modified GFCI

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JR.
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modified GFCI

Post by JR. » Wed Feb 11, 2015 12:47 pm

This may seem a little unusual for an audio design forum but it is peripherally audio related.

The problem I am trying to address is musicians being electrocuted by faulty mains wiring. In the US we have legacy 2 circuit mains wiring (hot Line and neutral return), that is now superseded by 3 circuit wiring (line-neutral-and safety ground). Many homes and facilities have replaced 2 wire outlets with 3 wire sockets. In those cases the safety ground is often left un-connected, but some perform a "bootleg" connection where safety ground is connected to neutral. In theory this safe and provides safety ground with a low impedance path to trip breakers during a hot chassis fault. In practice bootleg grounds are not acceptable for a few reasons. #1 the neutral line will have half the mains sag voltage riding on it, so will not be very quiet. #2 should the neutral wire back to the panel ever open up, the safety ground and equipment chassis is now energized. and #3 (the bigee), if a two-wire feed has hot and neutral reversed (like several outlets in my house) and bootlegs a ground from the hot line feed, the chassis is now energized with the full mains voltage.

This actually does happen. While I was working at Peavey we got sued once because a guitar player was killed while holding two guitars plugged into different amps, that were plugged into two different outlets. One amp was plugged into the outlet on the kitchen stove so properly grounded. The other guitar amp was connected to a mis-wired "reverse polarity bootleg ground" outlet so that guitar had 120V AC on it's strings and ground. That musician was killed. Peavey survived the court case. The Peavey products were UL approved and UL even testified on Peavey's behalf. The house OTOH was condemned until the wiring could be repaired.

OK long story short, this hot ground situation can also occur is live performance venues when a guitar amp is plugged into one outlet and the guitar player grabs a microphone that is grounded to a mixer plugged into a completely different power drop. A musician in Argentina was just killed a couple months ago by exactly that. The newspaper reports never fully explained the accident's cause, he was holding a guitar and was killed when he grabbed a microphone.

OK that's the set up, now here is my solution.
gfci2.JPG
gfci2.JPG (55.92 KiB) Viewed 19379 times
GFCI outlet drops are protective devices that compare the current flowing in line and neutral wires and disconnect if it senses a difference of more than 5mA... Pretty clever and this does not even rely on ground so can be used on 2 wire circuits. (I have added GFCI outlets to replace the 2 circuit outlets in my bathroom and kitchen). While GFCI outlets will protect agains a fault coming from the guitar amp that is plugged into it, it will not protect a musician from an external shock hazard like the two examples I mentioned above.

So my strategy is to take a page from old school 2-wire guitar amps that grabbed a chassis ground through a "stinger" cap, named that for the occasional shocks received if the stinger cap was connected to line instead of neutral. The stinger cap holds current to less than lethal levels as long as the cap is not shorted, and safety agencies qualify "Y" caps to specifically fail open not shorted.

Adding a cap in series with the ground of a GFCI seems to protect against all hazards. My only question is can the cap coupled ground provide adequate ground impedance for shielding.

In the picture you will see a 3 pos switch so I can alternately lift the ground, patch the stinger cap in series with safety ground, or hard wire the ground. This is mostly for testing since I need to be satisfied this will not degrade guitar tone. Musicians have been known to play dangerous old legacy amps, ignoring safety for the tone.

I had to increase the value of the stinger ground cap to 0.15uF to insure that it would trip the GFCI device that requires 5 mA +/- 1mA. In practice I would prefer a smaller cap, but the 0.15uF is still less than 7 mA so well below the current a human will stick to and get injured.

I sent this to a friend who designs guitar amps to listen to.

As is this will current limit the ground current from a RPBG (reverse polarity bootleg ground) to < 7mA but will not shut down. I am looking at detecting RPBG and shutting down the GFCI power to discourage using such outlets.

JR

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Re: modified GFCI

Post by Speedskater » Wed Feb 11, 2015 3:24 pm

We let that nasty word 'ground' sneak into the conversation.

So does 'ground impedance' mean - impedance to:
a] Safety Ground/Protective Earth (EGC) connection to power company Neutral?
b] Connection to Mother Earth - Grounding Electrode Conductor (GEC)?
c] Connection to component chassis?
d] Connection to to circuit/power supply common?
Kevin

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Re: modified GFCI

Post by ricardo » Wed Feb 11, 2015 4:25 pm

JR. wrote:As is this will current limit the ground current from a RPBG (reverse polarity bootleg ground) to < 7mA but will not shut down. I am looking at detecting RPBG and shutting down the GFCI power to discourage using such outlets.
The device should announce, "A Ground Fault has been detected. This building will self-destruct in 5 mins. Please evacuate."

... and countdown in a sexy female voice of course. :D

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JR.
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Re: modified GFCI

Post by JR. » Wed Feb 11, 2015 5:29 pm

Speedskater wrote:We let that nasty word 'ground' sneak into the conversation.

So does 'ground impedance' mean - impedance to:
a] Safety Ground/Protective Earth (EGC) connection to power company Neutral?
Generally yes. While ground is bonded to neutral at the service panel.
b] Connection to Mother Earth - Grounding Electrode Conductor (GEC)?
Since power company neutral is supposed to be bonded to earth too, yes.
c] Connection to component chassis?
Component chassis for 3-wire line cord products will be bonded to ground pin in the plug so will have my "stinger" cap impedance in series between it and electrical service ground. (Note: at a minimum UL will require notice to the effect that the GFCI outlet does not provide a true EGC.
d] Connection to to circuit/power supply common?
What power supply common...? inside the product PS who knows?

AFAIK GFCI outlets are legal without any ground at all, my stinger cap hopefully provides better noise mitigation than lifting the ground while still protecting the humans from external shock hazards via that (cough) safety ground. As I mentioned musicians have been killed by UL approved safety grounded products plugged into mis-wired outlets. :oops:

JR

PS: While I am not making any such claims the friend of mine who is checking this out, suggested it might be helpful in breaking ground loops. I am not aware of ground loops there (perhaps mains powered pedals?). If there are other grounds connected to that input ground, it will defeat my safety strategy of current limiting the path to ground from the guitar. My friend says he is designing a very high gain guitar amp so has ground noise concerns.

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JR.
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Re: modified GFCI

Post by JR. » Wed Feb 11, 2015 5:38 pm

ricardo wrote:
JR. wrote:As is this will current limit the ground current from a RPBG (reverse polarity bootleg ground) to < 7mA but will not shut down. I am looking at detecting RPBG and shutting down the GFCI power to discourage using such outlets.
The device should announce, "A Ground Fault has been detected. This building will self-destruct in 5 mins. Please evacuate."

... and countdown in a sexy female voice of course. :D
I have semi-seriously considered adding an annunciator to sit there and make an annoying alarm sound when it detects a fault wiring situation, but even with a mis-wired outlet the musician will be safer using this than not... So probably better to not encourage them to plug in around it instead. At least I should blink a rude LED.

JR

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Re: modified GFCI

Post by Speedskater » Thu Feb 12, 2015 2:09 pm

Getting back to:
My only question is can the cap coupled ground provide adequate ground impedance for shielding.
So I looked in my Henry W. Ott book:
"Electromagnetic Compatibility Engineering"
http://www.hottconsultants.com/EMCE_boo ... _book.html

He wrote about 100 pages on the subject. Often with opposite points of view in the same paragraph.
Like:
6.18 Grounding of Shields
A shield does not need to be grounded.
A shield should be connected to the power supply common.

************************
Jim Brown also writes on the subject but it's spread out over a 1000 or more pages.
http://www.audiosystemsgroup.com/publish.htm
Kevin

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JR.
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Re: modified GFCI

Post by JR. » Thu Feb 12, 2015 2:39 pm

Speedskater wrote:Getting back to:
My only question is can the cap coupled ground provide adequate ground impedance for shielding.
So I looked in my Henry W. Ott book:
"Electromagnetic Compatibility Engineering"
http://www.hottconsultants.com/EMCE_boo ... _book.html

He wrote about 100 pages on the subject. Often with opposite points of view in the same paragraph.
Like:
6.18 Grounding of Shields
A shield does not need to be grounded.
A shield should be connected to the power supply common.

************************
Jim Brown also writes on the subject but it's spread out over a 1000 or more pages.
http://www.audiosystemsgroup.com/publish.htm

I'm not sure if it was in the Ott book or the Morrison book I bought back in the '70s, but cap coupled shields is as old as dirt (well not that old) but an old technique for providing RF shielding while avoiding LF ground loops, or mutual ground current contamination (of course RF shielding can use even smaller caps). One conflict from the old books on shielding is that the world has changed in the several decades since then with a lot more RF energy around us.

The use of stinger caps to ground guitar amp chassis (to neutral) is also a very mature technology, pretty much abandoned after we started adding ground pins to outlets.

I know this will work from a safety perspective, but I do not know how transparent it will be to a very picky guitar player. I want it to be completely unnoticeable.

Coincidentally the design engineer I sent this for testing is also named James Brown, but not that same Jim Brown you mention... This is "James Brown" and IMO perfectly qualified to test for guitar/amp tone issues. He has spent decades designing guitar amps and associated guitar effects for a living.

JR

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mediatechnology
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Re: modified GFCI

Post by mediatechnology » Thu Feb 19, 2015 3:46 pm

Any feedback yet?

This is very interesting.
For some reason I'm reminded of the EquiTec balanced power system that, because it had what is essentially a floating neutral, required a GFI.
IIRC they used the absence of a code requirement for a utility ground when a GFI was used to their advantage to provide a utility-isolated ground.
My memory on that is foggy so I may be wrong.
But as I recall the presence of a GFI gave them latitude within the code.

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JR.
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Re: modified GFCI

Post by JR. » Thu Feb 19, 2015 6:47 pm

mediatechnology wrote:Any feedback yet?
No, I heard that the unit arrived but my friend was swamped with NAMM orders (he makes and sells guitar effects pedals ). Besides the backorder his area was slammed with ice storms so lost power for a while too.

This is very interesting.
For some reason I'm reminded of the EquiTec balanced power system that, because it had what is essentially a floating neutral, required a GFI.
IIRC they used the absence of a code requirement for a utility ground when a GFI was used to their advantage to provide a utility-isolated ground.
My memory on that is foggy so I may be wrong.
But as I recall the presence of a GFI gave them latitude within the code.
My understanding is that UL will probably accept the cap in place of a hard wire ground bond with the GFCI as long as the outlet is marked "NO EGC" .

Now I just need to hear from my guitar guru how the noise floor is affected if at all.

JR

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mediatechnology
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Re: modified GFCI

Post by mediatechnology » Thu Feb 26, 2015 9:56 am

There's an interesting test circuit for a GFI in the AD8436 True RMS to DC Converter application note.

http://www.analog.com/static/imported-f ... N-1341.pdf begins on page 11.

http://www.analog.com/media/en/technica ... AD8436.pdf

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