[Petrochemicals (OP)]

Why during torrential rain do power lines not arc ?

[evan_au]

The breakdown voltage of air is about 3MVolt/meter in dry air. This drops to a quarter of this in high humidity and/or rain.
- If the wires in your street are (say) 30cm apart, the breakdown voltage is almost 1 MVolt, well above the 415V AC that it is carrying
- Even in rain, the breakdown voltage is still a safe value
- The wires have to be kept well apart, as the wires sag in hot weather, and they sway in a strong wind. You need to ensure that the wires don't touch on a hot, windy day (especially in hurricane-strength winds).
- Some streets carry wires for both 415V AC and 11kV AC (usually higher up the pole). But the spacing is not greatly different

Perhaps the wires could be closer, but then you have problems with birds reaching from one wire to the other, or bats flying between the wires, with deadly consequences for the critter, and potential blackouts for the consumers...

I suspect that the main reason for this wire spacing is so that a lightning strike between phases will self-extinguish. Once an arc is struck, the insulating resistance of the plasma arc is far lower than un-ionized air.

[Bored chemist]

Part of the answer is insulators that look a bit like a stack of umbrellas.
https://steemit.com/science/@asbonclz/power-line-insulation-using-ceramic-electrical-insulators

[evan_au]

insulators that look a bit like a stack of umbrellas

When designing electrical circuitry exposed to high voltages, the two important criteria are "creepage" and "clearance".

Clearance is the distance through air, which has a fairly well defined voltage breakdown characteristic
Creepage is the distance over a surface. Dust, salt spray, and other crud can be deposited on surfaces, and conduct electricity (especially in a moist environment).
Minimum creepage distances are much greater than minimum clearance distances, which is why insulators have that convoluted surface path - maximum creepage distance in a small volume.
https://forum.mysensors.org/topic/4175/clearance-creepage-and-other-safety-aspects-in-mysensors-pcbs

When designing circuit boards to carry mains voltage (exposed to lightning transients), we often cut a slot in the board to increase the creepage, without wasting precious space on the PCB.

Conformal coatings are another way to increase creepage without increasing clearance. One circuit board draftsman was confused when I intentionally placed arc-over points in safe extra-low-voltage circuitry - he automatically assumed that arc-over is undesirable, so he put a conformal coating over it (despite instructions to the contrary).

Many years ago, I remember visiting a seaside blowhole, which several times per minute projects droplets of salty water high into the air.
- The old-fashioned analog AM radio was full of static crackles from powerline insulators with a surface coating of moist salt.

[paul cotter]

On a misty night visible corona discharge can often be seen on high voltage lines often accompanied by buzzing hissing sounds. This is usually restricted to the support insulators and connections in their vicinity.

[Petrochemicals (OP)]

The breakdown voltage of air is about 3MVolt/meter in dry air. This drops to a quarter of this in high humidity and/or rain.
.

Tall vehicles are warned to stay away from the power lines to prevent arcing , I would have thought that the rain in a torrent would have made the distance to earth similar.

[Bored chemist]

Fundamentally, there just isn't very much rain in rain.
If you wanted to halve the effective breakdown distance, you would need to have roughly half the space taken up by water.
But water is about a thousand times denser than air, so that would mean that the density of the rainstorm was roughly 500 times higher than that or ordinary air.

How could you get that much water up into the sky in the first place?

[vhfpmr]

A quick Google tells me that a heavy rainstorm is typically 300mm/hr, and terminal velocity 10m/s, so by my reckoning that means that space during the storm is about 99.9992% air and 0.0008% water.