Grounding Do's Don'ts & Why Part 2

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF5LJW, Feb 28, 2012.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    Mark I am aware of soil resistivity variations around the country. Thanks for bringing it up as it is worth talking about

    As you know the NEC does not specify an impedance for earth ground other than to say must be supplemented by 1 additional rod if the impedance is above 25 Ohm's. You are also aware of the SPARKY JOKE: Drive Two Rods, Collect a Check, and Call it Done. The irony is the joke is true because that is default. No Sparky bothers with driving 1-rod then trying to prove it to the inspector, because the chances of it being 25 ohm's or less is astronomical. So you drive two rods and its done right?

    OK with that said on average a single rod will yield around 500 ohm's. However when it comes to lightning the actual impedance of a driven Rod is irrelevant and meaningless. The reason is because of the impedance of SINGLE GROUND CONDUCTOR at frequencies associated with lightning.

    For example If I were to give you a spec to drive 15 rods and keep driving them until you reach 5 Ohm's as tested with a 3 point Dead Fall of Potential Meggar. I then ask you to run a 750 MCM from the disconnect to the nearest rod underneath the disconnect box. And let's say that piece of 750 MCM is 10 feet in length once you complete the circuit.

    Would you say at that point we have an excellent ground? Most would and do. But I am here to tell you it is worthless as a lightning and RF ground, and just about worthless as an Electrical Ground. Are you surprised by that statement?

    Let's just start with the Electrical portion. Let's say I take a fault somewhere between the meter and main panel directly to earth. From the disconnect we have dual 200 amp fuses on a single phase 240/120 service. What happens if say L1 goes to earth? Nothing right? With 120 volts imposed across 5 ohm's we get 60 amps of current through a 200 amp fuse. Fuse does not see a fault with 60 amps, it needs to see at least 1200 amps before it will operate in less than 1 second. OK yeah I know the fault will likely happen in a raceway which is bonded and get a few thousand amps of current, the fuse operates and clears the fault. But Earth had nothing to do with it, as no current ever flowed through earth and not needed.

    OK now let's get real with lightning and RF. What is the impedance of a 10 foot long of 750 MCM cable? You cannot answer that without more information right? You need a frequency to determine that. So here are some answers at various frequencies for a single 750 MCM copper conductor. Will heck it could be silver, makes no difference.

    1 Mhz = 26 Ohm's
    10 Mhz = 260 Ohm's
    100 Mhz = 2.6 K Ohm's

    OK for lightning the K/D current rise time equates to more about or greater than 100 Mhz. Now answer this question What is the total series impedance of a 5 ohm earth ground in series with a 2.6 K Ohm's conductor equal? 2.6 K Ohm's right.

    Last comment after you pounded those 15 ground rods into the ground all day you measured the impedance at power frequencies of DC up to 300 Hz. Actual RF and lightning frequencies are considerable higher.
  2. K4FMX

    K4FMX Ham Member QRZ Page

    There is a saying that says: "a good lightning ground also makes a good RF ground".
    What you describe as a ground system with 15 ground rods and a rather long connection to the rods would not make a good RF ground because of lead length. To make that into a good lightning/RF ground it should be configured in a radial configuration with several radials going out away from the point where the panel is connected. Each radial connected as close as possible to the panel or a wide, short (low impedance) strap connecting the panel to the termination point of those radials.

    All cables, coax, phone, power etc need to be directly connected to a common panel (SPG, single point ground) to keep all equipment/lines at the same potential during a lightning strike.

    A long "bonding" wire run over to the power panel that connects the radio grounding system to the power panel will not do. That long bonding line has the same high impedance problem as a long (or not so long) ground lead.

    Gary K4FMX
  3. KF5LJW

    KF5LJW Ham Member QRZ Page

    Gary I have not gotten into what needs to be done inside the shack as of yet. All I am talking about so far is just the GES. The 15 ground rods was just to demonstrate earth impedance is meaningless. It's methods and planning with an understanding what you are up against that count.

    As for the SPG system you refer to is what I (Telephone companies, Broadcast, ect) have been doing for decades. However it is almost impossible to implement in a Ham shack because those services do not enter in the Ham shack. However what can be done but a bit expensive is to implement Surge Reference Equalizer which you can find details about in IEEE Std 1100. Or hold your horses until I address inside the Shack in part 3. Basically it is a spin-off of using a Hatch Plate.
  4. VA3MYB

    VA3MYB Ham Member QRZ Page

    Wondering if one of the mods should sticky these (need to go find Part 1, have yet to read it)? I for one, especially being a complete newb am finding their contained information priceless. I'm still trying to figure the best way to apply the information to my situation but I'm (slowly) getting there :)

    quick ? .... is NEC code int'l? Would Canadian code be very much different? Reason I ask is i just had my house completely re-wired just under 2 years ago (knob/tube removal...upped to 200a...70yr old house...apparently brought up to code, sticker is on the box) and having a quick look at the mains Im not sure i see a entry ground? I do tho see (and it scares me a li'l) a nice shiny twisted copper attached at the main water line. I'll be the first to admit I'm a little electronically challenged (others might say a lot :) ).

    Anxiously await Part 3...I'm looking for the best way to bring everything into a 2nd floor shack

    Thanx for the gift of your knowledge KF5LJW

  5. KD0CAC

    KD0CAC Ham Member QRZ Page

    Yes some linking between the parts would be a good idea , rather than just stumbling across them ?
    Thanks John
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    No they are not. NEC codes are some of the best in the world. Many countries have no codes

    CEC and NEC are almost identical with a few nuances. Typically CEC follows (lag) NEC by 1 code cycle. In other words CEC adapts NEC 3 years later.

    Just keep in mind NEC are the minimum requirements for safe operation. There are things Ham operators can do to enhance the performance but they are not required, but are permitted and is referred to supplemental grounding and protection techniques. For example NEC does not require any lightning protection systemsnor does it address lightning protection except for the use of air terminals in 250.60. For that you have to consult and comply with NFPA 780, and UL 95 Master Label certification. FWIW NFPA writes the NEC

    NEC 250.60 Use of Air Terminals.

    Air terminal conductors and driven pipes, rods, or plate electrodes used for grounding air terminals shall not be used in lieu of the grounding electrodes required by 250.50 for grounding wiring systems and equipment. This provision shall not prohibit the required bonding together of grounding electrodes of different systems.

    FPN No. 1: See 250.106 for spacing from air terminals.
    See 800.100(D), 810.21(J), and 820.100(D) for bonding of

    FPN No. 2: Bonding together of all separate grounding
    electrodes will limit potential differences between them and
    between their associated wiring systems.
    Last edited: Mar 2, 2012
  7. AD5MB

    AD5MB Ham Member QRZ Page

    KF5LJW, do us all a favor. Post a link to your other tutorials in every tutorial. makes things much easier when you reference things explained in other tutorials
  8. KF5LJW

    KF5LJW Ham Member QRZ Page

    Wished I could do that.
  9. WA9SVD

    WA9SVD Ham Member QRZ Page

    Down, boy, DOWN! (My house has a DIY nightmare of an addition by the original owner, and it's a wiring nightmare!)
  10. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page


    This looks like very good reading and to make it easier, I have made both parts "sticky" threads for now.

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