Ode to a Fried Egg Insulator

Lightning is a perpetual risk. As our towers get higher, the antennas larger and the footprint of the antenna farm expands, the risk increases. When you have a big station you will be hit by lightning. Don't build a large station unless you are prepared to face the risk.

Lightning does not negotiate. Attempts to appease it with exemplary grounding practices, protection devices and emergency disconnects don't keep it away. All you can do is minimize the damage. Injury and property damage can be largely avoided when you follow best practices.

There is more that I can and should do in my station, steps that I've deferred to the priority of putting up towers and antennas. Now that I've been struck by lightning twice, each time to the Beverage system, it has percolated up my priority list. With summer largely in the rear view mirror I have time to plan and execute before next year's storm season.

You are probably familiar with the myth that lightning is attracted to high points. It isn't. There isn't anywhere near enough charge in a tower to equal that in the atmosphere. The real reservoir is in the ground. It is only when the cloud and the ground have sufficient electrical potential to bridge the large air gap that our towers may come into play. 

The height of an amateur radio tower is almost negligible in comparison to the distance between cloud and ground. Its role is secondary. If a conducting structure is within the "zone" of a nascent strike a primary or secondary channel will take advantage its presence to sate its appetite, travelling through it and other attached conductors to better reach the charge reservoir in the ground.

When lightning strikes a tower there are many paths to ground. It can, of course, travel down the tower to the ground. In a direct strike the ground rod is almost an afterthought since lightning will bridge the gap through the air as well for a channel large enough to carry up to 100,000 amps. Many of our protections are really only sufficient for secondary strikes and nearby strikes that induce high currents in the tower and other conductors.

In addition to the tower itself, lightning will recruit the coax and cables on the tower and, for a guyed tower, the guys and their anchors. The anchors are natural ground rods, and pretty low impedance ones at that. But what of the insulators we use to break our guys into non-resonant lengths? That brings me to the crux of this article.

The adjacent photo is a ceramic insulator on a guy cable for my 150' tower. Pre-form guy grips thread through the insulator. There is no DC electrical connection between the sections of 5/16" EHS. The capacitance between the pre-forms is no more than a few picofarads, which is an effective barrier at HF frequencies.

I use a mix of 502 and 504 insulators on my guys. Although they are designed for utility service, they do well in this application. But there is only so much they can do in the event of direct lightning strike. While I haven't checked the specs, I expect that they can withstand at least 20 kV, assuming they are in good condition and not coated in dust and pollutants. They are no match for a direct strike.

The 502 insulator at right used to be on the guy of a tall tower at a friend's station. The tower suffered a direct strike years ago and the guy cables conducted a substantial amount of the current. Some of the insulators shattered and the tower only kept upright when the now slack pre-forms made direct contact.

Notice how the lightning flashed around the insulator to travel between guy segments. The flash heating cracked and charred the surface. Even that wasn't enough so it punched through the insulator to reach the next guy segment.

Let's look a little closer.

Impressive, isn't it? Apart from the extensive lightning damage you can also see an ordinary abrasion mark of the pre-form on the insulator (top centre). That can happen even when the pre-forms are installed correctly since there will always be some guy motion that will scrape material over the years. This is a good reason to exceed the minimum requirements for guy components.

He had to replace all the guys on that tower. It couldn't have been a fun job but it can be done while keeping the tower standing. Knowing his background I am certain he did it properly. That, however, is beyond the scope of this article. I know how it's done but I've never done it, and I hope I never have to!

Lightning is intensely powerful. For most hams and their smaller antenna systems the greater risk is by induction from nearby strikes and secondary channels: lightning is an RF phenomenon. There are usually more enticing paths to ground in the area. That's why you should never stand under a tree during an electrical storm.

Use lightning protection measures but never deceive yourself of lightning's power and reach. Good protection against direct strikes is possible but absolute protection may be out of reach of ham budgets. Do it well, follow the regulations and make sure that you're insured.