Hot chassis circuit made safe?

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PRR

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https://content.fluke.com/promotion...dmm/fluke_dmm-chfr/files/safetyguidelines.pdf
While most people are aware of the danger from electric shock, few realize how little current and how low a voltage are required for a fatal shock. Current flows as low as 30 mA can be fatal (1 mA=1/1000 A). Let’s look at the effects of current flow through a “typical” 68 kilogram (150 pound) male:

• At about 10 mA, muscular paralysis of the arms occurs, so that he cannot release his grip.

• At about 30 mA, respiratory paralysis occurs. His breathing stops and the results are often fatal.

• At about 75 to 250 mA, for exposure exceeding five seconds, ventricular fibrillation occurs, causing incoordination of the heart muscles; the heart can no longer function. Higher currents cause fibrillation at less than five seconds. The results are often fatal.

Now let’s calculate the thresh hold for a “hazardous” voltage. The approximate body resistance under the skin from hand to hand across the body is 1000 Ohms. A voltage of only 30 V across 1000 Ohms will cause a current flow of 30 mA. Fortunately, the skin’s resistance is much higher. It is the resistance of the skin, especially the outer layer of dead cells, that protects the body. Under wet conditions, or if there is a cut, skin resistance drops radically. At about 600 V, the resistance of the skin ceases to exist. It is punctured by the high voltage.

For multimeter manufacturers and users, the objective is to prevent accidental contact with live circuits at all costs.
 
But then we have the worst cases. George comes off the surfboard saturated in seawater and sweat, stands barefoot on concrete, grabs the guitar.........case of electrocution - actual death - caused by the 9 volt battery

Let's just say that the big dumb blonde one is working on his first car, a 1949 Plymouth. One of the first things a kid does to his first car is to install a stereo, so the original 6 volt battery had been swapped for a 12 volt battery, followed shortly by replacing every light bulb in the car.

Now the car was over 30 years old, and a bit rusty. It was Florida in the summer so I was plenty hot and sweaty, no surfboard needed. We did have a lake in the back yard and I was in it a lot for a quick cooling off. I was wearing only a pair of shorts, and reaching anything under the hood of that car required leaning across the fat front fenders.

My grandfather who had given me the car taught me to disconnect the battery before messing around under the hood, so I grabbed a shiny Craftsman and touched it to the positive battery terminal to remove it. Upon doing this I felt a mild jolt and saw a flash of light.

Could I receive an electrical shock from 12 volts? Well the big dummy had to try it several more times to believe that it was actually possible. It did seem to require a large portion of bare sweaty chest to be in contact with the car body. Craftsman in each hand across the battery didn't do anything. This experiment actually produced the "metallic taste" in my mouth that some people report after being zapped.

I had learned the hard way about hot chassis radios at a much younger age. Much of what I brought home from the trash has to pass inspection by one or both parents before being allowed into the house. I had a small area of bench space outside in the carport (an unenclosed garage) for testing and deciding to attempt repair or strip for parts and return the rest to the trash. It had concrete floors and I was usually barefoot, and often sweaty. I got zapped a few times, once or twice rather severely before I understood how and why this happened. Then I got a piece of plywood to stand on and a pair of flip flops to wear when playing with such things. This was the early 60's, today we know better.

Looking back......I'm lucky to be here.
 
> called HPFI in Denmark. My entire house is protected by one of those.

And RCB in the UK; and they too sometimes do it whole-house. In more casual US wiring, GFI has grown from per-outlet to per-circuit, but whole-house seems dubious to me. How do you find the problem and reset the breaker if ALL the lights are out? But whatever works.
AU/NZ has moved to "all final subcircuits" for domestic/residential. That's a pile of RCDs which need testing every six months (push the leetle red button) and regular replacement.

> I'm not sure how common it is for GFCIs to trip by mistake.
It's a common complaint. Especially when a specific technology (there have been several) is new. Also cascaded GFIs seem glitchy.
To the point that AS/NZS 3000 has specific exceptions from the RCD (as they call them) requirements for certain non-domestic applications under certain conditions. But the rules get tougher with each revision of the standard.
 
...felt a mild jolt and saw a flash of light...Could I receive an electrical shock from 12 volts?
When I was maybe ten or eleven, I scrounged a small 12V mains step down transformer from something my big brother had built and then discarded. I had just learned about (selenium) bridge rectifiers from a magazine article, so I excitedly soldered together the transformer, a selenium bridge rectifier, and a big electrolytic filter cap, and mounted the whole lot into a plastic soap-dish my mom wasn't using.

My first DC power supply! (Now I wouldn't have to steal the D-cells out of my dad's emergency flashlight every time I wanted to do an electrical experiment.)

I didn't have any money to buy terminals or connectors, so the output of the power supply was just two insulated wires poking out of the soap-dish enclosure. The wire ends were bared, and I would twist them onto the wires of whatever contraption I was building at the moment.

One small detail: I didn't have a wire-stripper either. So I used my teeth. This needed to be done periodically, as the constant twisting and untwisting of the wire ends would break them off after metal fatigue set in.

You've probably all figured out what's coming next. Confident in my previous experience that 12 volts was entirely safe, I bit the insulation off one wire - without unplugging from the mains first. While I was doing this, the second bare wire touched the outside of my cheek.

I got zapped right in the mouth, and saw the bright flash of light that George described. (It's the optic nerve or some part of the eye/brain vision system going nuts - the person receiving the shock "sees" a bright flash, but there isn't actually any light there.)

So I too found out that twelve volts isn't necessarily safe. It turns out that twelve volts inside a moist human mouth can produce a pretty unpleasant shock.

I'd completely forgotten about his long-ago adventure until George's story brought back the memory. Thinking about it now, I was in much more danger than I realized: this was two-wire AC mains wiring, and there was no safety ground to the body of the transformer, or to the DC output terminals. :eek:

Both my parents were too obsessed with their own problems to notice any of the stuff I did, so neither of them ever knew about any of this.


-Gnobuddy
 
saw the bright flash of light that George described. (It's the optic nerve

I could understand a zap to the head, face or mouth causing the flash ....but mine was hand to chest, not sure which hand, but probably left since that's where the battery was. I had to repeat it several times before I actually believed that it happened.

Both my parents were too obsessed with their own problems to notice any of the stuff I did

That was the usual case in my house since most of my early electronics and other experiments were forbidden inside the house. Somewhere around age 10 or 12 the carport was enclosed and became my bedroom.

More of my stuff became permitted inside the house until an incident involving one of those old paper and wax covered electrolytics common in 50's vintage radios, a shorted silicon diode and the wall outlet created a fireball that burned my new workbench (an old door). This happened within a week of my brother's DIY fireworks experiment involving some shotgun shells, nearly set his bedroom on fire.......ALL "science experiments" by the "kids" were immediately banished from inside the house.

Thinking about it now, I was in much more danger than I realized

Same deal, I spent much of one summer inside an aluminum storage shed with a bare dirt floor and a 2 wire extension cord from the house for power tinkering with some old TV sets trying to make myself a working one. Does it rain in Miami in the summer, UH....every day. I did finally become the only "kid" in the neighborhood who had his own TV.

My first DC power supply! (Now I wouldn't have to steal the D-cells out of my dad's emergency flashlight every time I wanted to do an electrical experiment.)

I had a 6 volt car battery charger and a couple of 6 volt car batteries, several 6 volt "lantern" batteries along with some 45 volt and 67.5 volt "B" batteries from old radios, but my favorite source of "raw power" was the Lionel Train transformer......It could light up just about any tube I found.......some of them quite brightly too!
 
I could understand a zap to the head, face or mouth causing the flash ....but mine was hand to chest
In the book "Your Inner Fish" ( Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body: Neil Shubin: 8601400082775: Books - Amazon.ca ), author Neil Shubin points out that the human nervous system has some very bizarre cross-wiring, left in place by evolution as ancient ancestral species evolved new body parts (like teeth and jaws), while retaining the existing nerves already running through that general area of the body. A bit like hacking new electrical wiring onto the existing old wiring in an old house when you build an add-on room, creating a horrid (and probably illegal) mess.

Perhaps some of the nerves in the human chest or hand are connected somewhere in our heads or bodies to one of the nerves also involved in vision?
...a fireball that burned my new workbench...my brother's DIY fireworks experiment...
I would tell you about the time I built a hot-air balloon out of large sheets of tissue paper - in my bedroom - and what happened the first time I tested it with a wad of alcohol-soaked cotton suspended under it. But it's getting too far off-topic!

Perhaps we should start a new thread in The Lounge about childhood science "experiments" we shouldn't have tried, and shouldn't have survived? I'm betting there will be quite a few people who have some tales to contribute.


-Gnobuddy
 
When we were kids, the Walgreen's drugstore at one of the local malls had one of those large, floor-standing tube testers that were common back then. It was parked in the front of the store, right next to one of the aluminum floor-to-ceiling window frames. This thing had such a hot chassis that if you happened to be leaning against the window frame as you touched the front panel, you got one hell of a JOLT!! And I mean it would mess up your day, if not your pants.

We mentioned it to management more than once, but it never got fixed. Eventually we started taking our friends there and tricking them into electrocuting themselves, because you know, fun!

After a few more years(!) it was taken away to the Tube Tester Graveyard along with all the others. Dunno if it ever actually killed anybody, although I suppose that would've made the papers.
 
One thing that occurred to me was how you could have a 2 prong plug on a guitar amp and with the polarity switch the "wrong way" with respect to a mic attached to a PA. Well if you are sinking AC through the mic/player/guitar amp and that path happens not to be on the fused side of the guitar amp..I could see that welding strings to a mic stand..most PA amps could pull a lot of mains current before popping their fuses. Usually the mains panel breaker would be first to open if you were running a system at "eleven".
 
nothing to do with fuses or polarity(although there's more to that) it's floating potentials vs gnds and how they are connected, caps pass ac readily that's why "unpolarized" plugs are the problem to begin with if they are the wrong way around in the receptacle (wall socket) you have hot and neutral reversed so which way do you flip the line bypass cap??guessing wrong...and well you know...
and not to be alarmist but even on a single phase service, unbalanced loads can produce voltage offset between neutral and ground sufficient to cause a shock potential so it does not surprise me to hear that a professional musician takes care to ensure his safety.
i've also witnessed the opposite performers who got a "static" shock and got so scared that grounding was BAD and refused to touch equipment!
a quick check with a high impedance meter between guitar strings and a mic is a "must" (and do not neglect a dc potential test it not just "ac" that can be dangerous!)
 
Sometimes a high-impedance meter can produce a false-high reading here. If we consider 120VAC house current and a modern DMM with a 10 megohm input impedance, it only takes maybe 1 nF of capacitive coupling to the chassis to produce about a 95V reading on the meter, even though this would only figure to about 10 uA of leakage current if I've done the math correctly.

In their service manuals, one of the manufacturers whose equipment I service recommended adding a 10K, 2W resistor paralleled with a .01 uF ceramic cap, across the meter inputs. With this in place, the leakage voltage should measure no more than 4Vrms, assuming 120VAC house voltage. (This works out to 100 uA of leakage current.) I have fitted an MDP dual banana plug with this little network so I can just plug it in between the test leads and the meter input jacks.

Of course the above only deals with false-positives. Looking at it from the other direction: If you can't get a reading any higher than a few volts on a standard hi-impedance meter with no shunt network, it's probably safe? What do you think, sirs?
 
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PRR

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Joined 2003
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One thing that occurred to me was how you could have a 2 prong plug on a guitar amp and with the polarity switch the "wrong way" .... that path happens not to be on the fused side .....most PA amps could pull a lot of mains current before popping their fuses. ....

On 99% of 2-pin-plug amplifiers with "polarity switch", the connection is a 0.05uFd capacitor. This is like 50k at 60Hz. Or 2.4mA. Nowhere NEAR fuse-popping. Legally not lethal.

As mentioned, the common failure mode on these caps is "short", so bad things did happen.

Yes, a 10Meg meter will show "phantom voltage" that no person could mind. A chum was measuring 42V on the UN-connected leg of a 3-way switch run, and consideration of capacitance and meter loading comes to a similar answer. House-length cables will *never* have enough capacitance coupling to affect a person. (Gets very different on a run like my neighbors new feeder: 2,500 feet of 20,000V. A parallel run "unconnected" could induce a lethal shock, and such lines are routinely hard-grounded when not energized.)

10k-100k in shunt with a hi-Z meter will make it more-like a real person.
 
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> Sometimes the outlets in bathroom are a little isolation transformer.. That's how it is where I live

That is different. Common in the UK, unknown in the US. Dedicated low-voltage supplies for lighting and shaver.
You are correct, sorry my bad.

> I think it's a good idea to have such tester then make sure to check the outlet wiring is correct, those are warning you when the ground is missing or when the hot neutral is reversed

Yes; but there are at least two faults these will NOT reveal.
The circuit tester I have can test Open Ground, Open Neutral, Open Hot, Hot/Ground reverse and Hot/Neutral reverse. I'm so curious to know about the 2 others.
 
The music store/audio dealer where I worked for a long time had a Wiremold outlet strip installed underneath the front lip of the workbench (not by me, heh). We had no problems with it for years until one day we were power-testing a McIntosh MC2500*, and for some reason just couldn't get it up to rated output.

After a lot of head-scratching it was determined that the 110 VAC was fluctuating quite a bit. Further investigation revealed that the outlet strip had its ground & neutral wires reversed, and the full supply current had been going through the CONDUIT that the strip was wired to. Sure enough, the box couplings etc. all had little arc burns at contact points! After many years of testing large power amps, these eventually developed resistance, causing the voltage fluctuation - not to mention a potentially lethal voltage between the conduit and actual ground. Whew!

Ground/neutral reversal is one of the faults that an outlet tester won't catch. :(

*I believe this amp was one of a pair, owned by one of our more well-endowed regular customers, I think he was a doctor? He used them with a pair of Bozak Concert Grands in bi-amp mode! Those were the days... :rolleyes:
 
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Not sure but I think the neutral and grounds are bonded only in the main panel, any sub panels for example must have the grounds and netrals isolated, same goes for outlets. I presume it's not a good thing to have them bounded at any other place than inside the main panel, maybe the tester just can't reveal the fault in such situation?

I wonder how Neutral/Ground reversal can be tested, by reading resistance using a DMM from the outlet?

What other fault the circuit tester won't find...
 
Yup, that's the reason. The tester can't discriminate between ground and neutral, since they're ultimately at the same potential. But it's still important not to swap them, as the above example illustrates.

I had another customer at that store. This guy was a musician, and claimed to be trained as an electrician, although I began to doubt this after I went to his place once to check out some problems with his PA system, which was set up in his living room. :)

"Johnny" (not his real name) showed me an adapter he'd constructed. He said he'd been playing in a lot of crappy old bars, some of which didn't even have grounded outlets. So he whipped up the following: a 3-wire female inline receptacle, connected with a short cord to a 2-wire male plug. The neutral & ground wires from the receptacle were CONNECTED TOGETHER to the neutral blade of the 2-wire plug. Think about that one for a minute. :eek:

He explained, "Hell, they're connected together at the breaker panel anyway. I just connect 'em up right here, and now my stuff is grounded." (I swear to god this is a true story.)

Resisting a powerful urge to just snatch the thing out of his hand and beat him with it, I took a deep breath and calmly explained that if the 2-wire plug was connected the wrong way, every piece of gear in his rig would have 110 VAC on every exposed metal part. "Oh yeah, I know about that," he replied. I always test it to make sure it's plugged in the right way." At the time it didn't even occur to me that he would have a damned tricky time finding a proper ground to which to connect his tester, on a stage without a grounded outlet. :confused:

Instead, the following scenario came to mind. I breathed again and said, "OK, so what if some stumbling drunk accidentally kicks out the plug while you're taking a break. Maybe he has the presence of mind to plug it back in, but it's a flip of the coin as to whether he gets it right or not!" This was admittedly a weak example, but it seemed to get his attention. "Yeah, I guess that wouldn't be good," he agreed. I wanted to yell, No, that would not be very bloody f890ing good at all, man!! :mad:

After a bit more "tech-talk," combined with some straight-up begging & pleading, I finally convinced him to decommission his death cable. Good lord...
 
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> and now my stuff is grounded.

I wouldn't let Johnny touch any of my gear for sure, anything in my house as well!

> Resisting a powerful urge to just snatch the thing out of his hand and beat him

:boggled::smash:

Once I had a 'certified electrician' to check for a 'melted fuse odor' that I noticed in a building I own, at first the guy told me the problem was 'inside' the main panel and that it should be replaced to prevent fire.. and guess how much he wanted for the job... But my nose was telling me the problem was not coming from the main panel, but instead from another room (next to the main panel room), in that room I had a little sub panel holding fuses for the garage heaters.. Well the problem was there because of a loose screw.. After fixing it I called another electrician to be sure that everything complies with standards.. everything was fine, cost me about nothing. Take the money and go Johnny go! :D
 
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