Arrived to no low voltage at the thermostat. Found a failed low voltage transformer. Replaced the transformer and added an inline 3 amp fuse. Fuse blew immediately and the technician began to troubleshoot the short. Found the pressure switch in the outdoor unit had rubbed on the side of the casing and the copper had been exposed. Reinsulated the wire and performed function check. Systemnrunning normally in cooling and in heating upon departure.