modified GFCI
Posted: Wed Feb 11, 2015 11:47 am
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 In the US we have legacy 2 circuit mains wiring (hot Line and neutral return), that is now superseded by 3 circuit wiring (line-neutral-and safety ground).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. 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
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. 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