Residential Wiring Grounding Questions

Discussion in 'General Electronics Chat' started by sallysensation, Jan 9, 2014.

  1. sallysensation

    Thread Starter New Member

    Aug 12, 2011
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    I know this has been discussed a lot and I've done some digging, but I never seem to see the questions asked in quite the way I would like, so here it goes:

    1. The neutral bus on the panel is connected to the neutral wire coming from the utility. This is a wire that is run down the pole to ground. This is required to complete the circuit with one of the two energized "hot" wires that also come from the utility that can be traced back to the power plant. When that circuit is closed, 120V is produced. And if you connect the two hot wires, the neutral is not required, and 240V is produced. If this is correct, how can this bare neutral wire be safely run down the utility pole? Is't is part of a circuit with current flowing through it?

    2. I understand the concept of a ground for personal safety: if a malfunction occurs and the metal casing of an appliance becomes energized (en extension of the "hot" wire), the ground which is connected to the casing, becomes the path of least resistance to complete the circuit rather than a person touching the case. Correct? I guess the real problem is the discontinuity in the circuit which does not allow the current to travel back through the neutral wire, right? So why have a separate ground wire? Why not just connect the neutral wire to the casing as well? This would do the same thing, i.e. create a path from the case to ground. Where am I wrong?

    3. Why are electrical panels grounded to water supply pipes or other grounding rods? Doesn't the neutral wire coming in from the utility do the same thing? Is this just a redundant safety measure in case the neutral is accidentally disconnected from the meter and the hot wires are still active?

    4. Is the panel required to be grounded to the water supply or grounding rod? Are both required? I have heard that the connection to the water piping is actually to ground the piping itself. Is that true?


    Thanks for the help!
     
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    First things first.
    The neutral wire is a HOT wire so it must never be used as a substitute for a ground wire.

    1. The bare 'ground' wire down the side of the pole is not a current carrying conductor unless there is a fault. You might see a bare neutral at the service entrance to the meter.
    [​IMG]
     
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  3. sallysensation

    Thread Starter New Member

    Aug 12, 2011
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    In that diagram it shows that the circuit operating without a fault also utilizes the grounded neutral terminal which is, if I'm understanding this correctly, the bare wire down the side of the utility pole. The hot wires that come from the power plant are seeking ground, correct? It's this potential that creates the power. So unless you are wiring a 240V circuit, you need the ground neutral to complete the circuit. How is that not hot?
     
  4. MaxHeadRoom

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    Jul 18, 2013
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    In N.A. the residential supply is from a 240vac centre tapped transformer, the centre tap of which is connected to earth ground as a 'neutral'
    All this does is make the centre tap conductor a reference point to earth ground.
    IOW, if you come in contact with it no current will flow as you happen to be at the same potential as both the earth cond. and the neutral
    The current only flows back to the centre tap through the neutral unless ground a fault occurs on one other conductor then sufficient current would flow in the earth grounded conductor to blow a fuse etc.


    Yes, also the reason is is to re-reference the neutral to earth ground and ensure a low impedance to ground at the final installation.
    Max.
     
  5. #12

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    Nov 30, 2010
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    The planet grounds are redundant, and not very good for carrying current. The wire down the side of the pole is not then run back up the pole and inside your house. There is a dedicated wire to bring neutral to your circuit breaker box. That is the one that does the real work.
     
  6. MaxHeadRoom

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    The wire down from the pole is not seeking ground at all, IOW it does not carry any current, it is just a reference.
    Any conductor can be made a neutral, all this means is that this conductor is the same potential as ground and does not under normal circumstance conduct any current, the current carrying conductor called the neutral serves this purpose.
    In N.A. for a 240v circuit, you do not require a neutral in order for the circuit to work.
     
  7. sallysensation

    Thread Starter New Member

    Aug 12, 2011
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    I'm still missing something. Correct me where I'm wrong. Two hot wires come to our homes from the power plant. They are hot because they are charged relative to the ground. It's this potential that creates electricity and causes current to flow. The current never flows back to the power plant to complete the circuit because everything is relative to ground.

    So for a 120V circuit. One hot wire and the ground neutral is required. The ground neutral must be grounded somewhere in order to cause current to flow trough the circuit and energize an appliance. If it's not connected at the utility pole, where is it connected ? The power source terminals of the circuit in an example schematic would be positive at the power lines and negative at the earth (ground). The remainder of the circuit is within the home.

    If there is no current flowing through the ground wire on the utility pole, then where does the current flow? What is the circuit path?
     
  8. nsaspook

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    The neutral doesn't have to be grounded to earth to operate as a current carrying conductor from a purely electrical point of view as current should never flow in ground wiring unless there is a fault. As I said the neutral is a HOT wire (and is usually not exactly at the ground reference point at the outlet when loaded) but on US wiring it's connected to earth ground at the main panel for the protection of people and property in the house because it allows us to detect a current path other than the HOT connectors by providing a common reference zero voltage point for wiring and equipment enclosures. The ground at the pole does the same but it's mainly used to detect faults on the utility line side.
     
  9. #12

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    No. The power from the generating station goes through more than one transformer on the way to your house and each current loop is complete in its local neighborhood. The last transformer is on a pole in the back yard and its secondary forms a local current loop. It could act completely independent of any connections to the planet, but we add the planetary connections as a safety measure. The simplest reason would be to discharge static electricity. The more involved reasons are about fault current safety measures.
     
  10. MaxHeadRoom

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    Forget about the ground, it is not needed to complete a circuit, it is there purely for safety reasons, nothing else.
    In N.A. the supply is from a local single phase transformer, that has a 240vac winding with a centre tap in order to obtain dual 120vac circuits, outer ends to C.T.
    Look at the transformer in circuit terms before connecting any earth ground conductor. then it should be clearer, it is the earth ground that appears to be getting in the way of your thinking?
    Max.
     
  11. nsaspook

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    The actual ground potential is mainly irrelevant to the flow of power from the utility. What is relevant to current flow is the difference in potential between two points. The two wires are HOT in respect to 'each other' as power flows down both from the utility when a load is attached. The neutral wire is simply a transformer center-tap reference point to a voltage divider for two 120V circuits from the 240V.
     
  12. sallysensation

    Thread Starter New Member

    Aug 12, 2011
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    Ok. So I guess my misunderstanding is more fundamental. The actual potential in a 120V circuit is generated from the voltage difference between either end terminal on the transformer and the center terminal. I suppose a simple way to describe this is because the sine wave of the voltage at the center of the transformer is crossing the axis, so it has 0. I don't fully understand that, but it's clearer.
     
  13. MaxHeadRoom

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    Two 120vac Identical resistive loads connected from each outer end and C.T. What is the current in the C.T. conductor (neutral)?
    Max.
     
  14. inwo

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    I like to explain residential 120/240 usa mains with battery analogy.
    Works pretty well, as a transformer is a "separately derived system".

    Picture 2-12volt batteries in series.

    You may use any of the three connections as common.
    You may also ground any one of the three if you wish to have a grounded system.

    The neutral (as in usa system) would be the center tap. 12v to each side. 24v between the other two.

    By choosing to ground the center tap, the maximum voltage to ground is less.

    Choosing not to ground would mean that there is no voltage limit.

    A fault from a high voltage system, or even stray capacitance, could raise voltage to 1000s of volts.

    Of course this ignores ac. theory.:cool:

    ps.
    Analogy works with this too.
    "Two 120vac Identical resistive loads connected from each outer end and C.T. What is the current in the C.T. conductor (neutral)?
    Max. "
     
  15. sallysensation

    Thread Starter New Member

    Aug 12, 2011
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    Thanks for the help, everyone. I still need some clarification though. If for instance the main breaker panel was not grounded...a ground fault would still find a path to ground through the ground conductor, into the panel ground bar, which is connected to the neutral terminal which is grounded at the meter. So is the ground at the panel just a redundancy? Is it required by code? Are two ground connections required (water inlet piping, and ground rod)?
     
  16. MaxHeadRoom

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  17. inwo

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    At least wo ground connections are required by code. In MN.

    Water system grounding/bonding insures that the water system is at grounded neutral potential. So there will be no shock hazard from contacting, say range and water faucet.

    " If for instance the main breaker panel was not grounded...a ground fault would still find a path to ground through the ground conductor, into the panel ground bar"
    True! If you mean a ground fault to a ground conductor.
    But not a fault to ground, water pipe, furnace duct work, or other metallic system.

    Shunting fault current is not the only reason for grounding/bonding.
     
  18. sallysensation

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    Aug 12, 2011
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    INWO: So the connection from the panel to the water inlet pipe is to ground the piping system rather than the reverse?

    MaxHeadRoom: the first link you provided confirmed what I was saying earlier: 120V circuits are relative to the neutral ground. So the neutral conductor MUST be connected to ground somewhere. Right?
     
  19. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Woah there, that description could lead to a serious bang in many countries.

    In the United states there is a transformer between you and the incoming hot wires from the supplier.

    This is not true in Europe and any attempt to ground the incoming line will result in disaster.
     
  20. MaxHeadRoom

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    I don't think I have disputed that.
    In the case of a N.A. C.T. transformer, all the secondary conductors are relative or referenced to earth ground by virtue of the supply transformer C.T. being taken to earth ground.
    At the point of the service entry, the neutral is again re-referenced to earth ground by way of ground conductor in the form of metallic water supply and/or ground rod.

    In the case of other counties the supply is often a star point neutral and one phase of a three phase transformer.
    In this case there is only one voltage supply delivered, 240v live and neutral which is grounded.
     
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