
« Reply #299 on: June 25, 2015, 07:18:31 pm »


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