Apparently, every other avionics technician on the planet knew this except me:

 

Electro-mechanical switches and relays that operate at extremely low current passing through the contacts develop an oxide layer on the face of the contacts. this causes high resistance accross the poles.

There is a possibility of rehabilitating the contacts  by passing the normal rated current through the contacts and cycling the switch frequently.

This is known as WETTING current.

 

Reports of Thrust Reverser sensing switches being treated this way.

 

Who Knew?

technican

Whetting, Wetting, Switch, relay, resistance,