"N_Cook" wrote in message
...
I can see the mechanism for dust ingress to switches , not so much
for relays. Switch-casings are not hermetically sealed and the internal
pressure, inside the kit, transfered to the switch innards is probably
negative with a spinning blade, and sucks in fine dust from the air
outside.

The following is not directed at Mr Cook, but at the way people use
fancy language when simple words will do. (I'm guilty of it myself, and
often have to edit my own writing.)

"I can see the mechanism for dust ingress to switches, not so much for
relays."

should be...

"I can see how dust gets into switches, but it isn't so clear for relays."