Rain Protection for Substations

Could it be anything to do with their ingress protection rating?

All Sub Station and transformer are design for either indoor or outdoor use. For transformer that is located for outdoor will have special design to withstand the weather condition. It is not necessary for rain protection.

Believe it or not, rain water does not conduct electrical current as water from your tap does(too pure). Contamination with a little water is the problem. When rain first hits the insulators there is some tracking, but this soon dissipates once the rain continues to flow, washing away contaminates. Lower voltage panels do not have the luxury of having the exposure to rain at a frequent cycle, therefore they are dirty and when a little water falls on them they short out.