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Floating Earth leads to dangerous case voltages - neutral OK

  1. Apr 30, 2015 #1
    I moved to Australia from the US and took my Crate amp and Ibanez electric guitar with me. To deal with the 240 VAC, I needed a step-down transformer. So I have this setup:
    The ground connector of the socket on the 120 VAC side got weak and stopped making connection (Note 1). So the guitar floated. Problem is, I feel a mild shock when I'm touching a metal part of the guitar with my bare foot resting on the metal case of my PC (I live an exciting life).

    If I measure that voltage (Note 2), it's north of 100 VAC sometimes. Should an open ground cause this? What piece of equipment is at fault and how do I fix it? I'm unnerved that a simple open ground caused a voltage so high.

    The house's bond between neutral and ground is nearby and I'm sure it's good - I've just had it re-done and tested.

    Help much appreciated.
    Last edited: Apr 30, 2015
  2. jcsd
  3. Apr 30, 2015 #2


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    You have a very dangerous condition now, because if the supply is reversed, say the socket is wrongly wired, the metal case will be connected to live. I suggest you need to remove the neutral link shown under the transformer and ensure the ground connection is 100%. Ensure you have a double wound transformer, not an auto transformer i.e. one with one winding with a tap.
  4. Apr 30, 2015 #3


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    Your use of “A” on an Earth lead is confusing since it is the symbol for Active.
    In Australia the 240VAC is provided by a common neutral with a single active phase.
    Your circuit also has an Earth connection that you must repair to make things safe.

    The 240VAC is symmetrical about the neutral conductor, so you can expect capacitive coupling to produce about half the 240VAC = 120VAC on the ungrounded chassis. There is little power behind that floating voltage. It is the high impedance of your DMM that is detecting the voltage. That does not mean that it is not a problem. Voltage can damage insulation and cause power supply failures.

    The link across the transformer is not a problem, do not remove it. It keeps the voltage of the secondary within range of the primary and so breakdown potential to Earth is not exceeded.

    You must repair your Earth connection between the wall, the external transformer and the amplifier chassis.
  5. Apr 30, 2015 #4

    jim hardy

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    Very likely it would. A Baluncore pointed out, a tiny current flows through your amp into the guitar and your DMM and your foot are both sensitive enough to detect it. That current needs to be intercepted and returned to earth by the "ground connection" at your Note 1.

    You already said it's the socket on the 120VAC side of the transformer. Replace it.

    You've shifted tense here, past to present. .
    Are you saying you've already fixed the socket but still have the problem ? I'm confused.

    If your question is ,
    'why does my (now) grounded amp still shock me?' ,
    you should make this test:

    1. Unplug amp from transformer.

    2. Connect DMM leads :
    one to the ground prong of amp's power cord
    other to metal on guitar

    3. select DMM to ohms
    if you do not read almost zero, then your amp is not providing the "ground" connection that you think it is. That bottom wire in your sketch isn't right.

    If you do read almost zero,
    4. .string a long wire out the window to your house ground rod . Use that for "ground" and measure voltages from there to inside receptacles' ground point., to case of your PC, and to guitar .
    http://www.thearrivalstore.com/media//Type_I.JPG [Broken]
    who knows - it could be your PC or the house service panel itself.
    Last edited by a moderator: May 7, 2017
  6. Apr 30, 2015 #5
    Agreed, thanks. Have modified the diagram.

    Interesting, that explains it. Does it make sense that I can easily feel it though? Jim (below) thinks so. I understand the high-impedance of the DMM picking it up, but my impedance is quite a bit lower. It feels strong enough to be nearly dangerous. Wouldn't this situation have been common before they started grounding chassis?
  7. Apr 30, 2015 #6
    So that one fault is enough to explain the phenomenon, and the strength of what I'm feeling (subjective, sorry) doesn't indicate anything more serious.

    The fault HAPPENED and NOW I can feel the described effects. I have not yet fixed the ground.

    Measured voltages to the socket, so it's not anything special with the PC (good point). And it's not the house panel because I've had a good electrician over here checking things and re-doing the house earthing (as mentioned).

    So the consensus seems to be: Capacitive coupling explains the measured voltage and the felt effect. Earthing isn't just for safety in case a fault makes the case live, but also to eliminate this coupled voltage - desirable to prevent insulation breakdown, for comfort/sanity and possibly safety if the current is high enough.
  8. Apr 30, 2015 #7

    jim hardy

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    Thanks for the clarification.

    one step at a time. Hopefully that'll fix it.
  9. Apr 30, 2015 #8


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    Chassis have always been grounded to protect the transformer insulation. Originally the Earth used was the water pipes in a house, that was replaced by providing an Earth terminal on power outlets. A good Earth increases the chance that a fuse will blow before the live chassis kills someone.

    Voltage is always dangerous, it kills when the current rises sufficiently to trigger muscular activity. The human body can sense very small currents. I can best sense 50Hz AC on floating equipment by brushing it with the top of my fingers. When I do sense AC, I always identify the leakage and fix the Earthing system.

    With a good Earth, if the leakage current is sufficient then it should trigger your structure's residual current protection, RCD. You will then know that the AC insulation of your amplifier has broken down and needs repair.

    You know there is an Earthing problem. Failure to fix the known faulty equipment represents the legal liability of a manslaughter charge, with a period of imprisonment or a coffin for you. You must get the Earth connection fixed before you continue to use the equipment.
  10. Apr 30, 2015 #9
    In the days of two-pronged outlets, was the chassis just tied to neutral?

    I have RCDs. They're not triggering on this, presumably because of the isolation provided by the step-down transformer?
  11. Apr 30, 2015 #10


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    No. There is not always a neutral for a two wire system, and the Active and Neutral often get swapped in the plug. In the USA for example, both conductors could be live and symmetrical about Earth, so the electric field tended to cancel toward zero. Those two wires are twisted together so the magnetic field due to the current flow also cancels toward zero.

    It is not an “isolation transformer”. The link across the step-down transformer enables a fault on the amplifier side between Active and Earth to trigger the RCD. The link also prevents high voltage breakdown of the stepdown and amplifier transformer's insulation.

    An RCD looks at the difference between the Active and Neutral currents. When that AC current exceeds 30 mA it will isolate the circuit. The typical way to get a difference is to have a current from active to ground. That will happen if you accidentally cut through a power cord because the Active in the cable may short to the Earth = RCD, or to the Neutral = current limit fuse or Circuit Breaker.

    Your leakage current is probably well below the 30 mA RCD threshold.
  12. May 1, 2015 #11

    jim hardy

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    Hmmmm, well, sometimes.

    Two prong plugs nowadays have one wide and one narrow blade so they'll only go in one way but that was not always the case.
    Those old radios from 1930's to 1950's , the cheap table models in plastic or wood cases often were wired as you suggest.
    They were called "All American Fives" because they all used the same 5 tubes.
    AllAm5.png AA5sch.png

    To make them cheap they didn't have a power transformer. One side of the line cord was tied to chassis, and the enclosure was supposed to keep you from getting shocked. Before 1950's the plug was not polarized and could go in either way, so the chassis might be connected to either hot or neutral.

    Better equipment like quality console radios and TVs always had a hefty power transformer that isolated chassis from house power.

    ..................................... boring anecdote............

    My freshman year a guy in a room down the hall had a really nice stereo by 1964 standards.
    But no tuner, just a turntable. That was not uncommon then for FM stereo was relatively new as was transistor Hi-Fi
    He and his roommate scrounged a couple of "All American 5's" and decided they'd run wires from their speaker terminals to the aux inputs on the stereo. That'd give them big sound. In those days "Clear Channel" AM radio was popular and broadcast rock 'n roll music nationwide at night. In central Missouri we could get AM stations sometimes as far away as Quebec.

    Their hookup worked okay the first time. They'd got lucky and plugged in both radios with chassis at neutral.
    But they unplugged one radio to resolder a wire, and when they plugged it back in the plug was other way. Needless to say it made a lot of sparks, noise and smoke and scared the daylights out of them.
    I think the stereo survived. Tubes are tough.

    Things are safer today. If you tinker with an antique radio , be aware of a likely shock hazard.
  13. May 4, 2015 #12
    Thanks for all the info guys. For the record, I replaced the socket and the "effect" is gone. Now, no more excuses for not practicing...
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