Potentials at house outlets

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Tommy volts
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Potentials at house outlets

Post by Tommy volts » Thu Jun 05, 2008 1:15 pm

Dear Forum Members:

Two days ago my son was severely shocked when he grabbed two electrified guitars, one in each hand. His hands locked up around the necks and he fell unconscious. His brother pried one of the guitars from his hand and he is now o.k.

The strings on both guitars were properly grounded to their respective amplifiers.

Each guitar was plugged into its own 100W amp which was in turn plugged into a different garage outlet. Each outlet on a diffenent breaker circuit.

Is it possible for the "neutral" wire connections at outlets on different breaker circuits to be at different potentials? Possibly due to differences in wiring resistance to outside "ground"?

Robert Reed
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Post by Robert Reed » Thu Jun 05, 2008 3:16 pm

The only thing that should (and will) change the neutral's potential from "zero" volts is the load that is actually on that circuit. By Ohms law there will be a voltage drop due to the IR drop in that line. It would not be unusual to see a volt or two of drop in heavily loaded long lines. This is not of much concern. However the ground lines carry NO current and should remain at "zero" volts all the time Except breifly under fault conditions. What you experienced has got to be a miswiring problem in your out lets or Amplifier AC wiring. A few simple checks with a DMM should clear this up.

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Post by dyarker » Thu Jun 05, 2008 5:42 pm

Both hot and neutral should be isolated from the guitar side circuits of the amplifiers by the power supplies. So even if you find a fault at the outlets, I would also suspect something wrong in the amplifier AC wiring.

Or, was the voltage between guitars DC?

Glad your son is okay.
Dale Y

Dean Huster
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Post by Dean Huster » Thu Jun 05, 2008 8:04 pm

Go buy one of those little outlet testers at your home supply place or electrical store. They're cheap (under $5) and will tell you if the polarity is correct, the ground's good, etc.

Do the guitar amps have two-wire line cords? If so, I'd change to a 3-wire grounded cord as well as checking to make sure the power transformer is good and that any primary filter caps are good.

Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).


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Post by haklesup » Thu Jun 05, 2008 9:19 pm

That sounds just a bit frightning.

Since you say the Amps were plugged into different circuits it coiuld be the case where each circuit was the opposite phase and you end up with 220V across the two hot wires. I cannot justify how that alone would cause a shock, the AMP should isolate from the mains unless he is into ancient tube amps perhaps.

A good test would be to measure the AC and DC voltage between the two guitar's strings while pluggedinto the same circuit and the two in question. Same AC voltage but opposite phasing might as well be double the wall voltage.

I'm not an expert but I understand that some kinds of guitar pickups require the strings to be biased. I have heard of people getting shocked by guitars before. Its obvoius that each of the two guitars had a different voltage (instantinious differential). I don't think ground had anything to do with it.

As to wether this is a fault or just unfamiliraity with a certain hazard associated with particular electric guitar equipment. A bit of research into how electric guitar pickups work should reveal the reason. They do not work like microphones.

http://www.guitarnuts.com/technical/ele ... /index.php

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Post by MrAl » Thu Jun 05, 2008 10:19 pm

Hi there,

As a band member who played electric guitar and sang in various bands
through the years, this is no surprize to me at all.

The reason i say this is because when we did outdoor gigs we started
to get used to the occasional shock from the microphone metal, which
at the time was not wireless. The solution (we learned to test before using)
is to reverse the plug, that is, pull the amplifier 120vac plug out of
the wall and rotate it 180 degrees. This always worked.

The problem is that many older amplifiers tied one side of the line
to the ground of the amp (or sometimes through a hum adjust circuit)and
thus the ground of any instruments and microphones, which get held in
the human hand when using. This always caused a shock, sometimes
stronger than other times, sometimes just a tingle.

The problem outdoors is that feet on the ground means you become
a path to ground for current, although if two amps are plugged in
with 'ground' side of the line 180 degrees apart (one rotated and
the other not) you could get a shock between the amp grounds.

One safe way around this is to use ground fault interrupters, which
should trip if this happens. We didnt have them back then though.
The GFI's sense current both ways and if the current in one line is
not equal to the current in the other line it trips, so it should trip
even if the fault is between two amps.
LEDs vs Bulbs, LEDs are winning.

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Bob Scott
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Post by Bob Scott » Fri Jun 06, 2008 7:19 am

I agree with Dean. I think one or more of your outlets is miswired and you should get one of those inexpensive outlet testers from a hardware store. They are $5 to $11 at Home Depot depending on whether they test GFCIs or not. If you are in any way concerned about your family getting shocked, install GFCI receptacles in place of the old ones.

Also , there may be more than one problem. Use an ohm meter to check the resistance of the guitar amp grounds to both sides of their plugs. There may be a shorted capacitor wired from line or neutral to guitar amp chassis right where the AC enters the chassis. I've seen that before in older amplifers.

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Post by Lenp » Sat Jun 07, 2008 9:14 pm

Many older amps used a DPDT polarity reversing switch on the AC power, for hum reduction. This can complicate any line to chassis fault one of the amps might have, either AC or DC!


Tommy volts
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Post by Tommy volts » Mon Jun 09, 2008 8:31 am

Thanks to all of you for your kind and very helpful replies. This is a very serious matter to me because my boys are using this equipment.

Over the weekend I tested both outlets at the garage where the accident occured. One of the outlets tested o.k., but the test on the other outlet showed the hot and neutral wires to be reversed.

These I believe are the facts(I don't have the circuit diagrams nor have I dissassembled the amplifiers...yet):

The amplifier DC power supply (transformer) is grounded to the internal amplifier circuit which is grounded to the ground side of the the signal input and speaker output which are both grounded to the chassis (I verified signal input and speaker output are grounded to the chassis).

Here is my theory:

By reversing the polarity of the house voltage to the power supply, the output of the power supply changed polarity and placed the full DC power supply voltage on the chassis.

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Post by haklesup » Mon Jun 09, 2008 8:50 am

Sounds like the two best things you can do to protect them is install GFCI where they practice and buy them some wireless units for the guitars. You might also buy a couple GFCI extension cords and teach him to always use it on the road. Might get them a multimeter and their own socket tester too.

I guess you need to be their roadie for a while and learn to inspect for safety.

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Bob Scott
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Post by Bob Scott » Tue Jun 10, 2008 1:41 pm

Tommy volts wrote:By reversing the polarity of the house voltage to the power supply, the output of the power supply changed polarity and placed the full DC power supply voltage on the chassis.
You didn't say whether these amps were old/tube or new/solid state. The output of the power supply should not be effected if the amp has a power transformer. A transformer provides ISOLATION.

In the amplifiers, the hot and neutral should be isolated from ground. It looks like you have a chassis to neutral short circuit. If I'm right then if you try to plug these faulty amps into a GFCI, the GFCI will be triggered, shutting down the power and protecting your kids.

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Post by rshayes » Tue Jun 10, 2008 9:32 pm

It sounds like at least one of the amplifiers was an old piece of equipment, with a two wire line cord. New equipment should have a three wire cord with any exposed metal connected to the safety ground (green wire) and should not be a shock hazard.

Old equipment (possibly referred to as "vintage" equipment) might have only a two wire line cord. In this type of equipment, the chassis was sometimes connected to the neutral line conductor through a capacitor in the .005 to .01 uF range. Normally, this capacitance is small enough to limit the current to a few milliamps even if the plug is reversed and the chassis is grounded in some other way. If this capacitor shorts, the chassis will be directly connected to the neutral line conductor with nothing to limit current. If the plug is reversed, or if the outlet is improperly wired, the chassis will then be connected directly to the hot line conductor.

Another possible fault is a short in the power transformer between the primary and the iron core, which can also connect the chassis directly to the one of the ine conductors. this might not be noticed if the cord was oriented such that the connection was to the neutral conductor. Reversing the plug would make the chassis hot.

It is also possible that the insulation failed on one of the transformer primary leads. These are often routed through a hole in the transformer shell or a hole in the chassis. Sometimes the insulation hardens and breaks off, of is chafed through by vibration.

If the other amplifier has a three wire cord, its chassis would be directly connected to the neutral conductor via the safety ground.

This combination could easily result in what actually happened, where someone was shocked when he touched both chassis at the same time.

If a part is defective (capacitor, transformer, etc), it should be repaired or replaced. The outlet wiring should also be corrected. Finally, any equipment with two wire line cords should have the line cord replaced with a three wire line cord with a direct connection from the chassisl to the green safety ground. Any exposed metal oblects should also be connected to the safety ground. If there isn't one already, a fuse should be placed in the hot lead, so that any future short to the chassis will blow the fuse.

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Post by jollyrgr » Wed Jun 11, 2008 12:16 am

I am going to venture a guess that at least one of these amps is a tube amp. In the "old" days chassis of tube equipment was tied to neutral; there was no ground plug. You had a 50/50 chance of making the chassis hot when you plugged it in as there were very few polarized plugs. If the chassis was hot touching it and ground at the same time could be deadly.

If one amp was plugged in normally, and the other with a hot chassis or the plug were wired with the neutral and hot flipped, then you would have one instrument at GROUND the other at AC potential.

Now let's examine your situation: If the amps have ISOLATION TRANSFORMERS something is wrong with one or both of them. There are some tests you should perform on all of this equipment and the outlets. Since you already discovered one outlet miswired I STRONGLY urge you to check ALL of the outlets in your home. Hopefully you will find that only one was wired incorrectly. If you have one of the socket testers (a molded plug with several indicator lights) then this is a simple matter to do. If you are using a meter or test lamp, check this way:

Using an AC meter or 120V test lamp, connect the two probes to the two flat holes of the outlet. You should see 110V to 120V or the light should come on. (For the rest of this section assume if I state the light should come on that you get a voltage reading.) Remove the probe from the wider of the two slots; this is the NEUTRAL. Connect the probe to the ground hole and the light should light. Connect it to the screw that holds on the cover plate, the light should light. Next place the probe back in the wider slot to confirm connection. Move the probe from the narrower slot (HOT) to the ground and screw. The light should not come on.

Now to test the amps. BE VERY CAREFUL DOING THIS AS YOU ALREADY KNOW THERE IS A PROBLEM! You may want to consider wearing thick gloves for this test!

Plug in each guitar amp. Place a probe from your lamp or meter on each of the audio ground connection and go to the ground plug on the outlet. You should not see any voltage or light. Do the same to the metal chassis of the amp. Again, you should not see anything in the way of voltage. If the plug is not polarized and can be reversed, reverse the wall plug. Test the connections again. Do this for both amps.

The last test is to test between both amps. Plug each amp into its own outlet as before. Connect the meter or lamp from the audio ground between the two devices. Do the same at the chassis. If the plugs can be reversed, reverse one at a time until you have tried all combinations.

This type of problem can be fatal! I posted a story some years ago about a pastor being killed. (Not the exact link as that one is PAGE NOT FOUND, but the same story):
http://www.cbsnews.com/stories/2005/10/ ... 5829.shtml

My original posting was here: http://forum.servomagazine.com/viewtopi ... hlight=amp
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!

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