I'm getting shocked Electricity grounding with 2-prong?

Join the discussion
Registration is free. Start your own thread to ask a follow-up.
Sign up Log in
109 replies · 28K views
Just don't touch the chassis!
 
Engineering news on Phys.org
TurtleMeister said:
So when turbo used the word "reference" he was referring to the capacitor coupling between the chassis and neutral/ground or between the chassis and hot (depending on the plug or switch position).

Did I get that right turbo?
Yes, that is correct. A chassis ground is NOT necessarily at the ground potential of the AC supply wiring. It can easily be at the same potential as the hot leg.
 
Averagesupernova said:
I am curious to see what the authors of the book recommended have to say. I don't feel like buying one, but I do have an interest in older technology and it just might be worth it.
Dave Funk's book is very popular among amp repairmen, and he addresses the many mistakes that Fender made over the years in their power supply configuration, including putting the fuse on the neutral leg, instead of before the power switch on the hot leg. Unfortunately, Marshall and other amp-makers often copied Fender's mistakes, so dangerous guitar amps are all over the place. If you're interested in older guitar amps, take a look at Dave's book. You will buy it - it's that good.
 
turbo-1 said:
Yes, that is correct. A chassis ground is NOT necessarily at the ground potential of the AC supply wiring. It can easily be at the same potential as the hot leg.

Yes- it could be at the same potential but, with almost j1MΩ in series, how is it going to kill you - except if it causes you to fall off a ladder in surprise?

An isolated / insulated chassis could find itself at any potential - so can a human body.
 
You might not be killed, but if you have ever had sweaty hands on the guitar strings and touch your wet lips to a grounded microphone, you might wish you had died. :biggrin:
 
sophiecentaur said:
Yes- it could be at the same potential but, with almost j1MΩ in series, how is it going to kill you - except if it causes you to fall off a ladder in surprise?

An isolated / insulated chassis could find itself at any potential - so can a human body.
If the amp's chassis is at 120V (not neutral) and you're playing guitar, your strings and you will be at 120V, too. Touch a properly grounded mic, and your body becomes the path to ground. Musicians have died from this, so grounding/polarity is not trivial. It is likely that the OP was in a similar situation.
 
Turbo, there is an important distinction that you have not made here. If musicians have died from the use of one of these amplifiers (that we have shown schematics for in this thread), it was not because of their design. It is more likely that a fatality would have been the result of a component failure. Or even more likely, someone who did not know what they were doing making internal modifications to it. If you disagree then tell me under what circumstances someone could be killed from one of these amps solely because of their design.

I'm not advocating that these amps are as safe as modern amps. They're not. But they are not going to kill you because of a design flaw. The improved safety standards of today's equipment simply means that your chances of personal injury is decreased when something goes wrong. If you drive a car without seat belts you are more likely to be killed if you are in an accident. But the lack of having seat belts does not cause the accident.

There's a Barns & Noble in a shopping center not far from my home. I will try to remember to check out that book you mentioned the next time I'm there. I'm not really into guitar amps but I am into old tube type equipment.
 
Last edited:
turbo-1 said:
It is likely that the OP was in a similar situation.
No, the op was about a piece of equipment with a grounded three prong plug where the ground prong had fell off. Actually, I find it very unlikely that the prong would just fall off. It's more likely that someone cut it off. I've seen people do that when they want to use a two wire extension cord with a three wire grounded device. They're in a hurry and don't have time to go out and purchase the proper cord. Anyway, the point is that the op situation is not a case of improperly designed equipment. It's a case of improper and dangerous user modification. The simple solution is to replace the plug, which someone had already suggested.
 
sophiecentaur said:
I agree with your analysis of why they put the capacitor where they do. Isn't it usually described as 'coupling'? But where does the "referencing" come into it and where does it become a matter of 'safety'?
There is still no theoretical justification for those statements that turbo is making. The reactance of a 0.047uF capacitor at 50Hz is about j0.8MΩ. That won't let any more than a mild 'jolt' of current through you, even if your wet feet were planted on a copper floor at Earth potential. How can it be lethal?
I thought these fora were expected to vaguely scientifically rigorous so that the uninformed reader could rely a bit on what appears here. AC electricity not a mystical force and it obeys all the normal rules - so let's use them where we can.

Of course, we are relying on the integrity of those capacitors to provide isolation!

I make that reactance about 67.76 K and the current from 250 volts would be 3.7 mA rms, neglecting body resistance.
So, you would feel it, but it might not kill you.

But, that is talking about a relatively safe amplifier as long as no components in the primary of the transformer fail. The bad ones are where they connect one side of the mains to the chassis and attach a reversible plug to the power cord.

Incidentally, the original poster of this thread was last seen on April 27th. We all agree that it is bad to electrocute people and we can't correct the bad mistakes of the past, so maybe it is time to wind this one up?
 
TurtleMeister said:
Turbo, there is an important distinction that you have not made here. If musicians have died from the use of one of these amplifiers (that we have shown schematics for in this thread), it was not because of their design. It is more likely that a fatality would have been the result of a component failure. Or even more likely, someone who did not know what they were doing making internal modifications to it. If you disagree then tell me under what circumstances someone could be killed from one of these amps solely because of their design.
There are many older Fender amps (and others) that could easily end up with 120V Hot on the chassis through improper modification, internal failure, etc. When a guitarist using such an amp touches another instrument or a mic that is properly grounded, their body becomes the easiest path from hot to ground. Mouth to hand, hand to hand... both can put the heart in the path and it doesn't take much to stop a heart.
 
vk6kro said:
Incidentally, the original poster of this thread was last seen on April 27th. We all agree that it is bad to electrocute people and we can't correct the bad mistakes of the past, so maybe it is time to wind this one up?
Probably a good idea. I made it a campaign of mine to make every amp that I serviced as safe as possible, and took offense to people who minimized the risk of shock, and that seems to have ticked off a few people. Like Dave Funk, I have dissected amp designs and have refused to service amps if the owners did not let me modify them to current safety standards (fuse on the hot leg before the power switch, 3-prong plug securely grounded to chassis, etc, and NO "ground switch" allowing the chassis to be referenced to either hot or neutral). I was a small operator, but was not about to allow people to be put at risk because I ignored the mistakes of the amp designers. I highlighted Fender stuff (and Marshall) because that was the bulk of my business, but you could find some pretty "interesting" configurations in Gibson, Gretsch, and Supro amps, too, that were worse than Fender's missteps. Get into some of those amps, that are true point-to-point amps (no tag-board or circuit-board) and you've got to learn to "read" the circuit without a map.
 
vk6kro said:
I make that reactance about 67.76 K and the current from 250 volts would be 3.7 mA rms, neglecting body resistance.
So, you would feel it, but it might not kill you.

But, that is talking about a relatively safe amplifier as long as no components in the primary of the transformer fail. The bad ones are where they connect one side of the mains to the chassis and attach a reversible plug to the power cord.

Incidentally, the original poster of this thread was last seen on April 27th. We all agree that it is bad to electrocute people and we can't correct the bad mistakes of the past, so maybe it is time to wind this one up?

I wouldn't kill this yet quite simply because it is not resolved. I figured 120 volts and not 250. Not sure if any of those came in a 250 volt option. I also cannot imagine ANY guitar amp design with a true hot-chassis (no power transformer). So let's throw that one out of the discussion. I am going to try to have a look at the book and post based on my findings.
 
Turbo, I did not take offense. I was just confused by some of your statements. You made it seem as if the chassis was hot under some circumstances because of a design flaw. You also mentioned something about the DC voltages of two amps causing a problem - which I still don't understand. But I'll let it go if every one wants to end the thread.

By the way, if I were servicing amps I would do the same as you. I would want to bring them up to modern safety standards. It's a really simple matter to remove the capacitor and ground the chassis with a three wire power cord. So there's no conflict there either.
 
TurtleMeister said:
Turbo, I did not take offense.
Nor did I. Amp-techs who deal with vintage amps have to talk about ground (chassis ground and string/player ground) knowing that user-actions, mods, and component failures can conspire to put that ground at any potential from neutral to hot and anyplace in between.

BTW, Leo Fender and his crew "made do" and they did some stuff that would never have been allowed today. Back then, the progression from non-polarized power supplies to polarized power supplies to polarized and grounded power supplies was VERY slow. The conversion of music venues often lagged by decades, and Fender did nothing to keep up with the modernization until they had to, because musicians would have been "inconvenienced" by the more stringent safety standards and might not have bought new Fender amps because of it.
 
I thought this was interesting: http://www.highvoltageconnection.com/articles/ElectricShockQuestions.htm

Also, I tested a GFCI tonight with a 47K resistor. Not enough to trip. Should have passed about 2.5 mA. Allen Mottershead second edition Electricity and Electronics claims it takes about 5 mA to trip a GFCI. Notice that is the same current as stated in the table as maximum allowable safe current. I didn't test the GFCI with a 5 mA load since I don't have a resistor rated at enough wattage. All of the ones I have on hand would smoke. Please don't take this post as saying that 'if it won't trip a GFCI then it must be perfectly safe'.
 
Averagesupernova said:
I thought this was interesting: http://www.highvoltageconnection.com/articles/ElectricShockQuestions.htm

Also, I tested a GFCI tonight with a 47K resistor. Not enough to trip. Should have passed about 2.5 mA. Allen Mottershead second edition Electricity and Electronics claims it takes about 5 mA to trip a GFCI. Notice that is the same current as stated in the table as maximum allowable safe current. I didn't test the GFCI with a 5 mA load since I don't have a resistor rated at enough wattage. All of the ones I have on hand would smoke. Please don't take this post as saying that 'if it won't trip a GFCI then it must be perfectly safe'.
I wrote the electrical safety portion of a program that my employer (2nd-largest training company in the world) presented to the electrical supervisors of what might have been the world's largest chemical company at the time. It is frightening how little current it can take to kill you under ideal circumstances. People who minimize safety risks ("get used to it", or "the capacitor will only allow XX amps") really get under my skin, because people can die from this stuff, while the nay-sayers go "la-la-la".
 
@Turbo
You seems to be changing your tune a bit here. Are you accepting that there is, in fact, a path for only a very few mA through 'the capacitor' and are you, in fact, accepting that this is the only relevant source of danger? Can we now also eliminate the ideas of "peaks" occurring in antiphase etc. and causing large differences in DC voltage?

No one on this thread is claiming that there is no harm in unearthed and mis-connected equipment. There is universal condemnation of shoddy design and construction but a realistic appraisal of the actual risks involved. I think your crusade for 'improving' this old fashioned gear may actually be a bit of two edged sword - encouraging the preservation of stuff which should be on the dump by now. If they turn up with broken stuff and you mend it for them then they will only put off further the day that they bite the bullet and go out and buy something properly engineered.
 
The OP was playing an ungrounded amp and got shocked by a mic. It could just as well have been through contact with another amplifier or another musician whose strings were at a different ground potential. If his chassis is hot and he touches a properly grounded piece of equipment, he becomes the path to ground.

If you want to imply that I am "changing my tune" have at it. If you read my posts you'll see that I was quite consistent, and your misconceptions regarding chassis grounds were likely the cause of your confusion.

As far as my "mending" broken stuff, I have explained quite patiently that I will not repair, tune, or modify any vintage amp unless the owner gives me permission to bring it up to modern code. Or did you miss that too? In older Fender amps, that requires disconnecting the ground switch (if present) and running the hot lead to the fuse-holder, and THEN to the power switch, as well as establishing a proper chassis ground (at supply ground) for the amp.
 
If you just disconnected 'that capacitor' there could be no more trouble. Why not just do that? There is nothing to stop someone changing the mains plug wiring as soon as they get it home - you just can't trust them. However much you get their permission to do what you want, you have no control on what goes on subsequently.
You still make statements that are not relevant to the problem. The fuse: Until it actually blows, how does the siting of the fuse alter the shockability of an amp when the mains leads are swapped? Fuses aren't there to protect against shock - they are there to prevent excess current flowing. (No need to explain the reason for putting the fuse in the correct place - I already made that point earlier - it's not relevant here though.)

You have not explained how, if his amp has a mains transformer, he can get a lethal shock, however he connects it to the mains, if the only path is through 'that capacitor'. What do you mean by "hot"? On its own, the value of voltage measured on an object is not relevant. You can touch a Var Der Graaf generator (several 100kV) and get 'a shock' but I know of no examples of people being killed that way. They even let Schools operate them, in these litigious times, with no special safety instructions attached. If he really is getting a shock then I can only suggest that the transformer isolation could be breaking down and I think that a PAT tester or Megger would probably reveal that. Does your upgrading to modern code include testing and replacing possibly faulty transformers? As I said in my last post, the sooner such aged amps are out of circulation the better. If there were evidence that modern amps suffer from the same problem then things might be different. After all, the basic theory doesn't include age in the parameters.

I am not going LA LA on the subject of safety. I am referring to the internationally accepted levels of health risk which have been established by much more extensive testing than your limited examples of anecdotal evidence. Anyway, if you were really that concerned then I can't see that you'd still be keeping these amps operational.
 
I'm keeping my mouth shut 'til I have a look at the book. You all (should) know where I stand.