I recently cooling checkups on 5 carrier RTUs.
I opened one to find a shredded fan belt and all the wiring in the blower compartment completely melted.
apparently the belt broke during the heating season and the limit failed to trip. the unit ran until the wiring shorted out. Both the 24v fuse was blown and the 50A breaker tripped.
I ended up replacing the limits on all 5 units. The units were 4 years old but this was the first season they had been used. The building had sat vacant for 3 years and there was signs of significant moisture buildup inside the units.
Anyone seen a limit fail closed before? I never have.