So, like 6 years ago(was one of my first years being a tech) i had a call at a new client ive never been before, unit was not cooling. Roof access was in the electrical room on the 2nd floor of the building. After a short diagnostic, i Found a leak in the condenser so i repaired it. It was pretty hot outside. I did my leak test and put on vacuum pump on the system. Since it was hot inside the client office i tought they would be more comfortable with the blower running so i turned on the unit(while doing vacuum) It was the 1st time i worked on that machine, i didnt know if the compressor was good or not, didnt know for how long it was empty of refrigerant either. So i tought, i should try to bump the contactor of the compressor(still under vacuum) to see if its turning. I never knew what would happen, neither hvac school or collegues told me what could happen. The contactor instantly welded after i bumped it. The unit shorted out reeeeeaaalll bad, smoke started coming out of the roof access from the electrical room, then fire and sparks. Whole building power went out and the power lines from the pole melted and fell in the street! I was freaking out. I was stuck on the roof with possibly a fire inside. Firetruck got there like 15 mins after and i went down with the firetruck ladder. Lesson of the day. Never put power on a unit when under vacuum. Electricity has zero resistance inside compressor so it arks from winding to body and the ground was so direct it passed over breakers and melted everything.

Posted by Deleted (cd14679b) at 2021-03-12 02:34:00 UTC