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Trying to understand the finer points of current flow

  1. Oct 19, 2015 #1
    • Touching live wires is inherently dangerous and should not be attempted. This thread is ONLY about the physics behind why it is dangerous, and not about how to do it safely. Do not touch live wires.
    I'm looking at an ungounded 3-phase system here for starters. In a perfectly ungrounded system, grabbing a phase seems safe in theory, but in practice (because perfect ungrounded doesn't exist), this is a bad idea. I'm trying to determine exactly WHY.

    If I have a perfect ungrounded system (zero potential between any phase and earth), and I measure the voltage of a wire, have I not begun to move the earth into the same potential reference at the phase that I have connected to it? I guess it is easier to explain if we are looking at a shipboard system. I'm going to assume that we are looking at this ship at a single instant. If I have a phase and connect it to the ship, then the ship will "polarize" and become equivalent of the phase connected to it. What is physically happening here? Is there initially current flow, in order to polarize the object to bring it into the same reference? I'm trying to understand the difference in voltages (i understand they are all relative to a point)... However, I don't see how physically a difference can exist between a ship touching a branch, and that same ship touching another branch at that same instant. What changes? can I measure a potential difference? If I have a honda generator sitting in the bed of a pickup truck, and I measure the potential difference to the warehouse floor, why do I get a voltage reading? Is this only temporary as the warehouse is undergoes constant "hysteresis"? Is that word even allowed to be said in this conversation?

    I have conducted this test before and it has given me a voltage reading on the meter at the potential difference of the generator's terminals.

    Also, I ended up measuring the potential voltage difference between one branch of a 440V 3-phase terminal and literally 10 feet of wire in a coil sitting on a rubber matted table, and I measured 30 volts... how does this make any sense? I would get current flow through the terminal to the wire as well... this makes no sense to me. I initially measure a low current and it slowly goes up until reaching a maximum.

    Can anyone clarify all these crazy current flow questions? I have a formal education in circuit analysis, however these things aren't exactly covered in class. I see these all as open circuits and my brain tells me zero current... or connecting anything to a terminal of an open circuit just transfers the voltage across the resistor/object, and gets
    Last edited by a moderator: Oct 23, 2015
  2. jcsd
  3. Oct 19, 2015 #2


    Staff: Mentor

    I do t want to discuss your question because others who read this thread in the future might be led to believe that they can touch a live wire without consequences. Dangerous topics are not allowed on PF.
  4. Oct 19, 2015 #3
    The purpose of this question isn't to prove that you can touch a live wire without consequences, it's to help me understand the current flow between dissimilar grounds in a system. There is pretty much never a case where you should be safely touching a live wire. Why am I getting these readings on a meter?
  5. Oct 23, 2015 #4


    Staff: Mentor

    The mentors have discussed this and decided that a discussion on the physics of why it is dangerous is OK. There must be no discussion of how to touch live wires safely. Any such posts will be deleted with an infraction!
  6. Oct 23, 2015 #5


    Staff: Mentor

    We all get into trouble when we combine concepts about ideal circuits with real life measurements.

    Insulators are never perfect, they leak current. Even your voltmeter is drawing nonzero current from the wire to ground. The reading you get is only measuring the relative imperfections of one flaw against another. The answers may vary from year to year as things deteriorate. They might vary with humidity. They might vary from ship to ship and circuit to circuit. It is really hard to attach meaning to measurements you describe.

    If you connect a voltmeter to ground and to any isolated point, it has to read something. No matter what it reads, including zero, you can ask why that value?

    To repeat, it is hard to attach meaning to the measurement because it depends only on the degree that the isolated point is not truly isolated.

    Does that make sense?
  7. Oct 23, 2015 #6
    Scott - your inquiry is all over the place - and not very specific. Considering the hazards ( outside of the touch the live wire issue) - you indicate you are literally measuring a 440 V system - without understanding what you are dealing with, this is a REAL hazard and I support the closing of this thread. ( If you are in a ship you are exempt from OSHA - but in the states, you are not a "Qualified Individual" and should not be exposed to this without proper training - this is called Live Line work in the US)

    A) Your observations are based on a real world system that is no where close to a "perfect ungrounded system" - I can only imagine this existing in a vacuum / space - and by making a measurement you are affecting the system ( Schrodinger will tell you you Meter Matters!) .

    B) As for the specific case of the 440V Supply measured to "coil of wire" - there are legitimate reasons for this but it is an observation - not an experiment, and by that I mean there can be many reasons and contributing factors. Most likely - if the coil is completely isolated and slightly capacitivly coupled to ground ( are you on a ship, surrounded by grounded metal - including the table? - then the Coil and all of the grounded metal world around you make a capacitor) -- also the meter you are using probably has very high internal resistance, AND you are dealing with AC (not DC)---- the meter possibly can "see" an AC voltage difference between the Live source ( which is probably grounded somewhere) and the coil. ( DO NOT DO THIS -- BUT -- if you connect a high resistance from the coil to ground - then you will probably see the full 440V, conversely if you were to connect the coil to the 440 with the high resistance the V would go to zero - and the coil measured to ground would be 440.) So do discuss "absolute" issues like there should be no voltage - is not realistic. Your coil and source are not perfectly ungrounded>

    C)- Voltage and Current are different (but related) things, you can have voltage with no current, and you can have current with no voltage ( I know I'll be called out on this but there are cases for both situations). So you need to be specific about what is voltage (potential) and what is current, (again leading to my comment about a qualified individual) - So your statement "I initially measure a low current and it slowly goes up until reaching a maximum." makes very little sense since you started talking about Voltage.

    You may have studied DC circuit analysis - but this is AC and we can often we rely on our DC principals, but must always keep in mind AC analysis items - like capacitance, so when we get an unexpected measurement we have the theory to step back and re-think our initial assumptions. When you are working with 10-50 V DC and basic resistors you will probably never notice these issues.

    Sorry if this comes across as harsh - I really do not mean it that way - but I do want you to take notice! I am seriously concerned about your safety. While your observations and questions seeking understanding are valid - it is involving many different aspects of a real world system.
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