Take a look at high-voltage power lines, or peer into a neighborhood substation, and you’ll see uninsulated high-voltage lines attached to supports or components via non-conductive standoffs. Depending on the voltage involved, the standoffs may be several feet long to prevent arcing. Notably, these insulators appear to always be deeply ribbed, like these.
So why are they ribbed?
[sub]Please wait for a few legitimate responses before answering “for her pleasure.”[/sub]
Electrical current will pass along the surface. By adding the ribs it increases the distance the current would have to pass to reach the ground.
Why would a high-voltage arc want to pass along the surface, rather than taking the much shorter open-air path to ground?
Or are you talking about a specific situation in which the insulator’s surface has been wetted by rain/snow?
Her?! What about HIS pleasure?
Sorry, I just couldn’t resist! <runs away, laughing hysterically>
contamination on the surface. same reason that back when cars still had distributors in the ignition system, they could eventually suffer from “carbon tracking” on the inside of the distributor cap which would shunt current from the spark down to distributor body.
I’m sure I was told it was so that rainwater wouldn’t form a continuous covering on the insulator and thus create a conductive path. But that could be wrong.
Here you go:
Electricity doesn’t take the shortest path. It takes the path of least resistance. Air has a lot of resistance to it and only very high voltages can jump more than a few inches.
But any surface eventually gets contaminated by dirt film, salt, dust etc and that by itself or combined with water can create a path with less resistance.
Ribbing makes that path much longer in a smaller amount of space. it has the added benefit of somewhat shielding the inner surface from contaminants.
Even with ribbed insulators it’s sometimes difficult to keep insulators clean. In beach side communities the insulators have to be washed periodically as the salt air quickly contaminates them. If you happen to be near a power line on a quiet dark night you can sometimes hear the buzz or see the corona of small leakage current on contaminated high voltage insulators.
small insulators might be ribbed too if used for high voltages, electric fence insulators are made to have a long path.
It won’t if your assumption remains true, and if we substitute “lower resistance” for “shorter.”
The purpose of the ribbing is to keep the surface path higher resistance than the air path, even when it is wet and dirty.
In a vacuum environment, even a completely clean surface can have a much lower resistance than the through-space path. I work with 25kV electrodes in vacuum, and I have observed 4cm arcs along the surfaces of incredibly clean insulators, even when there is no direct arcing where the electrodes have only 2mm of vacuum separation.
If anyone has information about the onset threshold for surface currents in a high purity vacuum environment along a vacuum quality insulator like MACOR or PEEK, I would be much obliged.
Linky brokee. I fixee: Insulator (electricity) - Wikipedia
The original insulators weren’t only ribbed, they were sort of umbrella-like. It was discovered in the early years of telegraphy that when the poles holding up the wires got wet, it conducted electricity to the ground (actually, ions in the water covering the post did). But if you put up a glass or ceramic insulator with somewhat mushroom-shaped “caps” that overhung somewhat, there would be dry areas underneath the caps that prevented this flow.
Later it was realized that using such ribs alone, without the asymmetry of the mushroom shape, also helped because (as noted above) it lengthened the path along the surface. Even after they had good insulating coats on wires and breakdown was less likely, it could still happen if voltages got large enough. Breakdown preferentially happens along surfaces, not through the air or through the body of the insulators (I speak from experience here, having ha to scour out carbon scoring from breakdown paths along a non-ribbed surface where current arced along the surface of a plastic part).
You can see the same ribbed shape on parts made of glass (insulators for wires) ceramic (the same, but also spark plugs) and plastic (high voltage probes for oscilloscopes and the like).
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I’ve argued that it was the analogy between high voltage phenomena and the high energies associated with “ray guns” that lead to both the “Zap!” sound of ray guns and to the ribbed appearance they used to have in the 1940s and 1950s – How the Ray Gun Got Its Zap, indeed.
(You won’t see this on ray guns these days, unless they’re “retro”. Styles change. For some reason, a popular one lately is the Grossly Oversized Gun that looks like a Submarine Sandwich
)
