Today, I was working on a carrier Heat Pump. The unit was doing nothing and I suspected a fuse. I pulled the fuse and it was so hot I couldn't touch it but it tested good.
What I found was a neutral wire that had come undone. On these units, the neutral wire connects directly to one terminal of a combination fan/compressor start capacitor.
It seems that if the neutral was broken, and the ground started to carry the load, it would still not heat up the fuse.
20A fuse and the unit once fixed only drew 13A. What happened here?