Ground at the lightning rod?

I'm investigating upgrading (to full code compliance) an off grid DC system and have a very special situation: the structure is literally in the middle of a harbor surrounded by salt water. So going outside and pounding a copper rod into the soil to ground the system isn't very practicable. Not without scuba gear at minimum.

We do have an existing lightning rod bonded down the side of the structure down to the base. Is this an acceptable or preferred grounding point, or should it be avoided like the plague?

BTW: I am in no hurry to do any work, and as I'm not certified I can't perform the work anyway. I am just doing research to understand what is involved.

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Ground at the lightning rod?

    welcome ernie,
    is this a boat or house boat or something that is afloat on the water? if so, then it is impractical to try a ground rod and most will try to have the wire make contact with the water. this will be a good deal of contact area involving many feet of the wire, but if the very bottom is made of metal, then the ground wire could attach to the bottom. i doubt the bottom would be made of copper so the use of proper transitioning for differing metals must be made.
  • BB.BB. Super Moderators, Administrators Posts: 32,921 admin
    Re: Ground at the lightning rod?

    Probably need to know more about the exact application (now--could I give you an exact answer knowing more--probably not--hopefully can guide you to where to get the answers/what questions to ask).

    Here is a pretty nice and quick explanation of the basics of grounding a boat (and shore power). There are 4 (or more) specific types of grounds--And each has its own requirements (DC, AC, Lightning, RF). Salt vs fresh water, galvanic corrosion, etc...

    http://www.westmarine.com/webapp/wcs/stores/servlet/WestAdvisorView?langId=-1&storeId=11151&catalogId=10001&page=Marine-Grounding-Systems

    I would start there.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ErnieMErnieM Registered Users Posts: 3
    Re: Ground at the lightning rod?

    This ain't no boat and there ain't no shore power. It's a lighthouse, and there is an existing lightning rod on top of the roof with a run of 2/0 uninsulated copper down the structure side and down below the rip rap.

    Do lightning rods make good or bad electrical grounding points?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Ground at the lightning rod?
    ErnieM wrote: »
    This ain't no boat and there ain't no shore power. It's a lighthouse, and there is an existing lightning rod on top of the roof with a run of 2/0 uninsulated copper down the structure side and down below the rip rap.

    Do lightning rods make good or bad electrical grounding points?

    Lightning rods make bad grounding points. So would the lead from the ground to the earthing rod. The earthing rod itself, however, might be a viable ground connection point providing it is a good ground and a separate wire is run to it from the electrical equipment. What you want to avoid is the possibility of your electrical safety ground becoming a pathway to introduce lightning/stray Voltage inside the structure and to the equipment.

    Another choice where accessing deep into the ground is difficult is a grounding plate like this: http://www.homedepot.ca/product/black-ground-plate-c-w-connector/910157
  • BB.BB. Super Moderators, Administrators Posts: 32,921 admin
    Re: Ground at the lightning rod?

    I am not sure I go as far and state that a lightning ground is a bad ground... And if this is a fresh vs saltwater light tower, earth grounding characteristics will look different (I would guess).

    Back to the question of what/why are you talking about grounding here... My guess is you want to add some radio gear of some sort (repeater? Radio Beacon?).

    Sharing a lightning ground plus "other grounds" raises issues. If you have a lightning strike, you don't want the energy coming into the building/equipment.

    On the other hand sharing a lightning ground means that all electrical gear is at the "same voltage"--Not a bad thing if the electrical system is "stand alone"... A bit more of an issue if there is shore power (get a voltage difference between AC power and DC battery bus).

    The reason I mentioned the boating ground article was because of the differences between RF ground and AC coupling over a short "spark gap" (antenna ground plane/noise suppression) and also the whole issue of galvanic corrosion.

    If you have two ground rods with XX feet distance between them--The can pick up a voltage gradient in the ground too... I.e., the voltage is "high" closer to the strike and lower farther from the strike--Giving you hundreds to thousands of volts difference between the two ground points. So--You would not want to have anything "close" between the lightning ground cable and the rest of the power in the structure.

    If everything (radio gear, charge controllers, etc.) is in a metal box (Faraday Cage) or screened area/back board--then you only have to address the cables that "cross" the boundary (battery power, solar power, antennas, lights) with surge suppressors, and such.

    At that point, it probably does not matter too much if the local electrical ground is connected to its own ground rod or to the lightning ground cable near the base of the tower (I would not bother going under water). Keep "your grounding" insulated from the rest of the structure and "gear that is not yours".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ErnieMErnieM Registered Users Posts: 3
    Re: Ground at the lightning rod?

    The only reason for grounding the system is this is intended to be a NEC compliant system, and that means a ground is required.
  • BB.BB. Super Moderators, Administrators Posts: 32,921 admin
    Re: Ground at the lightning rod?

    You can design ungrounded power systems. It is not that difficult and done many times.

    Ground becomes more important with radio gear for antenna patterns and noise control. But a ground rod is almost useless from an rf point if of view.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.