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Author Topic: Stray Voltage - Rossen Report  (Read 9420 times)

Mike Sokol

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Re: Stray Voltage - Rossen Report
« Reply #20 on: November 17, 2014, 08:59:44 am »

"Fault voltage", Leakage voltage,

JR

Seriously, the problem with calling this sort of thing a "leakage voltage" is that there's three distinct types of hot-chassis fails. One is indeed a high-impedance/low-current "leakage voltage" that's measurable whenever there's a disconected EGC. As I often demonstrate in my seminars, even an iPhone with a Apple wall charger develops between 40 and 80 volts on its chassis (depending on which way the charger is plugged into the receptacle). But the available "leakage current" is way below 1 mA, perhaps a few hundred micro-amps at most for a properly build and operating "wall wart". The same holds true for a properly operating guitar amp, even if its ground pin on the power cord is broken off. There may be a little fault current available, but typically not enough to be noticed.   

On the other hand, without an EGC to drain away any fault currents, you won't know when this turns into a mid-impedance (~1,000 ohms)/medium-current "leakage voltage". That's when there's 10 to 100 mA of ground fault current available. There's a pile of ways this can happen from an old, leaky "stinger cap" on a antique Fender amp, to an aging power transformer in your bass amp, or even a bunch of "leaky" gear plugged into the same power strip. Hey, even MOV based "surge strips" leak a small amount of current from hot-to-ground, so once you get around 5 to 10 mA of leakage to the chassis, then it gets VERY noticeable when touched with wet hands. And 20 mA through you is enough current to overcome your hand's opening muscles, which then causes you to clamp onto a hot mic or wire and not be able to let go. Just 30 mA though your chest cavity for more than a few seconds is almost a guarantee of ventricular fibrillation and death by electrocution if help doesn't arrive in time. Ouch. 

The final (and most dangerous of all) "leakage voltage" isn't really a leak at all. It's a hard, low-impedance fault with a hot-to-chassis short which can result from a wire pinched inside a chassis,  cut insulation on a power cord draped over a metal stair on a stage, or even RPBG miswiring in an "upgraded" outlet. Those are the conditions that can cause the OCP breaker to trip or make a mic wire to glow red and catch on fire, and will certainly draw sparks from a grounded screwdriver like the videos above. However, these hard faults are less common than the first two, but not unheard of.

The problem is that without additional testing you really can't determine if you have a low, mid or high-current hot-chassis voltage. But if a properly earth-bonded EGC is in place, then it's impossible for any of the above fault conditons to create a shock hazard. A low-current fault will be drained away without you ever knowing about it. A mid-current fault will likely cause your GFCI to trip for no apparent reason, but saving your life in the process. And a high-current fault may trip an OCP in your distro,  but can kill you in heartbeat (literally) if you get between it and a solid ground.

So my mantra is that your should NEVER feel a sustained shock. A one-time static jolt from walking on the carpet isn't the same thing. I'm talking about a tingle when touching your guitar or mixer or mic and a damp floor or grounded gear at the same time. Or ANY home appliance or power tool for that matter. If you feel a shock from your refrigerator at home while touching the stove or sink, then something has gone seriously wrong with your appliance ground and the next touch could kill you or someone you know/love. So don't let a shock situation exist for longer than it takes you to unplug it from AC power. Fix it NOW before it kills somebody (even you). 
« Last Edit: November 17, 2014, 12:27:24 pm by Mike Sokol »
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Jonathan Johnson

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Re: Stray Voltage - Rossen Report
« Reply #21 on: November 17, 2014, 12:15:43 pm »

Slightly (yeah, maybe a lot) OT, but here's an interesting prediction.

"...it will be possible for a business man in New York to dictate instructions, and have them instantly appear in type at his office in London or elsewhere.  He will be able to call up, from his desk, and talk to any telephone subscriber on the globe, without any change whatever in the existing equipment.  An inexpensive instrument, not bigger than a watch, will enable its bearer to hear anywhere, on sea or land, music or song, the speech of a political leader, the address of an eminent man of science, or the sermon of an eloquent clergyman, delivered in some other place, however distant.  In the same manner any picture, character, drawing, or print can be transferred from one to another place.  Millions of such instruments can be operated from but one plant of this kind."

Nikola Tesla, in 1908 - on his never-finished wireless communication tower, AKA a modern-day cell tower


In the 1940s, back when radios were still the size of iceboxes, my grandfather would tell people, "some day we will have radios you can fit in your shirt pocket" and they would laugh at him. He was just a farmer from the Dakota prairie, but he didn't let that limit his expectations of the world and its future.

(I never knew my grandfather; he died before I was born. My father told me that story.)
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Stop confusing the issue with facts and logic!

Stephen Swaffer

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Re: Stray Voltage - Rossen Report
« Reply #22 on: November 17, 2014, 01:35:54 pm »

Seriously, the problem with calling this sort of thing a "leakage voltage" is that there's three distinct types of hot-chassis fails. One is indeed a high-impedance/low-current "leakage voltage" that's measurable whenever there's a disconected EGC. As I often demonstrate in my seminars, even an iPhone with a Apple wall charger develops between 40 and 80 volts on its chassis (depending on which way the charger is plugged into the receptacle). But the available "leakage current" is way below 1 mA, perhaps a few hundred micro-amps at most for a properly build and operating "wall wart". The same holds true for a properly operating guitar amp, even if its ground pin on the power cord is broken off. There may be a little fault current available, but typically not enough to be noticed.   

On the other hand, without an EGC to drain away any fault currents, you won't know when this turns into a mid-impedance (~1,000 ohms)/medium-current "leakage voltage". That's when there's 10 to 100 mA of ground fault current available. There's a pile of ways this can happen from an old, leaky "stinger cap" on a antique Fender amp, to an aging power transformer in your bass amp, or even a bunch of "leaky" gear plugged into the same power strip. Hey, even MOV based "surge strips" leak a small amount of current from hot-to-ground, so once you get around 5 to 10 mA of leakage to the chassis, then it gets VERY noticeable when touched with wet hands. And 20 mA through you is enough current to overcome your hand's opening muscles, which then causes you to clamp onto a hot mic or wire and not be able to let go. Just 30 mA though your chest cavity for more than a few seconds is almost a guarantee of ventricular fibrillation and death by electrocution if help doesn't arrive in time. Ouch. 

The final (and most dangerous of all) "leakage voltage" isn't really a leak at all. It's a hard, low-impedance fault with a hot-to-chassis short which can result from a wire pinched inside a chassis,  cut insulation on a power cord draped over a metal stair on a stage, or even RPBG miswiring in an "upgraded" outlet. Those are the conditions that can cause the OCP breaker to trip or make a mic wire to glow red and catch on fire, and will certainly draw sparks from a grounded screwdriver like the videos above. However, these hard faults are less common than the first two, but not unheard of.

The problem is that without additional testing you really can't determine if you have a low, mid or high-current hot-chassis voltage. But if a properly earth-bonded EGC is in place, then it's impossible for any of the above fault conditons to create a shock hazard. A low-current fault will be drained away without you ever knowing about it. A mid-current fault will likely cause your GFCI to trip for no apparent reason, but saving your life in the process. And a high-current fault may trip an OCP in your distro,  but can kill you in heartbeat (literally) if you get between it and a solid ground.

So my mantra is that your should NEVER feel a sustained shock. A one-time static jolt from walking on the carpet isn't the same thing. I'm talking about a tingle when touching your guitar or mixer or mic and a damp floor or grounded gear at the same time. Or ANY home appliance or power tool for that matter. If you feel a shock from your refrigerator at home while touching the stove or sink, then something has gone seriously wrong with your appliance ground and the next touch could kill you or someone you know/love. So don't let a shock situation exist for longer than it takes you to unplug it from AC power. Fix it NOW before it kills somebody (even you).
+1

The one thing I would add to this is to treat tripping GFCI with the same concern-don't just go find or install a non-GFCI receptacle to fix the problem.  The latest GFCIs should not reset if they are faulty-so more than likely if it keeps tripping the problem is with the attached appliance/equipment.  I just dealt with a situation where a year ago we had a tripping GFCI on a horse waterer at a stable.  I refused to remove  the GFCI and told them to fix the waterer.  Called me to look at it Saturday-a tenant "electrician" that works for a competitor replaced the GFCI with a standard duplex-and now it doesn't work again.  This time the heater is completely open-I guarantee you it was defective a year ago. Fortunately no person or animal got hurt-this time.
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Steve Swaffer

ProSoundWeb Community

Re: Stray Voltage - Rossen Report
« Reply #22 on: November 17, 2014, 01:35:54 pm »


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