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Hot chassis circuit made safe?
Hot chassis circuit made safe?
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Old 23rd March 2019, 12:47 AM   #31
aut0m4tic is offline aut0m4tic  United States
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The 30mA is a rough number. The typical resistance for a man is different than the typical resistance for a woman. (Typical man and woman???) Things change as we age, and rough hands from manual labor increase this resistance. The resistance of a person is roughly 500 to 1000 ohms.

Most protective devices are labeled in amps, but that's rarely the technical reality. A gfci provides no current limiting devices, so when my body ends up in series between the phase and neutral, I am exposed to the entire fault current available in the circuit. In reality it's a time curve that we're working with. How fast the fault can be cleared cleared, or in the case of the gfci, how fast the circuit can be deenergized. A 10A breaker just trips faster than a 20A breaker during a fault, but I receive the same amount of current till it trips.
Fuses are the same way. Grab a 1A fuse, a rheostat, and a 12v battery. Slowly increase load through the fuse and you will get it to hold more than 1A. In some cases, 150% more. It eventually melts out instead of blowing. I see 40A fuses hold at 50A every summer as AC loads grow. A 10A difference is well within the margin of error of my ammeter.

If you look at the chart, the big box before DEATH is from what 10 to 100 milliamps? That's a very wide range of currents. It's there to take into account the differences in our bodily makeup.

I am in no way an expert, and I'm not arguing with the experts, but I have spent considerable time with industry experts being trained on this exact subject.
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Old 23rd March 2019, 01:47 AM   #32
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by aut0m4tic View Post
I am in no way an expert, and I'm not arguing with the experts, but I have spent considerable time with industry experts being trained on this exact subject.
I'm grateful for your contributions, and I'm sure most people on the thread feel the same way.

Lots of smart and knowledgeable people here, so there is always some discussion and back-and-forth. I don't think any of it is hostile, just a healthy part of the scientific process. (But unsettling to people used to polite non-technical company in which nobody disagrees with anyone else, because that would be rude. Scientists and engineers and mathematicians don't follow that rule, the only way to keep things honest is to have debate and disagreement and scribbled equations and experimental data until the facts emerge.)

So: we should be thinking " a GFCI limits the total energy (joules) delivered by the shock" rather than "a GFCI limits the maximum current delivered by the shock", yes?


-Gnobuddy
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Old 23rd March 2019, 03:43 AM   #33
aut0m4tic is offline aut0m4tic  United States
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Gnobuddy, I appreciate you looking out for me!! Upon rereading, I realize I didn't sound the same way on paper as I did in my head. Hot chassis circuit made safe?
I wasn't feeling like things were hostile, this forum is amazingly helpful! I realize that I know just enough about electricity to understand that I have tons more to learn.

Most of us linemen are cautious about being too confident and forward outside of work. We have to work and understand things in such a way that we stay alive. Its common for us to argue and fight when someone has different ideas because I'm the one that gets killed for following or misunderstanding someone else's advice.

I also realize I'm a tradesman in the midst of engineers. Our engineer is the smartest man on earth! So I have serious respect for what many people on this forum know and understand!!!

I teach a transformer theory class, and a grounding class at work in addition to working on a line crew, but dang if I can make an amp that doesn't hum way too loud.
power line grounding and amp grounding could not be more different! I just have to make sure everything is connected, an amp actually cares where and how its connected....

Yes on the Joules. Its important to know what's actually happening with safety items, not just what the manufacturer tells you. I regularly take things apart at work to see how and where they fail.

We used to put 1A fuses in suspect underground cables, to see if they were actually bad. (Bang test) The thinking was that blowing a 1A fuse couldn't hurt, till the engineer showed me how we were actually putting 10,000A at 7200V for a short time, and were damaging very hard to replace circuits. We now use specialized testing equipment.
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Old 23rd March 2019, 05:46 PM   #34
russc is offline russc  England
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Rod Elliott has a new page dedicated to mains safety including guitar amps & death caps -
Electrical Safety Requirements
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Old 23rd March 2019, 08:16 PM   #35
wg_ski is offline wg_ski  United States
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Originally Posted by aut0m4tic View Post
I teach a transformer theory class, and a grounding class at work in addition to working on a line crew, but dang if I can make an amp that doesn't hum too loud
An amplifier that is properly grounded for safety will almost always have a ground loop the instant it is connected to another piece of equipment that is also properly grounded. You canít get rid of the ground loop without bending up the ground pin somewhere. The only thing you can do is arrange things such that the ground loop doesnít matter. It is not always obvious how, with the exception of balanced interconnects which is basically cheating (because neither conductor for the input signal is ground).
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Old 24th March 2019, 03:26 AM   #36
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by wg_ski View Post
...with the exception of balanced interconnects...
I've often wished that electric guitar pickups and wiring was balanced.

Wikipedia says the XLR connector already existed in the 1940s, so clearly audio engineers of the time already knew about the problems that go with small audio signals and single-ended cabling and circuitry.

Sadly, the pioneers of electric guitar were not engineers, but tinkerers with little to no technical education, and so the chance was missed. I'm sure the high cost of valve amplification at the time didn't help.

But imagine if Harry DeArmond or George Beauchamp had thought to centre-tap their guitar pickup coils, and bring out both the centre tap and the two ends to the guitar connector. We might all have been spared the next century of struggles with hum, buzz, radio interference, and so on. (And when Leo Fender later copied DeArmond's pickup, he would presumably also have copied the centre-tap arrangement.)

The opportunity surfaced again with Seth Lover's invention of the humbucker. The centre-tap was there for the taking, since the two coils in each humbucker had to be connected in series anyway. And balanced audio cabling in professional audio was well established by this time.

But Mr. Lover appears to also have been primarily self-taught in electronics, a technician and not an engineer, and may not have been aware of the advantages or even existence of balanced audio interconnects. At any rate, the opportunity was missed yet again. It would not have helped that existing guitar amplifiers all had single-ended inputs, and it would cost a minimum of one extra triode to process the balanced input signal.


-Gnobuddy
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Old 24th March 2019, 04:50 AM   #37
PRR is offline PRR  United States
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Originally Posted by aut0m4tic View Post
....The resistance of a person is roughly 500 to 1000 ohms. ...
Surely you have an ohmmeter? So you can show that "normal" is more like 10K-100K. On 120V, that's hardly dangerous (why Edison worked with 100V; fewer deaths).

As you say, everybody is different. I knew a guy could not feel straight 117V AC. His skin resistance was close to 1Meg.

But then we have the worst cases. George comes off the surfboard saturated in seawater and sweat, stands barefoot on concrete, grabs the guitar.

Also, and often key: with continued current the 10K skin resistance breaks-down. Cells get leaky. Body juice soaks the dry skin cells. Skin resistance can fall below 1K faster than a person can understand the danger. Now we are into lethal shocks.

Quote:
Originally Posted by Gnobuddy View Post
So: we should be thinking " a GFCI limits the total energy (joules) delivered by the shock" rather than "a GFCI limits the maximum current delivered by the shock", yes?
GFI *only* detects current.

It has line voltage across it but does not monitor it. We could say it simply assumes all the 120V may be going to a person. But the real danger is the nerves in the heart. The heart is a sack of salt-water and would be a low-low resistance, much-much less than skin resistance. From "outside" we can't know how much voltage is dropped in the skin and how much is dropped in the heart. It is reasonable to monitor just the current.

Since line voltage is usually 120V, yes in a sense it does monitor Watts. And the time-curve does approximate Watt-Seconds (which is Joules). However it will cut-off at the same current (there's a Zener reference) at any line voltage where it will "work". On the up-side, maybe 180V before a dropping resistor smokes and it goes dead. On the down-side maybe 60V before the circuit starves or the trip-coil gets too weak.
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Old 24th March 2019, 05:25 AM   #38
aut0m4tic is offline aut0m4tic  United States
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PRR

Ive often wondered why my multimeter doesn't jive with the standards, but lack the info to answer why.

IEEE Std 1048-2003 lists it as 1k but says it's currently under consideration.

That's also the standard listed in osha 1910.269 app c

They use that number to determine the required resistance of an equipotential grounding system, to make sure current across your body is low during a fault.

1000 ohms is an approximation that started in the 50s doing straw man tests.

If you read carefully though, you are absolutely correct. It's hard to tell skin resistance from body or internal resistance, and moisture makes a world of difference!

I've measured hand to hand, and gotten similar results as you did. I just figured that my $100 fluke was not the device they used to make the standard.

I can say from experience though, that whatever my real resistance is makes me pretty jumpy when I have a hole in my gloves hooking up house services in a transformer. Hot chassis circuit made safe?

What is super cool though is being 15ft from an energized 345kv line and having arcs across your steel toe boots, and drawing 4in arcs from the steel tower to your fingers as a result if induction. Looks like your a wizard, but feels like hornets!!Hot chassis circuit made safe?
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Old 24th March 2019, 05:34 AM   #39
Jim the Oldbie is offline Jim the Oldbie  United States
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Quote:
Originally Posted by Gnobuddy View Post
I've often wished that electric guitar pickups and wiring was balanced.
The best example of the effectiveness of balanced audio I've seen is in the Hammond organ. The cable from the organ console to tone cabinet carried 110VAC and balanced audio alongside each other in the same cable. Worked like a charm. Granted the audio was pretty high-voltage, low-impedance, but still.
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Old 24th March 2019, 07:41 PM   #40
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Jim the Oldbie View Post
The cable from the organ console to tone cabinet carried 110VAC and balanced audio alongside each other
Impressive, right?

Ever plug one of those cheap microphones sold for karaoke into a P.A. with better bass response than the karaoke machine? They hum like crazy, because they're usually single-ended. It's easy to see why balanced microphone cables were already in use early in the history of studio audio.

A couple of years ago I needed a vocal mic at short notice, so I bought a $6 dynamic mic at the local dollar store while waiting for my mail-order hand-held condenser mic to arrive. The cheap mic sounded better than the ubiquitous Shure SM58 - but it hummed at 60 Hz, though it had the usual XLR connector. When I opened it up, there was no internal transformer, and the XLR was wired to deliver a single unbalanced signal!


-Gnobuddy
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