*** Transformer burning up! Please help ***

To get right into it.The transformer burned out in the furnace.I called a service company out.They replaced the transformer and instantly it burnt out he tested the system and everything checked out.He called his boss and said it sounded like a neutral problem in my electrical somewhere,either in the box or out at the road.AEP electrical supply company said everything from the house out to the street check out ok.So called a another HVAC guy.I really liked this guy,Honest and upfront.He thought what the other company said was crazy.He installed another transformer and it burned up quickly.He bypassed the outside air unit and tried again and again burned up(5 Transformers!!!).He did a through check of everything as far as i could tell.The crazy thing is,He always has an inline fuse and it never reads over 0.8 amps,BUt it still burns up!I feel this guy is good and has been doing this 22 years and said hes never ran into this situation!The only thing that i have done recently is change out a light fixture,and the wiring was oddly set up,wires split off and went up stairs and it did take me a while to get it wired correctly but when i did the light worked fine.2 days later i get a call saying the heat was out.Could the wiring have anything to do with the transformer burning up?I do know that the Furnace is on its own breaker.Please any suggestions or any thoughts would be great full.Its like 40 degrees in the house and its a rental!