A couple simple questions about AC electricity

Discussion in 'General Electronics Chat' started by Jigawatt, May 11, 2018.

  1. Jigawatt

    Thread Starter New Member

    May 11, 2018
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    Hi,

    Up until now, the vast majority of my work has been with microcontrollers and low voltage DC circuits. I've recently started working with AC quite a bit more and have a couple of questions that I've been trying to work out in my head but I'm stuck. I was hoping someone could help get me pointed in the right direction.

    1) In a 240V/120V residential application, I understand the neutral wire is connected to the ground bar in the load center which is then connected to the ground rod that is buried in the ground outside your house. I also understand the neutral coming off of the step down transformer and into the house is grounded at/near the power pole. My question is, if the neutral somehow loses it's connection to the ground rod outside the house, will you be able to be shocked by the neutral since it is no longer connected to earth? Or does the ground at the transformer keep it connected to earth? If you also somehow lost the ground connection at the power pole, could you be shocked by the neutral in that case?

    2) My second question is somewhat related to the first. How is it possible for the neutral to be connected to the earth without major problems? Since the voltage between the neutral and one of the hot leads (black or red) is varying 120V AC, the "polarity" of the source is switching each half cycle. In the negative half cycle, can the neutral be considered a "hot" lead (due to polarity switch) and if so, how does connecting it to ground in that case not cause a ground fault?

    Thank you
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    18,248
    5,604
    If the neutral loses connection at the service panel end, you still have one the centre tap neutral grounded at the transformer, some jurisdiction only use this method and do not re-connect to earth at the panel.
    The conduction will be through the earth which is considered one homogenous mass that is electrically Neutral' i.e. an equal number of negative and positive charges are distributed throughout the earth at any given time.
    Being electrically neutral, earth is considered to be at zero potential at any location and establishes a convenient reference frame for voltage measurements.
    Noting that voltmeters read only the difference in potential between two points, absolute measurements can be made, by using Earth as a reference.
    2) There is normally no current flowing into or out of ground, it is just used as a reference point, current does flow in the case of a related conductor coming in contact with it.
    If the supply is not grounded or loses all earth reference points then there will be no current flow in any conductor coming in contact with it, (in theory).
    Max.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Yes, but the polarity is switching with respect to ground.
    Thus it goes from +170Vpk to -170Vpk with measured to ground.
    As Max, noted no current flows into or out of the ground connection during normal operation.

    In residential wiring the "hot" lead is not the positive lead, it is the lead that has AC voltage which swings positive and negative with respect to ground.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    In almost all power systems (not all -- HVDC monolines are the main exception) the ground connection is not needed to supply power, but is there to increase safety. But that increase is on balance -- it actually decreases safety in some situations, but the net effect is improved safety overall.

    You could remove all of the ground connects everywhere and then your single-phase 120 VAC would just have two wires, probably called L1 and L2, and there wouldn't be any significant difference between them. Plus, you could walk up and touch either one of them and be completely safe (provided no one else was touching either of them at the same time). But don't touch both. So from that perspective, grounding one of the leads (forcing it to always be close to the same potential as the ground you are standing on) decreases safety. But what if you did touch both at the same time? Well, that would be the same as touching the Hot (Line) wire now and it is dangerous and can be fatal (is fatal quite a few times a year), but people do it all the time and the vast majority survive with no permanent damage. So why ground one of the leads? Consider what happens if the transformer out on the pole develops a short between the primary and secondary? Now you might have 13 kV or so between the L1 and L2 wires and that might well be enough to start a fire in equipment and appliances intended for 120 V and if you came in contact with that there is a really high likelihood that is sayronara. How does grounding one wire prevent this? Well, as soon as that short happens it will almost certainly cause so much current to flow immediately that the breakers/fuses pop before anything bad happens. Also, if an appliance has a fault and shorts the power to the case, the fact that the case is grounded to the neutral via the ground connection will probably cause the breaker to blow right way instead of waiting for you to come complete the circuit between L1 and L2 at some point.

    In any system you can take a conductor to some other object and connect it anywhere in that circuit you want and, in theory, it has no effect because it isn't part of any closed circuit. In practice, it can have significant noise implications because the radio frequency fields couple other parts of the system together to make closed circuits at sufficiently high frequencies, but that isn't a relevant factor in this discussion.

    The real potential problem is that the power grid is tied to ground at many locations and that DOES open up the possibility for ground loop currents, which do happen all of the time. But, except for the case of a significant fault, the ground currents are essentially residual currents from load imbalances and are pretty local.

    Where you see large (intentional) ground currents is in systems like high-voltage DC monolines where ALL of the return current is through the ground all the way from one point to the other, which is often hundreds if not thousands of kilometers.
     
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