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Transformer and potential difference

  1. Aug 12, 2015 #1
    1. The problem statement, all variables and given/known data
    So i was in a course yesterday and the "teacher" told us the following (which i had a hard time believing/understanding)

    There is a load connected to a transformer. Applied potential difference is 230V. The load is malfunctioning and the insulation isn't working anymore. The load is now at 23V. Now the guy told us there is no harm on touching the load, there will not be any current through the person into the ground.

    2. Relevant equations
    U=R*I

    3. The attempt at a solution
    His explanation was that there can't be any current because there isn't a closed loop.

    imo the only requirements needed for current is a potential difference and a non-ideal insulation.

    i asked him how he would explain thunder as there isn't any closed loop during the discharge of the cloud. Suffice to say, he didn't convince me.

    Now can someone tell me what to believe? And maybe throw in a kind of explanation of why i'm wrong?

    Thanks!
     

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  2. jcsd
  3. Aug 12, 2015 #2

    BvU

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    This is a tricky one. There simply are too many implicit assumptions to exclude life threatening situations. Teacher isn't all wrong in all cases, but I really didactically abhor the way in which he tries to bring it across. Easy to describe the exact same situation and have a deadly accident on hand:

    A transformer can be used to separate the consumer loop from the mains. Mains is on the primary side, usually one connection is 230 V wrt gound and the other is at ground potential. Mains has verly low internal resistance and can deliver huge currents . Transformer secondary windings usually (but not necessarily) have reasonable internal resistance that somewhat dampens output current. As in case described: transformer can't maintain the 230 V when there is something wrong (e.g. short circuit) in the load.

    Potential difference between secondary (output) connections is 230 V but there is no difference from either of the two wrt ground. IF load is completely isolated from ground, which isn't guaranteed at all: may well be part of the malfunction.

    So if load is on a wet floor and a broken wire makes contact with the puddle, all of a suddden the hot side does have 23 V potential difference wrt ground. If you are in contact with the hot side and the malfunction is interrupted (vibration, whatever -- malfunctions are erratic) you can easily get the full 230 V shock and a few tens of milliamps are enough to kill you. So you don't even have to be standing up to your knees in a bathtub to ensure good ground contact.

    There's simply too many things that have to be favourable -- and Murphy's law makes sure that won't always be the case.

    Theacher apparently is still alive. Tell him/her to make sure his/her pupils stay alive by not challenging their understanding in this way.
     
  4. Aug 12, 2015 #3
    Thanks for the quick reply.

    I changed the drawing a bit. It will help with my understanding.
    According to the teacher Ic will be 0A.
    According to me earth is not at 230V. As there is a potential difference and a very small resistance Ic will not be 0A.

    So, is the teacher correct? and can someone explain?

    PS: wrt = with regard to?

    thx :)
     

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  5. Aug 12, 2015 #4

    BvU

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    wrt is with respect to (I'm a non-native english 'speaker').

    In the circuit you draw one of the leads from the transformer is now connected to ground. So there will be a potential difference of 230 V between the other lead (the bottom end of the load) and ground. As long as the malfunction lasts that would mean your maximum shock is from 23 V -- you probably just get, well, shocked. But if the malfunction is canceled for a fraction of a second: Poof ! and you're toast.

    However, teacher is right in claiming Ic = 0. Which was probably all he/she wanted to bring across in the first place.

    The 230 V doesn't mean the earth is at 230 V. Only that the difference between the two leads is 230 V.
     
  6. Aug 12, 2015 #5
    Ok, i think i'm beginning to understand.
    To say that there is a potential difference is only relevant to their system only.
    There is no "global" zero potential. All potentials are relative.

    Only the difference is know between the two poles, that way it is impossible to know the difference between one of the poles and something exterior to the system.

    Does my logic make more or less sense?

    I was confused because i like to compare electricity current with water current. The behaviour is comparable imo. But the similarity ends here.
    With water there is clearly a global zero potential level ie sea level.
     
  7. Aug 12, 2015 #6

    BvU

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    Good. The thing about potentials is that they are somewhat imaginary. What drives everything in the world are the differences in potentials. That's real physics: forces are the derivatives of potentials. (I should say conservative forces ...)

    And we just agree that e.g. earth is at zero potential. Easier that way.
    And zero sea level isn't as easy as you make it seem. Tides, currents, winds, ...
     
  8. Aug 14, 2015 #7

    rude man

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    If the load has 23V across it you might or might not get a shock if you touched both ends of the load simultaneously. The pk-pk voltage would be 23*2*√2 = 65V. I remember the old days when I had a tube portable radio & it had a 67 1/2 V battery for the plate supply which I could sense when placing one finger on each battery terminal.

    However, with a 230V primary voltage and god knows what the condition of your transformer is, I would not exclude the possibility that you could get 230V rms = 650 V pk-pk between you and ground by touching the load. Even tho' your impedance to ground is on the order of megohms, that would be like sticking your finger in the high side of a 230V outlet which is really nasty as I found out when I once did it (in Sweden) as a kid.

    BTW I like your sense of humor! Need more like that.
     
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