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Old 4th March 2003, 04:21 AM   #1
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Default Electric shock risks

I want to make my next amp with -35-0-35 rails and I'm wondering how dangerous a 70VDC shock would be. I read somewhere that 60hz voltage is over 5 times more dangerous than DC to humans. I really don't want to get into using lethal voltages in my projects. The AC component of the amp's power supply would be sealed off but I'd be using the DC side pretty frequently. Do you guys ever get shocks from your rails?
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Old 4th March 2003, 04:40 AM   #2
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70VDC is definately potentially lethal if you get a belt directly across the supply capacitors. I've been zapped by ~300VDC almost directly across the chest, it was incredibly painful and in hindsight i'm probably lucky to have survived (fortunately it was only a 120uF capacitor). On the other hand, if you get shocked by 35VDC the worst your likely to get is a fairly small burn that might require a few days of attention.
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Old 4th March 2003, 04:49 AM   #3
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Default Re: Electric shock risks

Quote:
Originally posted by Solid Snake
I want to make my next amp with -35-0-35 rails and I'm wondering how dangerous a 70VDC shock would be. I read somewhere that 60hz voltage is over 5 times more dangerous than DC to humans.
Dunno about the five times figure, but an AC voltage is more lethal because the body's impedance is lower under AC conditions. A little like leaky capacitor. At DC, the impedance is quite high. But under AC conditions, the impedance drops depending on frequency. The higher the frequency, the lower the impedance and the more current that will flow through the body.

Suffice to say that it would take a quite unusual situation to find yourself connected between the two supply rails. Touching either rail and ground would only come to 35 volts. So you'd have to have something like one hand on one rail and the other hand on the other.

Quote:
I really don't want to get into using lethal voltages in my projects. The AC component of the amp's power supply would be sealed off but I'd be using the DC side pretty frequently. Do you guys ever get shocks from your rails?
Shocked? Hey, yer talkin' to a professional here!



se
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Old 4th March 2003, 05:44 AM   #4
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Working on high output R.F tube amps, I hate to admit one or two hits at 800 volts D.C. and one real memorable hit from 4,800 volts D.C. at about 1.5 amps from a real tube rig running 4 4-1000's in a grounded grid config. For some reason I thaught I'd check the D.C. + at the top of 2 6000 volt D.C. oil filled caps with my good meter. Don't remember much after the blinding ark. Burned a good mark in my knuckle and out my right foot to the concrete. Meter was fine but I was not.
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Old 4th March 2003, 06:07 AM   #5
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Quote:
Dunno about the five times figure, but an AC voltage is more lethal because the body's impedance is lower under AC conditions. A little like leaky capacitor. At DC, the impedance is quite high. But under AC conditions, the impedance drops depending on frequency. The higher the frequency, the lower the impedance and the more current that will flow through the body.
I seriously doubt the human body has impedence. Doesn't impedence require some kind of magnetic property? Anyway, I read that the higher the frequency past 60hz, the lower the danger. It has to do with the rapidly contracting and expanding muscles in your body. The safe limit of DC through your body is 2mA, while the limit at 60hz is only 0.5 mA. The whole frequency thing has to do with what frequencies your muscles will respond to, 60hz just happens to be one of the most deadly frequencies that can go through your body. The bottom line is how many mA go through you. 2ma @ 60hz will be painful while 2ma at 1Mhz probably won't even be noticed.

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On the other hand, if you get shocked by 35VDC the worst your likely to get is a fairly small burn that might require a few days of attention.
I'm working with -17-0-17 rails now and I can't even feel anything across 34vdc, let alone getting burned. Although supposedly people have died from as low as 25vac.
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Old 4th March 2003, 06:27 AM   #6
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The safe limit, IIRC, is considered to be 70V. Much more and enough current will flow (and a delta V that the nerves will pick up) that you'll feel a shock.

Kinda interesting how tubes start about where shocks do.

Tim
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Old 4th March 2003, 06:29 AM   #7
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70vdc is the safe limit? Where is this written? If so I guess that's good news for my project.
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Old 4th March 2003, 07:43 AM   #8
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Ok, I should have better explained my comments with regard to 35VDC. If both points of the circuit touch your body within about 1 foot, you'll certainly feel a tingle. Within, about 6" and it'll hurt. Within 2" and it'll probably leave a burn. If you talking across both arms and chest then 70VDC is right on the edge where given the right conditions, it could be fatal (or do serious injury) or if your lucky, you'll be ok.
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Old 4th March 2003, 07:46 AM   #9
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Quote:
Originally posted by Solid Snake
I seriously doubt the human body has impedence. Doesn't impedence require some kind of magnetic property?
Not sure what you mean by magnetic property.

Impedance is basically a frequency-dependent resistance brought about by inductive or capacitive reactance.

At DC and low frequency AC, the body behaves as a simple resistance. But as frequency increases, the body behaves more capacitively. And like any other capacitance, as frequency increases, impedance decreases, allowing for greater flow of current.

Quote:
Anyway, I read that the higher the frequency past 60hz, the lower the danger. It has to do with the rapidly contracting and expanding muscles in your body.
Dunno. Haven't heard anything along those lines. I just know that the body's impedance decreases with increasing frequency. There may be some cutoff point where for whatever reason the increase in current becomes less dangerous.

Quote:
I'm working with -17-0-17 rails now and I can't even feel anything across 34vdc, let alone getting burned. Although supposedly people have died from as low as 25vac.
Yeah, but try putting your fingers across the binding posts when your amp's swinging a +/- 17 volt sinewave.

se
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Old 4th March 2003, 07:47 AM   #10
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Oh, by the way, when you include quotes in a single post from more than one person, it's helpful to note which quote came from which person.

Thanks.

se
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