Grounding

With lightning season in high swing lightning protection is an important topic.  At the radio cabinet we are following the generally accepted practice of a single point ground system. This means we connect all devices to a single point so that at the moment of an “event”  potentials will rise and fall simultaneously for all devices. Potential differences are eliminated and therefore damage is eliminated.  For the single point we have chosen the ground rod at the equipment cabinet.  An 8 foot ground rod has been driven and all devices in the cabinet are connected to it.  Below, the ground rod can be seen as well as various wires connected to it making it the single point of ground. The frame of the solar panels is also connected to the ground rod. (Bonding the frame is not in compliance with NEC 2014.  See “Section 690.47(D)” for details. We feel we might be ok here because we are not connected to the grid).

2016-04-09 14.02.07

An appropriate surge protector is installed where each cable enters the cabinet and the protector is in turn connected to the ground rod.  This is to provide a path to ground through the protector and not through the equipment.

The antennas are located 100 feet away from the cabinet (to solve interference issues ).  If the two were closer the antennas and associated devices should be bonded to the same ground rod using 2″ copper strap.  No. 6 copper ground wire would likely “fuse” and melt upon a direct hit.  We could have driven a ground rod at the base of the antenna.  If the antenna was closer to the equipment that would create a dreaded ground loop.  At 100 feet separation it’s probably far enough not to hurt anything. The function of the ground rod is performed by the radial ground screen acting as an electrode.

Ward Silver, N0AX, says in an article in September, 2015 QST, Grounding and Bonding Systems, p69, “…if the tower is not located close to the house and it’s bonded earth connections..the radial ground screen can help spread out the charge.”

We’re going on the principal that we have two systems not bonded together except by the coax cables and the cable will “fuse”  (and become an open circuit) upon a direct strike. We also expect the long cables will provide enough inductive reactance before they fuse to isolate the two systems.  I believe NEC refers to this concept as “impedance” grounding. The two systems are defined as follows.  One is the cabinet and a ground rod for a single point of ground.   The other is the antenna and it’s radial screen acting as an electrode.  There is no ground rod at the antenna.  We expect the system that gets hit to dissipate 90 per cent of the charge.  We expect the other system  to suppress the other 10 per cent without damage. We further expect the cables in between to suppress a considerable portion of the charge before fusing.

Another important feature not to be overlooked is the shape of the antenna.  A pointy vertical tends to put up feelers which lightning step leaders search for as they come down out of a cloud.  A flat surface tends not to put up feelers.  Our inverted vee flat top at the top of the vertical antenna provides that flat surface and reduces feelers.

Laughable inconsistencies.  On the other hand the 20 meter EF-20 half wave vertical has everything wrong about lightning protection.  It is pointy, likely putting up feelers during a thunderstorm.  It has no ground rod nor ground screen.  It is a culprit waiting to cause lightning problems.  On the positive side it is a thin wire which will fuse quickly and it has only a coax running back to the equipment and it runs though a Polyphaser surge protector at the entrace to the equipment cabinet.  Even though it’s a thin wire it can put up feelers and attract a lightning bolt. Once the EF-20 is vaporized the charge will need to find an alternate path to ground with an unhappy outcome. (I was checking the antenna one day as a thunderstorm approached, maybe 5 miles away.   The coax was biting me something fierce.  I hope those Polyphasers do their job.)

Another laughable inconsistency is the fact that the cabinet is not conductive.  It is fiberglass.  At the new remote base the cabinet will be made of stainless steel. Metal could prevent the charge from bypassing the protectors better than fiberglass. We just do the best we can given what we have.

 

 

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