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Author Topic: brain storm an optimal human safety system for back line  (Read 95742 times)

John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #50 on: December 20, 2014, 09:12:03 am »

Existing GFCIs are designed not to work if hot/neutral are reversed-but they might have to be in the tripped condition for this to work (obviously no one expects an electrician to remove a working GFCI and reverse the wires).
I don't quite follow...
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A lot would depend on the reliability of your detection methods-specifically IDing the hot/neutral with a non existent ground.  NCVD are not 100% reliable-and if you bootleg to the wrong wire you create a RPBG-how often is that failure acceptable?  IMO, a floating ground-on a GFCI circuit-is not a big issue for safety, it might be for noise in sound gear.

I have scratched up some simple circuits using passive components that I should be able to determine the obvious Hot or neutral, and neutral or floating. I am not sure how to reliably determine neutral from ground and may not bother.
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In a safety class a few years ago, a trainer explained a pet peeve of his-"Wet Floor" signs.  He pointed out you were aware of a hazard-but did nothing to correct it-ie dry the floor, etc.  I may be pessimistic, but I would lean towards letting it feed through power if hot/neutral were correct and ground was either floating or correct any other conditions cause a lock out.
A visual indication of a good ground could be helpful for gear that needs a good ground to run properly.
 
your vote is registered.
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If you have reverse polarity or RPBG, how confident are you that the circuit is otherwise safe to use?
I would prefer to not operate that way and my whole premise is that I can sense current in the ground and release if it senses more than a few mA.  I could also add another relay that fixes the RPBG so the outlets in the power strip are good, but my gut feeling is that RPBG should light a blinking strobe light and a klaxon alarm until it is fixed. Maybe invent a plug that melts blocking the outlet and prevents other people from using it.
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But then I got zapped last spring by an energized green ground wire, so I my tolerance for incorrect wiring practices is at a low point right now!
Yup many more people get shocked than actually killed but preventing shocks would be nice too.

JR
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #51 on: December 20, 2014, 09:23:14 am »

I may be off base but let me toss something out.  Rather than analog devices have you consider a small uP?  Certainly if a small RF tracer signal ,from an amplified TTL output switched on and off very fast, was induced on the hot leg the corresponding return signal would radically differ with a device that produced a hot chassis.  By producing a couple 100 samples of gear in distress the samples could be plotted and a distress signature identified.

I have written many thousands of lines of micro code so I could easily program a micro, I am not as certain that I could reliably learn anything from injecting a high edge rate signal onto a hot power line. If anything I would probably learn more about if the product has a line cord power filter like used for switching PS units with switching noise. Tell me more if you have a specific idea how this should work.

In my judgment the decision tree for a smart power strip is simple enough that a processor is probably not needed. Cheap modern micros have 12 bit A/D on board so I could use that for my precision current sense, and to maybe determine if ground is bootlegged.  At this point I worry it is already getting too expensive so would like to keep it simple.

Injecting some HF into the ground could be useful to measure ground inductance but probably TMI for an outlet strip. 

JR
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Stephen Swaffer

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Re: brain storm an optimal human safety system for back line
« Reply #52 on: December 20, 2014, 01:19:12 pm »

New (at least quality name brand-I won't use bargain basement junk for my installs) are shipped "tripped"-they must be reset after power is applied.  They will not reset if H-N are wrong-of course on a 2 wire circuit they can not test for polarity.  I have not tried swapping the H-N after power is applied-and the test may not catch that condition since teh primary purpose is to make sue the install is correct.  A "smart" power strip would effectively be "installed" every time it is plugged in-so the H-N test or reset would need to be performed each time to ensure safety.

Neutral-ground distinction would be tough/impossible.  If I install a correctly wired dedicated receptacle 50 feet from a panel you will have a 50 ft #12 chunk of copper for a neutral and another for the ground-the only difference being the color of insulation.  Determining a bootleg neutral should in theory be possible-for a minimum distance from the panel the distinction being 1" of wire vs X number of feet. In most real world cases the distinction should be significant, but you would have to determine a threshhold.
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #53 on: December 20, 2014, 01:41:54 pm »

I am not expert about GFCI but I think I have seen some literature where the differential current transformer is grounded somehow. If there is no ground connected it will probably still work as expected but if the hot-neutral is reversed perhaps not.

I am leaning toward adding two switches to the front end of my smart power strip. One a manual DPDT allows the user to swap hot and neutral if they indicate wrong. The second switch a SPST allows the operator to bootleg the ground to neutral if that ground indicates open.

Only after the smart circuitry determines that hot and neutral are connected properly, "and" a ground is present, then it actuates a relay to connect the power strip to the outlets. A standard GFCI outlet is used in the strip.

Further another sensor detects current flowing in the ground lead. If this ground current exceeds X mA the power relay opens up ALL THREE LINES..... I could sense after the GFCI also but that might require a turn on delay to reset the GFCI.

I could make the polarity and bootleg switching automatic with extra relays but IMO that is too much expense, the operator cannot get the relay to connect power without getting everything set right first so no risk of making that manual. If they try to change it while it's already working it will just shut off. 

Now the minor details of figuring out how to do all this but so far nothing looks like it will break the bank or amount to rocket science.

JR

PS: I ASSume a bootleg ground is only dangerous in the reverse polarity case, or if a neutral opens up dumping current into ground. My detection circuitry should protect against both of those scenarios. 
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Mike Sokol

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Re: brain storm an optimal human safety system for back line
« Reply #54 on: December 20, 2014, 02:07:14 pm »

PS: I ASSume a bootleg ground is only dangerous in the reverse polarity case, or if a neutral opens up dumping current into ground. My detection circuitry should protect against both of those scenarios.

Because an open neutral on a bootleg ground will back-feed the hot voltage/current back into the chassis ground via the resistance of the device, code will NEVER allow a bootleg ground of any kind. While I personally feel that a bootleg ground on an outlet that has proper polarity isn't that dangerous, the NEC/UL feels that it is and won't allow it. But if your smart box can detect and disconnect that condition, perhaps they'll allow an exception.
« Last Edit: December 20, 2014, 02:09:58 pm by Mike Sokol »
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Lyle Williams

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Re: brain storm an optimal human safety system for back line
« Reply #55 on: December 20, 2014, 02:57:56 pm »

If we are trying for backline safety:

() Backline must meet current electrical standards.
() Backline must be fitted with a GFCI/RCD.  Or at least powered from one.
() Backline must pass an annual/biannual/whatever test.
   plus
() Legal and financial penalties for noncompliance.

Serious financial penalties for venues without correct electrical installations too.  Got a RPBG? That'll cost you $10k and your venue is closed until it's fixed.   Taking this seriously now?  Still just want the barman to do your electrical works before the morning drunks arrive?

Dangerous stuff is only dangerous because we allow it to continue to be used in the name of art or expedience.  In any other industry gear that was unsafe would be scrapped or refurbished.

The level of safety society accepts is the level of safety you get.
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Lyle Williams

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Re: brain storm an optimal human safety system for back line
« Reply #56 on: December 20, 2014, 03:12:05 pm »

What I am trying to say is that this isn't a technology problem.  It is a bands-and-venues-willing-to-be-unsafe problem.

There is nothing electrically unique about the live sound environment, except that the quest for killer tone is being allowed to trump safety.

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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #57 on: December 20, 2014, 04:45:51 pm »

What I am trying to say is that this isn't a technology problem.  It is a bands-and-venues-willing-to-be-unsafe problem.

There is nothing electrically unique about the live sound environment, except that the quest for killer tone is being allowed to trump safety.

In fact my whole premise is that musical performance spaces are a unique case because musicians routinely find themselves touching two different power drop EGCs (back line EGC via a guitar and FOH EGC via a mic). The sundry dedicated protection devices do not specifically anticipate a hot external EGC. Of course if every outlet and every piece of equipment was wired and performing properly there never would be a hot EGC or any dead musicians.

I am doing this mainly as a mental exercise and I may build a proof of concept just to prove to myself it can be done, but I will not attempt to arm wrestle UL to take my tens of thousands of dollars so I can tool up and sell a tiny handful of these. That would be stupid, and I try not to be stupid.

Less stupid might be to wire a stinger cap into a GFCI power strip to provide a little extra human safety.

 JR
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Stephen Swaffer

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Re: brain storm an optimal human safety system for back line
« Reply #58 on: December 20, 2014, 05:41:39 pm »

The main reason I lean towards a lockout condition if the receptacle is not acceptable is that management tends have tunnel vision-if it works great of not fix it-but if it works with a work around it still works so ho cares?  IN an industrial setting I learned that a temporary work around often became permanent because who wants to interuupt production or spend money to fix something that is not broken?  If the only way to get power for the band is to run a drop cord across the managers desk and unplug his Kuerig machine so you have a good receptacle to use he might decide to fix it.

I realize that making it painful for management by not doing a work around is more doable as an employee vs as a guest sound provider (they might just decide to hire the guy down the street that doesn't care about safety), but that is why I don't care for work arounds.

Of course the other side of that coin is that allowing the operator to properly configure the wiring would encourage leaving the safety device in the circuit rather than just bypassing in a pinch-and I have always felt making safety convenient was a good thing..
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #59 on: December 20, 2014, 06:32:14 pm »

The main reason I lean towards a lockout condition if the receptacle is not acceptable is that management tends have tunnel vision-if it works great of not fix it-but if it works with a work around it still works so ho cares?  IN an industrial setting I learned that a temporary work around often became permanent because who wants to interuupt production or spend money to fix something that is not broken?  If the only way to get power for the band is to run a drop cord across the managers desk and unplug his Kuerig machine so you have a good receptacle to use he might decide to fix it.

I realize that making it painful for management by not doing a work around is more doable as an employee vs as a guest sound provider (they might just decide to hire the guy down the street that doesn't care about safety), but that is why I don't care for work arounds.

Of course the other side of that coin is that allowing the operator to properly configure the wiring would encourage leaving the safety device in the circuit rather than just bypassing in a pinch-and I have always felt making safety convenient was a good thing..
I am thinking of something for the musician to carry with him to protect himself from bad venues, not as a semi-permant fix for a venue. Hopefully any outlets he identified as dangerous will get taped over or identified as bad after the gig. The show usually goes on anyhow, just trying to make the musician's personal space a little safer. 

JR
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Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/
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