Grounding Do's Don'ts & Why Part 3

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF5LJW, Mar 19, 2012.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    Not true at all. Alaska uses SWER systems. NEC has nothing to do with a utility which is regulated by NESC. Utilities use Dirt as a conductor as they use high voltage.
     
  2. KF5LJW

    KF5LJW Ham Member QRZ Page

    You have answered your own question and do not understand electricity. On the utility the Neutral is in parallel with Ground. Very basic electrical fundamentals. What you are showing is a rural Multi-Grounded-Neutral. NEC forbids you to multi-ground your neutral or use ground as a circuit conductor. It would be extremely dangerous otherwise.
     
  3. W9JEF

    W9JEF Ham Member QRZ Page

    What I'm trying to understand, is this apparent contradiction:
    Since the neutral wire is in parallel with the relatively high resistance ground, what would cause “normal load currents to flow in the earth” (using a multi-grounded neutral)?
     
    Last edited: May 26, 2019
  4. KF5LJW

    KF5LJW Ham Member QRZ Page

    There is no contradiction. As your picture shows the Neutral is grounded via the Pole Butt Ground. If we assume 100 Ohms at each Pole Butt Ground and say 100 poles bonded between you and the substation yields a 1-Ohm Ground. The Utility uses ACSR conductors and over a distance of say 30 miles is going to be roughly 100-Ohms. Common sense will tell you 99% of the utility neutral current flows through dirt. So to answer your question if 100 amps is flowing, 99 amps through dirt, 1 amp on the conductor.
     
  5. W9JEF

    W9JEF Ham Member QRZ Page

    So the earth is huge; and all those pole butt grounds give earth resistance a huge cross-sectional area, and much less resistance than that at each particular pole. No wonder every power pole on the planet has a ground!

    So the purpose of the neutral conductor is to connect the individual pole ground resistances in parallel. Since current in the neutral conductor (from the pole grounds) can be thought of as flowing in from both directions, it seems the cancellation would mean much less current. Come to think of it, after a freezing rain, the ice always melts off the hot line much quicker than off the neutral. Now I know why. Thanks for your explanation, Dereck.

    73,

    Jim
     
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    No Jim the purpose of the Neutral Conductor is to carry any unbalanced current between phase conductors. But not in your case because you only one phase conductor. As you can imagine utility conductors are quite expensive and dirt is cheap. In your application it cuts the utility power losses in half.

    Utilities can use earth as a conductor due to the nature of high voltage. Example say the Pole ground is 10 ohms which means your service neutral is 10-ohms to ground. If you were to say use a 20 amp breaker and connect 120 volt line to ground, what happens? Absolutely nothing except you have 120 volts siting on a ground and can be electrocuted. 120 volts / 10 ohms is 12 amps which is a normal load on a 120 volt breaker. Understand so far.

    But what about the utility? Say you have a 20 kva transformer and the primary is 13.2 Kv which is a very common service. The transformer primary would be fed with a 3-amp fuse. What happens when you have a transformer fault to ground. 13,200 volts / 10 ohms = 1320 amps. What happens to a 3-amp fuse with with 1300 amps flowing through it?

    All the physics change when you go from low voltage (600 volts) or less, to high voltage of a utility. NEC forbids dirt to be used as a conductor, and NESC allows for dirt to be used as a conductor. Why? High voltage!
     
    Last edited: May 26, 2019
    NE1U likes this.
  7. W9JEF

    W9JEF Ham Member QRZ Page

    Apology if I seem to be argumentative, but more reflection raises doubts as to the 99% of the current flowing through dirt.

    From the Wisconsin Public Service Website:

    Electric current always flows from a power supply to the equipment that is using it and then back to its source, completing a circuit as it moves along an electrical supply system. Every electrical supply system uses a neutral conductor to return the current to the supply source. In addition, this neutral conductor is grounded.

    This approach creates a "path of low resistance" for the current to follow in returning to its source. Under normal conditions, when electrical current flows along the grounded neutral conductor, a small portion of the current also flows through the earth. This creates some neutral-to-earth voltage (NEV), a small but measurable amount of voltage at each point where the electrical system is grounded.


    https://accel.wisconsinpublicservice.com/business/farm_voltage/electricity.aspx

    [​IMG]
     
  8. W9JEF

    W9JEF Ham Member QRZ Page

    In our application a neutral conductor of the same size--and equal expense as the live wire--is used for return. In order to cut the utility power losses in half, 50% of the return current would need to flow through that ground wire going down the pole.

    [​IMG]
     
  9. KF5LJW

    KF5LJW Ham Member QRZ Page

    No apology needed. Real simple I over stated with a mathematical example, and Wisconsin Public Service is grossly understating NEV. Stray voltage aka NEV is a huge problem on rural distribution with a single phase wire. Ask dairy farmers in Wisconsin. That is why milking operations are done on raised steel platforms to combat Stray Voltage. .

    Where there is a Single Phase as you have pictured, Phase Current = Neutral Current. Not so with 3-phase. So here is what you have to consider. Impedance is going to determine how much current flows. I do not know how many miles you are away from the sub-station. The utility uses ASCR conductors which is aluminum clad steel. I will assume they are using 1/0 which has a resistance of .22--Ohms per thousand feet or let's say 3-ohms per mile. If you are 20 miles away from the sub-station there is 60-ohms in both phase and neutral. Wisconsin soil is fairly conductor and a conservative pole butt ground is 100-ohms. If every 4th pole has a pole butt ground with rural spacing or 300 feet. That is 5 pole but ground per mile x 20 miles = 100 pole grounds. So the Neutral conductor is in parallel with ground.

    So you have a 60-Ohms Neutral Conductor Impedance in parallel with 1 to 2 ohms ground. Let's say 2-ohms. So do the math. for every 100-amps of total neutral current would result in 3 to 4 amps in the overhead ASCR conductor, and 96 to 97 amps in dirt. Think of what you have observed in winter. Phase conductor ice melts first.
     
  10. W9JEF

    W9JEF Ham Member QRZ Page

    I'm trying to fathom how “100 ohm butt ground” resistance is figured. I have a tower tied to six 4-foot ground rods. The resistance between it, and our utility ground 100 feet away reads 40 ohms. At 0.4 ohms per foot, that would be over 2000 ohms per mile.
    Like the neutral conductor, is the resistance of dirt not directly proportional to the length of the path? Using my 2000 ohms per mile calculation, with 100 poles, at 20 miles distance (20 ohms per mile) it's 400 ohms. A 20 mile neutral conductor of 0.22 ohms/ft would be <24 ohms.
    Dereck, I certainly appreciate the time you're taking for my enlightenment.

    73,

    Jim
     
    Last edited: May 28, 2019

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