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HAM SHACK LIGHTNING PROTECTION
by Hal Myers K4JHM
First of all, lets get some basic concepts in mind. The thing that "gets" our equipment is the voltage difference between the various conductors coming into the equipment, not the magnitude of the voltage.
Lets just take a simple modem as an example. It has two sets of conductors coming into it: the power and the `phone line. If you get a transient on one of these conductors but not on the other, then there will be a potential difference between the two sets of conductors. This is what zaps the device.
In the case of our ham equipment, we will usually have three sets of conductors, the power, the antenna and the `phone or data line from a modem which is tied to the `phone line. It is important to keep the voltage difference between all three sets of conductors at a reasonable voltage.
You may have noticed that usually the `phone service drop terminates at the NID (Network Interface Device) which is usually mounted close to the power service drop at the electric meter. There should be a driven ground rod at that point and both the power common (neutral) and the NID (which contains a lightning arrester) are connected to this ground rod. The intention here is that if lightning hits the power line (or `phone line) at the street and the resultant transient comes down your service drop, since both the power and `phone lines are connected to that same ground rod with short leads, from that point on into your house, the potential difference between the power and `phone conductors will be low enough that nothing gets damaged. (Usually it works.)
Now, we add a third conductor, our antenna system. Ideally, we should route it into the house right there at the electric meter with a lightning arrester also tied to that common ground rod, now all three sets of conductors should ride at roughly the same potential (limited by the break over value of the various lightning arrestors) when ever a transient entered the system on any one of the three sets of conductors.
We hardly ever do that for a variety of valid reasons. But we go to other side of the house, drive one or more ground rods and bond all of our ham equipment together, we now have two (independent) ground systems. These two ground systems (one at the electric service entrance and one at our ham shack) are separated by the width of the house (maybe any where from 20 to 100 feet of earth). Earth is not a "conductor", it is a "resistor". The value of the ground resistance between the ground rod at the service entrance and the ground rod at our ham shack may be anywhere from 1 ohm to 1000 ohms, depending upon the distance, the type of soil and the amount of moisture in the soil.
Lets take a best case and say the resistance between the two ground rods is 1 ohm. When lightning hits the power line out front *or* the antenna out back, the strike may peak at 100,000 volts or more. This voltage difference shows up across the 1 ohm resistance between your two lightning rods. This voltage difference is going to show up at the equipment between the power connections and the antenna connections.
Fortunately most lightning strikes don't hit the power line right outside our house or our antenna directly. Most of the time, it hits some distance away so the transient is attenuated some as it travels toward out house. So, we have a smaller transient to contend with which normally comes in on the power service drop and is grounded at the meter. If your equipment is grounded at the other side of the house with a second ground rod(s), then there will be a voltage difference between the AC going to your rig and the rig rounding system as the transient dissipates through the earth.
If you have a three wire power plug on your equipment, most of the voltage difference will show up between the chassis and the antenna connections. If you have a two wire power plug on your equipment, most of the voltage difference will show up between the AC incoming lead to the rig and the rig chassis.
Take your pick where you want the transient to show up. I'd rather have it between the chassis and the antenna, there are usually more clearances and higher voltage insulation at that point than there is between the power cord and the chassis. Of course, bonding all the ham equipment together keeps the voltage difference from showing up between the various pieces of equipment.
If it is practical to bond the two ground rod systems together with a relatively short, straight, low impedance conductor, you have a chance of holding the voltage difference between your two ground systems at a reasonable level. Remember these transients contain high frequency components so that all bends in the ground conductors constitute series inductance which will tend to isolate the two ground systems and defeat the purpose of attempting to bond them together. This does, however have a benefit, other than lightning protection, in that it makes sure that there is no voltage difference between the two grounding systems and consequent ground loop currents from flowing under normal operating conditions.
When a lightning storm approaches, disconnect all your antenna leads and through them out into the middle of the back yard. Unplug all of your equipment from the power lines. Unplug all your modems, etc. Obviously, this seems to be ridiculous advice, unless you can see that the lightning is really, really close. Remember that sound travels about 1100 feet per second, so the time between when you see the flash and hear the thunder allows you to estimate the proximity of the lightning.
So, yes single point grounding is the ideal but remember that includes not only all or our ham equipment, it should also include our AC power and `phone systems at that single point. My preference is 3 wire ground plugs on all power cables, then you know where the majority of the voltages are going to appear rather than it being a crap shoot where it will show up using 2 wire power plugs.
Hope this helps,