Now I can understand how they work when they’re oriented vertically: when it rains, there are many pockets of dry surface area to prevent a flashover.
But these things are also installed horizontally. When it is rainy & windy, presumably, the entire surface gets wet. Yet they don’t flashover. Why is that?
(Yea, I know pure water is an insulator. But I would assume dirt on the surface contaminates the water, and the water becomes at least somewhat conductive.)
I have wondered about this for years, and have asked a number of people in the industry about it. Have never received a good explanation.
Strain insulators are typically used outdoors in overhead wiring. In this environment they are exposed to rain and, in urban settings, pollution. As a practical matter, the shape of the insulator becomes critically important, since a wetted path from one cable to the other can create a low-resistance electrical path.
Strain insulators intended for horizontal mounting (often referred to as “dead ends”) therefore incorporate flanges to shed water, and strain insulators intended for vertical mounting (referred to as “suspension insulators”) are often bell-shaped.
Well its the best that can be done… In the rain there is a lot of popping at the insulators.
But the main idea is that, the rain coming in mostly from the top is spread out across a large surface area.
And the wind driven rain, that is hitting the left side of the disks, is tending to leave the right hand side dry
and the wind is being forced to flow parallel to the flanges, so that the wind tends to push the rain through, rather than it accumulating inside in a no-wind zone…
The insulator is of course made with a hydrophobic surface, so that it doesn’t hold a film of water, the water forms drops.
Side note, a handy rule of thumb for determining the voltage of the wires is roughly one insulator per 12kV. It’s not quite linear, with more insulators at lower voltages and fewer at higher voltages, but it’s a good way to approximate.
Anyway, I recall reading that the operators of the Chicago ‘L’ can tell when rain was starting because they’d see an increase in current leakage. The sort of dust that accumulates on the third rail and insulators is highly conductive metal and carbon from the pickup shoes, brakes, and the third rail itself. When it’s dry there’s no pathway for current flow, but when it gets wet it creates a conductive slurry. It doesn’t take long to wash off however, and the current leakage drops when the hydrophobic properties of the insulator take over. Seems like something similar could happen in the overhead grid but to a lesser extent since the normal dust in the air isn’t as conductive, nor as concentrated.
