Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AI5DH, Mar 19, 2012.
Thanks for filling in some of the gaps .
Not understanding why the RF and safety grounds have to be bonded together. I do get it that the NEC requires it, but do not get the reasons why. If the outdoor wire antennas are grounded to an independent ground rod, and then the antennas are disconnected from any cables coming into the house when the radio is not in use, and if the radio is not used, ever, during periods of lightening/storms, and the cables coming into the house are physically moved away from their respective connection points to the grounded outdoor antennas, WHY does that ground rod need to be bonded to the house electrical ground rod??? Not trying to be difficult, but if there is religiously a separation kept between the (grounded) outside antennas and the radio gear inside when the radio is not in use, why bond the antenna ground rod to the house electrical ground?
Tom W8JI explains starting here why all ground electrodes (nee rods) should be bonded.
I think I understand, sort of. If lightening hits the power lines and enters the house, then the radio will connect the house wires to the station ground, and that will enable a path for lightening energy to exit out to ground via the station ground rod. I'm not sure how bonding the ground rods (station and electrical service entrance) together would prevent that. Electricity will flow through all available paths, not just the lowest resistance path, so electricity will still flow through the house wiring to the radio, out the station ground to the ground (via the station ground rod) even if that rod is bonded to the electrical service entrance. Some, but not all of the electricity coming down the electrical wires from a lightening strike on the power lines will shunt from the electrical service entrance ground rod to the radio station ground rod if they are bonded together, but not all of the electricity will go that way. Some will still flow through the house wires seeking out that return to ground via the station ground rod.
Anyway, if the antennas are physically disconnected from the cables that enter the house, and those cables are physically isolated from metal, ground rods, etc, then energy can not flow through them
Very very simple to explain and understand. As lightning enters the ground, extreme high current flow out in all directions away from the point of entry of earth. Dirt or earth is a poor conductor thus having resistance. Current flowing through resistance develops voltage. In the case of lightning very high voltages over very short distance. So high in fact it is is called Step Voltage Potential Difference which means the distance between your feet can develop more than enough voltage to kill you instantly. This is why you hear of people in proximity of where a tree or object is stuck by lightning were either injured or killed. Lightning never touched them.
When you have two different grounds isolated by dirt is a recipe to get fried. Either your equipment, or you if in between the two grounds. When you bond them together, you minimize potential differences to acceptable levels in most cases because the resistance of the wire is significantly lower than dirt and shunts the current through a much lower resistance, thus much lower voltage potential difference.
Here is a real common cause for house fires and cooked electronics that is very common, especially in older homes or after remodels. You have your electrical service for you home bonded to a ground rod as code requires. At some point you have CATV or SATV added or worked on. CATV or SATV installer puts up his block and knows it needs to be grounded but there is none nearby. So he pounds a rod into the ground and calls it good. Well a year or so later a storm comes around and lightning strikes somewhere in your neighborhood and your TV blows up or let's out it magic smoke and you wonder what happened? Can you figure it out now. Your TV AC power was at one ground reference, and your CATV at another. When lightnin gstruck both grounds were at several thousand volts potential difference and equalized by flowing in on the CATV ground shield, through your TV AC ground back outside to the AC service ground. If the two woul dhavre been bonded together nothing would have likely happened.
It is that simple.
It's explained quite simply on Tom's website. He has several TALL towers that each get several direct hits per year (he's in Georgia). He doesn't disconnect his equipment and says he's not suffered any significant equipment damage.
No doubt the best way to protect you rig is to completely disconnect it from the antenna, ground and power source. Take the radio from your shack and place it on your kitchen table with nothing connected to it. Your radio will likely survive a lightning strike nearby, assuming the lighting doesn't enter your kitchen window and directly blast your rig.
Bear in mind that electricity flows from one point to another point that is at a different energy potential. The reason to bond everything together is to make sure, as best you can, that everything connected is at the SAME potential, so energy has no reason to flow from one point to another.
At least that's my story, and I'm sticking to it!
That is also explained (and illustrated) by W8JI. With several tall towers receiving LOTS of direct strikes annually, and not suffering significant equipment damage, the man must be doing something right (or he's the luckiest ham in the world).
I don't think he's lucky - I think he's doing everything right.
Me?? This time of year I still lower my crank-up tower to "summer height," which means the antennas are down below the surrounding trees.
Yea I know, call me a wimp!! I still sleep better at night!
Mine's been at full height nearly a year, now. My Cushcraft AR10 10m λ/2 vertical is atop the mast that supports the Force12 C-4XL yagi boom. The top of the vertical is at 99 feet AGL. The nearest tree that's taller is a few hundred feet away. We get some lightning but nowhere near what you get. I follow the W8JI plan and sleep well. I'm working on adding almost 2400 feet of radials for my 160m vertical antenna so you can hear me better in FL (between lightning storms).