I would love to understand why HV lines don’t carry the voltage required to break the resistance to earth in very heavy rain. Is rain water pure water with low conductivity? Or are the air gaps too much for the voltage?
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Pure water does not conduct, and rainwater is pretty clean. The other half of the equation is the design of the insulators holding the conductors. You’ve probably noticed that they are either belled or finned…in both cases it greatly increases the surface distance over the insulator that the electricity would have to travel, making a flashover much less likely.
Counter-intuitively, a flashover is more likely to occur in a heavy dew or drizzle where the moisture mixes with accumulated dust/dirt on the insulator. Dirty water does conduct. Heavy rains tend to wash the dust off.
The air gap itself (the linear distance from conductor to a grounded surface or other phase), has significantly higher insulation properties and rain doesn’t really compromise it that much, since even in a deluge the drops are relatively far apart.
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