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Hot chassis circuit made safe?
Hot chassis circuit made safe?
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Old 26th March 2019, 02:28 PM   #51
turk 182 is offline turk 182  Canada
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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!)
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Old 27th March 2019, 02:47 AM   #52
Jim the Oldbie is offline Jim the Oldbie  United States
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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?

Last edited by Jim the Oldbie; 27th March 2019 at 02:56 AM.
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Old 27th March 2019, 03:14 AM   #53
PRR is offline PRR  United States
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Quote:
Originally Posted by shanx View Post
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.

Last edited by PRR; 27th March 2019 at 03:38 AM.
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Old 28th March 2019, 01:34 PM   #54
hex69 is offline hex69  Canada
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Originally Posted by PRR View Post
> 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.

Quote:
Originally Posted by PRR View Post
> 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.
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Old 28th March 2019, 07:07 PM   #55
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by hex69 View Post
...Hot/Ground reverse...
That's the terrifying one. If that happens, you're back to the hot-chassis era, but this time with a 100% chance of electrical shock if you touch the chassis.

I suppose we should really be testing every unfamiliar AC outlet before we use it, but I find it hard to be that obsessive.


-Gnobuddy
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Old 29th March 2019, 04:41 AM   #56
Jim the Oldbie is offline Jim the Oldbie  United States
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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...

Last edited by Jim the Oldbie; 29th March 2019 at 04:57 AM.
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Old 29th March 2019, 09:18 AM   #57
turk 182 is offline turk 182  Canada
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Originally Posted by Jim the Oldbie View Post
Ground/neutral reversal is one of the faults that an outlet tester won't catch.
would this be because ground and neutral are bonded at the service panel?
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Old 29th March 2019, 01:26 PM   #58
hex69 is offline hex69  Canada
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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...
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Old 29th March 2019, 01:55 PM   #59
Jim the Oldbie is offline Jim the Oldbie  United States
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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.

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.

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!!

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

Last edited by Jim the Oldbie; 29th March 2019 at 01:58 PM.
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Old 29th March 2019, 02:52 PM   #60
hex69 is offline hex69  Canada
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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



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!
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