ground fault

Discussion in 'General Electronics Chat' started by john_avery, Jun 12, 2011.

  1. john_avery

    Thread Starter New Member

    Jun 12, 2011
    I was reading Volume I, chapter 3, "Shock Current Path" of the e-book, and it says that grounding the neutral wire of a power system ensures that the wire is safe to touch. However, if you're holding just the neutral wire, and if the hot wire faults to the ground, can't a current go through the hot wire, through the ground and through your shoes, through you, and back through the neutral wire? If a tree knocks over a wire, there is a 50/50 chance it's the hot or neutral wire, so there's a 50/50 chance you will get shocked if you're holding the neutral wire.

    If you didn't ground any of the wires, there's still a 50/50 chance you'll get shocked, depending on if the wire you're holding is the one that didn't fault to the ground.

    So is it really beneficial to ground one of the wires? And is the statement that it is always safe to touch only the neutral wire since it's grounded false?
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    There are some fault scenarios where the existence of a ground connection may seem to increase the hazard. The situation you describe requires a victim to be holding on to a neutral wire in the immediate vicinity of a fault into the soil. Many other faults would not involve danger of this kind.

    I would agree that, although touching the neutral line is not usually going to give you a shock, it is not such a good idea either. There may be a fault, or you may make a mistake about which is the neutral conductor. An incorrectly installed cable may have the live and neutral conductors transposed, so don't risk your life on the fact that the wire is the right colour to be the neutral!
  3. n1ist

    Active Member

    Mar 8, 2009
    The neutral is a current-carrying conductor. Even without faults, it won't be at ground potential if there's current flowing (ohm's law). There are a number of fault conditions that will energize the neutral (hot-neutral short with a faulty breaker, open neutral, reverse-wired receptacle)
  4. john_avery

    Thread Starter New Member

    Jun 12, 2011
    The only advantage from grounding one of the power lines that I can think of is that it allows the use of a fuse in case of a fault. If there were no such thing as a fuse, is there an advantage to grounding one of the power lines?

    If the outer box of an appliance shorts to the hot wire, then grounding the outer box will cause current to flow from the hot wire, through the box, through the ground wire and finally though the neutral where they're tied together, and this large current will blow the fuse. However, the danger in this scenario was that the person was touching the box and the ground at the same time, which is only a problem because the neutral is grounded in the first place! So this fault scenario is not something that grounding prevents - grounding causes it, but also solves it via a fuse!

    I'm having a hard time figuring out the advantage to grounding one of the power lines, except for a fuse.

    I'm not sure I understand. Wherever the neutral is tied to the ground, they'll be at the same potential. Say the neutral is tied to the ground at points x=0 and x=L, the two points separated by some voltage V. From symmetry, I expect the voltage at x=L/2 to be V/2, no matter if I'm looking at the neutral wire circuit that goes from 0 to L, or the ground that goes from 0 to L. So if I'm standing at x=L/2, and touch the neutral wire with my hands and the ground with my feet, there should be no voltage difference, even though the resistance of the ground is a lot larger than the resistance of the neutral wire - all I'm assuming is that the resistances are uniform throughout the respective materials.

    Now I've heard that at very high voltages (before stepping down), a fault of the hot wire (such as lightning ionizing the air, causing the air to be a conductor, so that current from the hot wire can leak through the air and down the metallic pole into the ground) can lead to dangerous potentials on the ground, such that you can get shocked just by spreading your feet on the ground. If you model this as having a potential difference between 0 and L, then the potential drop per unit length G would be:


    or G=V/L

    So if your feet are separated by a distance d, then the voltage between your feet is:


    which is proportional to the voltage V. This potential drop is caused by currents in the earth which drop the potential by IR. However, it doesn't matter what R is for reasons of symmetry.

    So it seems the only way you can get shocked from a mismatch of voltages between the ground and the neutral wire is if the neutral wire is wounded a lot, so that although you touch the neutral wire and the ground halfway between where they are tied together, in actuality you might be touching the neutral wire at 3/4 its length due to windings, so there would be a potential of V/4. Also, do we really even know what V is?
  5. n1ist

    Active Member

    Mar 8, 2009
    If there is no fault, the neutral and ground will have the same potential at the point they are connected (either the meter can or first panel). At any other point, they will be different since there's current flowing through the neutral, causing voltage drop, while the ground is still at 0v (no current flowing, so no voltage drop). Sure, the difference isn't much (6v for 150' of 14AWG at 15A) but in an overload situation it can rise to hazardous levels.

  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    In a real life situation, you probably don't know if the neutral before you has continuity to ground. NEVER touch a neutral, assuming it is harmless. I've seen enough in the years that when I lock out, I measure all wires to ground before touching anything.
  8. someonesdad

    Senior Member

    Jul 7, 2009
    The rationale for grounding the neutral in US code is that then there's only one hot conductor in 120 VAC branch circuits. Then neutral throughout the house is essentially at ground potential. I say essentially because a load drawing 15 A of current can drop a volt or two across the resistance of the neutral wiring (I see a volt drop from the little heater I keep next to me in the winter). If you see more than a few volts with respect to ground on a neutral, you may want to check out the wiring of that branch circuit, as a properly designed and installed neutral shouldn't have significant voltage on it.

    And don't assume anything about how things are hooked up until you measure it yourself. The first step is to bring in an independent ground reference (i.e., don't trust the wiring) so you have a known-good reference potential to measure with respect to.

    To answer your original question, if there's a short to ground, you're not going to get a shock through a properly installed neutral -- because it's at ground potential. Besides, that short is almost certainly going to blow the breaker. This is for inside a house -- if you're talking about e.g. Cat IV conditions outside a house or up on a pole next to a transformer, then things can be different because of high voltage transients.

    Unless you're experienced and have the right equipment, you shouldn't be messing with this stuff...
  9. john_avery

    Thread Starter New Member

    Jun 12, 2011
    By overload do you mean drawing more current than 15 A?

    n1ist has calculated a 6 volt drop for 150 feet of wire with 15 A current. I suppose it depends on the size of your house, but for you to see a 1 volt drop between neutral and ground that would imply you were 25 feet away from where your system is grounded. But whether it is 6 volts or 1 volt, isn't this harmless? I heard that 12 volts from your car won't do anything to you.

    How would you do that? Do you have to stick your own metal stake into the ground, and attach your voltmeter between the line and that stake?

    The hot wire has some resistance Rh, as does the ground Rg, as do you Ry, and the neutral wire Rn. So if you imagine the circuit of the hot wire connected to the ground (through fault) and the ground to your feet and your feet to your hands and from your hands to the neutral wire you're holding, the current would be:


    The resistance of the hot wire and the neutral wire is negligible, so approximate this as:


    Assume there are no faults, and you're just holding the hot wire and standing on the ground. Then the current you would experience should almost be the same as this. In this case, the current flows through your hands, through your body, and through the ground until it finds the neutral wire. In this other case, the current flows through the ground, through you, and into the neutral wire you're holding. As far as resistances go, aren't the situations about the same?

    I'm definitely not going to mess with this stuff. I want to make my way up in the e-book to electronics, but I have to get through this stuff first, since it comes first.