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Thread: earth testing

  1. #1
    gnuuser's Avatar
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    Apr 2020
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    Default earth testing

    on the topic of grounding your tower or shed.
    As an electrician I used to regularly do earth resistance testing in preparation for driving ground electrodes.
    while the average operator does not have the equipment for this. basically you are using a high power megger and taking a resistance reading from 3 places and averaging them.
    you can get the soil tested by any linesman qualified to do the testing.( of course in other countries the spark's (electricians) normally do this test anyway as part of an installation)

    depending on the substrate of the soil in your area you may have a lot of difficulty driving rods.
    If you cannot drive a rod full depth due to rock substrate one alternative is to use a steel plate 2 foot by 2 foot placed directly on the rock and buried.

    If you are mounting a tower and need to ground all 3 legs or 4 legs dig your pillar holes down to the rock. place your plate with attached cables in the hole against the rock and pour your concrete pillars, leaving enough cable sticking out of the concrete to bond to the tower legs.

    this provides a more than adequate ground for lightning protection.
    too much blood in my caffeine system.

  2. #2

    Default

    Thanks for this informative note. Have a unique situation at my QTH regarding grounding radio towers, antennas etc. My problem is I'm on the summit of small mountain (good for radio communications) but 90 percent of my property is rock ledge and bedrock. The area I need to install my tower only has on average about 18 inches of soil depth in all directions out to over 300 feet.

    In fact the house's full basement required 5 days of blasting to make an 80 x 40 x 10 deep foot hole in solid rock. (I inherited this place and only recently moved here after my parents passed away). My research mostly points to possibly burying large plates at least 3 or more feet deep and using them as grounds. (similar to what you are talking about here) But I only have 18 inches of soil to work with.

    So what would be number of grounding plates and their size given that I only have 18 inches of ground. I am thinking of drilling into a large exposed granite ledge (about 15 by 20 feet by 1 foot above ground) and making a rebar cage from a form filled 1 foot concrete pad poured onto the ledge held in place by the rebars drilled into the ledge. Then also put a few threaded rods into the drilled holes sticking out of the rock and concrete base to hold the tower mounts. BUT .. how do I ground the tower which will be sitting on the top of the 15 by 20 foot exposed granite bedrock where the only "soil" available around it in hundreds of feet in all directions is only on average about 18 inches ?? Thanks for any ideas ..

  3. #3

    Join Date
    Oct 2018
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    Default

    Im confused, because you are talking about utility ground and I am taking about RF ground.

    RF likes lots of ground, as does lightning..

    The purpose of the ground rods is to spread out the power dissipated - as in a lightning strike.

    The cell tower people adopted the standards used by the electric generation / transmission people a long time ago.

    Basically when you put up a control building for a electric circuit recloser you clear the site for 20' in all directions - more than what the site needs for the pads and footers for the breakers and the transformers.

    You put down a mesh and you drive multiple ground stakes beyond the mesh and you physically bond them by cadwelding.

    If all you are doing is using clamps you are wasting your time!

    Then you cover all of it with a layer of limestone chips, of course there is some weed control - spraying that has to be done from time to time and there is some inspection that has to be done from time to time and there is someone with a lawnmower that has to be employed to cut around the perimeter of the fence.

    If a person is building a tower, it should be enclosed with a fence with a gate and a padlock.

    Or a cage built around the base of the tower with a gate and a padlock to keep unauthorized people from climbing the tower.

    If you spread out the ground, bond everything to bring it to ground potential - less than 1 or 2 ohms you shouldn't have to worry about lightning and you shouldn't have to disconnect the coax when there is a storm.

    You never put any ground rods in the footer next to the base of the tower, because when lightning strikes - and it will, it will fracture the cement in the base - causing damage.

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