Two Out of Four Must Go!

For the most part, tower insulators are pretty rugged. I’ve been in this business for more than 35 years and have never seen a base insulator fail — or even crack … until recently.

Porcelain is a rugged material that is very durable in compression; it does not crush. That makes it an excellent material for use in high weight-bearing applications (such as under a tower). The same material is used for guy wire insulators in AM applications, with the guy cable rigged through the insulators so that they compress the porcelain material and don’t pull on it.

In guyed AM tower applications, a cone-shaped or cylindrical insulator is placed between the base pier and the tower base plate with the full weight of the tower compressing the material of the insulator.

Free-standing towers are a little different because they have both downward and upward moments, depending on the direction and velocity of the wind. As such, base insulators under the legs of a free-standing tower have to work in both directions, both supporting the weight of the tower and holding the tower leg to the foundation, depending on instantaneous conditions. That double duty requires a special design, usually a pair of porcelain insulators held captive in a steel or iron frame. The lower insulator supports the tower leg’s weight while the upper holds the tower leg down.

In my company, we have a number of “legacy” AM signals, and some of these employ free-standing towers with this type of dual insulator design. Some of these towers and insulators date back to the mid-1930s, a testament to their toughness and durability.

Base insulators have to operate out in the elements, providing isolation between the metal members above and the ground below in all kinds of conditions — wet, dry, dirty and clean. It’s almost unheard of for a properly-selected base insulator to arc over due to RF excitation or a combination of RF plus static electricity. Ball gaps are usually provided across the base insulator to provide a discharge path for static and lightning away from the porcelain material of the insulator.

We were wrapping up a tower painting project awhile back when at the end of a workday, our chief engineer noticed paint overspray on the porcelain surface of two of the base insulators under tower #3. He notified the tower crew and they wiped the insulators down with a wet rag.

The next morning was foggy and everything was soggy-wet. When our engineer arrived at the site early that day, he was greeted by one of the tower crew asking, “Are those base insulators supposed to be steaming?” Our engineer noted that indeed there were copious amounts of steam rising from two of the insulators, and from his vantage point in the parking area he saw what appeared to be a sustained plasma arc dancing over the surface of the nearest insulator.