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Ground the bridge or not?

Discussion in 'Pickups & Electronics [BG]' started by KramerDon, Nov 5, 2013.


  1. megafiddle

    megafiddle

    May 25, 2011
    I don't think anyone was saying death was imminent.

    Every single phase voltage supply has two poles. The line transformer that supplies your house has two terminals,
    with 120VAC between them. One of those poles is grounded. We refer to that pole as neutral. The other pole is what
    we refer to as the 120V line. There is no other difference between them.

    When you are in contact with ground, you are in contact with one pole of a 120VAC voltage supply. The fact that you
    are very rarely electrocuted lies in the fact that you rarely come in contact with the other pole. If you do, and it's for
    a sustained period, it will kill you.

    I am not an expert in electrocution. But if I'm not mistaken, shocks below a cretain level cause a reflexive reaction,
    much like pulling away from a hot object. That is my experience, with direct contact with the voltage source. The shock
    lasted for whatever the reaction time was to jump back from it. Above a certain level, paralysis occurs and one is
    unable to remove themselves from the voltage.

    Most amp and equipment faults are due to missing grounds. You can measure a sizable voltage on an equipment
    chassis with a DVM in that case. That voltage will typically fall to a very small value when someone makes contact
    between that chassis and ground. The reason is that it is typically not a direct connection to the 120V line, but due
    to capacitive leakage in the transformer, for example. Very often these "shocks" are very mild or even unnoticed.

    If there was in fact 120VAC present, I can assure you that the shock would not be mild and that piece of equipment
    would be removed from service very quickly.

    One of the worst shocks I ever got was from a high frequency, high voltage supply when I was "ungrounded". Due to
    the high frequency involved, I was in fact AC grounded. By the time I pulled away, which was only a fraction of a second,
    there was already a wisp of smoke rising from my finger. Interestingly, the danger there was more from burns than from
    electrocution.

    Because the voltage in question is AC, you do not have to be in direct contact with ground to experience a shock.
    Your presence in space provides some capacitive coupling to ground. Even when fully insulated, you can feel a
    tingle from a 120VAC line. Also from the glass tube of an 8' fluorescent lamp. Many "shocks" are of this nature,
    and many are indeed harmless.

    Again, and as you said, grounds are not the danger. But in almost all cases of electrocution, the ground was one
    of the poles supplying the current.

    -
     
  2. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    No, they are not comical at all. They are a reaction to the notion that grounded bridge == death trap whereas ungrounded bridge == vital safety feature. The assumption behind the "you are going to die" statements is that all the other factors necessary for an electrocution to occur are are already present. All you have to do then is touch some grounded metal and you will die. An ungrounded bridge is not going to protect you under such circumstances because there are so many other grounded metal objects in your environment that you will touch one of them sooner or later and then you will die. The moral here is not that grounded bridges are death traps but that when death traps exist they should be addressed at their source. Relying on an ungrounded bridge to protect you under fatal circumstances is foolish. And though electrocution is rare it is still far too common an occurrence to win you a Darwin Award.

    I don't think fuses with save you either because when currents are high enough to kill you within milliseconds your fuse is only hundreds of milliseconds away from blowing! The main purpose of a fuse is to blow before a fire can start. A fuse only blows when something bad has already happened.
     
  3. bassbenj

    bassbenj

    Aug 11, 2009
    This is true. For real protection you need a ground fault system (as seen in bathroom outlets) which break the circuit electronically in microseconds if a ground fault current is detected. Now that I think of it, I wonder why these bathroom outlets are not installed as standard equipment on amps?

    Anyway, electrocution can take many forms. In the electric chair, a lot of current and voltage is used so that the real cause of death is thermal heating. You basically are cooked. And this can happen by accident too especially from power circuits.

    However, it is much easier to die than that. An electrical impulse applied to the heart can cause it to beat randomly in different sections stopping all blood flow. It's called "fibrillation". It takes an AC signal to cause this. A single large pulse (as seen on TV for "restarting" hearts) actually puts everything back in sync. A heart can be disrupted with mere millivolts and microamps applied directly to the heart! (I've done it) Luckily, our hearts aren't that exposed. It is enclosed in an insulating bag called a pericardium. This and the fact of the location of the heart in the middle of the chest means that it takes some voltage to drive a current to it. Add to that the skin resistance of your hands (if you aren't sweating) and it takes a pretty stiff zap to jolt a heart.

    However, if you are making good contact (sweaty hands on nice metal strings or metal mic stand etc.) and current flows from one arm to the opposite leg so it traverses the heart area, you can easily drive your heart into fibrillation. No, it doesn't "stop" your heart, but it doesn't pump either. And if there is a emergency defibrillator handy it's a simple matter to start it working right again. But if there is NOT one handy your heart won't reset itself and dare I say it? You die.

    You may have seen electricians "test" circuits with two fingers. As long as the fingers are on the same hand there isn't much of a problem if you can stand the pain. But holding bass strings with one hand and grabbing a hot mic stand etc. with the other is not especially safe.
     
  4. I think the phrase, "AC grounded" is a misnomer. What actually happens is you are touching two objects that have a difference in potential between them. It is not required for either of those objects to be grounded. All that matters is that there is difference in their potential. For example, touching two legs of a standard three-phase 120V supply qualifies. There will be 208V between any two legs.
     
  5. An AC signal is not required for electrocution. A DC voltage can do it also as long as the current is between 0.1 and 0.2A. See this page.
     
  6. khutch, your comments about fuses are absolutely correct: they do nothing to protect people. Fuses are designed to protect components, not people - same for circuit breakers. That is why both are referred to as "circuit protection." The only protection they provide to people is by helping to prevent buildings from burning down.


    As far as death by electrocution, your statements continue to be entirely false.

    "The assumption behind the "you are going to die" statements is that all the other factors necessary for an electrocution to occur are are already present. All you have to do then is touch some grounded metal and you will die. An ungrounded bridge is not going to protect you under such circumstances because there are so many other grounded metal objects in your environment that you will touch one of them sooner or later and then you will die."

    The above statement quotes you. It is patently false. If it were true, no one would ever be shocked without dying. If you change the words "you will die" to "you can die," THEN it would all be true, but "you will die" is absolutely false.


    You appear to understand enough about this that I find it a bit confounding that you can continue to repeat such illogical statements. I repeat: if your statement were true, then there would be zero survivors of electrical shock from 115v mains, and I have to believe that you know that this is not the case; rather, the opposite is true: most shocks from ac mains are not deadly, although the threat of death is always there.


    Also, for everyone talking about the amount of current that can possibly kill you, yes, that's all true. Just keep in mind, they are NOT talking about how much current is available. They are talking about how much current actually goes through your heart. You can receive a shock of much more current than that without it killing you because most of it does not end up traveling through your heart. The theoretical amount of current through your heart to kill you is well known. The actual amount that goes through your heart in any given situation is dependent on the current available, the conductivity of both contact points in your body, the location of those contact points, the comparative conductivity of you body surface vs. through your body, time, luck, etc.. Keep in mind, people survive being hit by lightning, with billions of volts and 100's of thousands of amps, while some are killed by miniscule fractions of those amounts of electricity. There are an enormous number of factors that influence whether an electrical shock is deadly or merely a nuisance.


    I'm beginning to think this whole thread should be moved into the currently active thread "Lets spread some musical misinformation!" :rollno:
     
  7. mech

    mech Supporting Member

    Jun 20, 2008
    Meridian, MS, USA
    I have worked for a major manufacturer and had the unfortunate experience of inspecting and testing amplifiers which were involved in electrocutions on two separate occasions. The amplifiers were boxed and sealed by local law enforcement and shipped to our facility where they remained sealed until a UL inspector arrived for the inspection/testing. Fortunately neither amplifier was at fault. The fault was determined to be in the wiring of the buildings concerned and was actually found before the amplifiers were tested.

    You can be electrocuted by anything that plugs into the wall. To think otherwise is unwise. I use a circuit tester for any new venue to make sure all the outlets used are properly wired.

    The bridges of all my electric instruments are connected to the output jack common and shielding applied as necessary.

    There was some misinformation in an early post. Non-ferrous shielding can be effective for electrostatic fields and rf (neon lights, light dimmers, switching power supplies, radio stations, etc) but will not be effective for electromagnetic fields (power transformers).

    Also, standing on concrete or the ground with bare feet while holding anything that ends up at a wall socket is asking for a shock.

    mech
     
  8. bootsox

    bootsox

    Apr 28, 2012
    Biloxi, MS
    Jazz basses in particular tend to benefit more from shielding than grounding.
     
  9. I stand by my statement that if the path is thru your heart or brain you'll be dead before a 20A breaker trips at 120V.That was in response to someones claim that the breaker or fuse would blow before you'd be electrocuted.That would only be true with a GFI breaker or receptacle.
     
  10. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    You are confounded because you are saying the same thing we are saying but you just don't realize it. I understand that in order for death to occur a number of factors have to exist that in combination produce a lethal level of current flow through your heart. We all understand that. Thankfully most of the time people touch a "hot" piece of metal one or more of those factors is missing and they just get a nasty shock. None of us is saying that isn't true. Chances are none of us would still be here if that was not true!

    We are discussing whether or not a grounded bridge is a safety hazard. This discussion is not worth having unless we assume that all the factors leading to a death are present except one: contact with a piece of hot metal. These are the only circumstances under which a "grounded" bridge which is really "hot" can possibly make a difference. If any of those other factors were missing then contact with a hot bridge or strings will not kill you. But for the purpose of this discussion we are not concerned about those cases, only those cases where touching hot metal will most definitely kill you.

    And that is why some of us are saying "touch that metal and you will die". We are not saying it because it is true in general. We are saying it because this discussion has no purpose except in those cases when it is true.
     
  11. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    Radio stations and RF circuits generate and receive electromagnetic waves. Electrostatic fields are electric fields that do not change with time. Everything you mention generates time varying electric fields. But I know what you are trying to say. So lets talk about electro-magnetics in more detail.

    Static magnetic and electric fields generate no audible interference because any effect they generate occurs at DC, zero Hz. So we don't need to worry about electrostatic or magnetostatic fields. Time varying electric fields generate magnetic fields, time varying magnetic fields generate electric fields. In both cases the generated fields are also time varying so in fact each one generates the other in turn and this is the foundation of electromagnetic wave propagation. In free space electromagnetic (EM) waves generate electric and magnetic fields that are in a known strength ratio to one another. When we say that interference is electromagnetic we mean that the electric and magnetic fields have the free space or plane wave or "far field" ratio to one another. This condition only occurs when you are located several wavelengths from the source. The wavelength of 60 Hz EM waves is about 3000 miles so you are basically never at the free space distance from any significant source of 60 Hz interference.

    When you are close to a source of EM interference the ratio of electric to magnetic fields can have almost any value. So we often speak of these fields as being electric or magnetic depending on which one predominates but as long as the fields are time varying they are EM, just "near field" EM fields which have E to M ratios different from the far field values. There is no fundamental difference between the two although there are differences in detail that have to be considered.

    Non-ferrous but conductive shields can be effective against all EM interference, far field, near field electric, and near field magnetic. Conductive shields reflect a lot of the incident energy, they absorb some, and they transmit the remainder. The effectiveness of a shield depends on the nature of the energy (magnetic or electric near field or far field) and its frequency. You can find a shield effectiveness calculator from Clemson University here. It will do magnetic and electric near field calculations and if you click a link near the bottom the page there is a far field calculator too.

    So for example pick magnetic field, set the source distance to 36 inches, pick copper for the shield and leave the conductivity and permeability at their default values of 5.8 and 1 for now, set the shield thickness to 4 mils (0.004 inches or about 0.1mm), set the frequency to 0.06 kHz (60Hz) and click calculate. You should get 32 dB of reflection shielding and total shielding. Far from perfect but not exactly chump change either. Shielding will help with magnetic pickup. Now leave that all the same but click electric field. Now you get 269 dB of shielding effectiveness! Clearly it works much better for electric fields. If you now put those same values into the plane wave (far field) calculator you will get 150dB of shielding effectiveness so the shield works quite well for far field waves too.

    Now go back to the near field calculator, put in the same values but set the frequency to 60kHz. You just gained a bit more than 30dB of shielding effectiveness. Conductive shields reject high frequency magnetic fields better than low frequency fields. Set it back to 0.06kHz but change the thickness to 1cm. You gain 10dB of shielding. Thick conductive shields reject magnetic fields better than thin ones. Go back to 0.01cm but set the permeability to 20000, the value for Mu metal. Woah, it gets worse?!? Yeah the Mu metal actually concentrates the field rather than rejecting it but it does absorb a lot too. Now change only the thickness to 0.1cm (1mm) and the shielding goes to 133dB! If you try 1mm thick copper you get only 33dB so Mu metal needs to be a little thicker than copper but it can work very well against magnetic fields if you can make it thick enough. If you try it again with Mu metal you will see that 0.3mm of Mu metal is equal to 0.1mm of copper but Mu metal gets better a lot faster than copper as you increase the thickness.

    Shielding is a huge topic, this only scratches the surface. But you don't need magnetic materials to make a good magnetic shield.
     
  12. uOpt

    uOpt

    Jul 21, 2008
    Boston, MA, USA
    Getting zapped in the fingers when poking around in amps is one thing. I got that done dozens of times as a kid (with 220V actually), and just recently in a mother-in-law related incident.

    However, the strings of a guitar that is on a strap on your body being under power is more dangerous. Your muscles tense up, and in the poke-in-amp case that makes the fingers break contact as the hand jerks upwards. In the case of the guitar on the strap you cramp up the left hand around the strings, so you do not break contact. In addition, instead of shorting the circuit through the fingers you have the current flow through yours arms toward the floor, and that means your whole body including the heart.
     
  13. megafiddle

    megafiddle

    May 25, 2011
    In the case I was referring to, the only contact was with the arc from the high voltage terminal.
    There was no physical contact with ground or any other part of the circuit.

    The path to ground is through the capacitance between a object with significant surface area
    (like people) and the surrounding space and earth. The plasma lamps that were popular some
    years ago are a good example of current flowing to ground through this capacitance. The arcs
    inside the globe travel to the glass surface, which is at a lower AC potential (nearer to ground
    potential) than the center terminal but otherwise insulated from ground. Touching the globe with
    your finger provided a better path to ground and the arc intensified and followed it.


    "AC ground" refers to a ground path that exists for AC, but not for DC. So there is no path through
    a conductor. In the case of space capacitance, it is a relatively poor path to ground at 60Hz, but it
    can be quite good at high frequencies.

    -
     

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