Hey guys. So have this weird problem. Any auto electricians on here?

Last Saturday my alternator stopped charging my battery. So on Wednesday I pulled it out of the car and dropped it off at a local auto electrics place. Had the choice of a new one for $430 or to get mine reconditioned for $250. He said he couldn't guarantee it would work afterwards (don't know why?) Anyways. I picked it up on Thursday and installed it that night. All seemed well. Car off battery read 12.99v which is normal, car on 14.4v which is normal and with high beams on it was 13.4v which seems normal. Took it for a drive around the block and all was fine. Not the next morning I drove to work. Around 20 minutes in the car died at a set of lights. Completely dead had no ignition or anything. Thought it was just a loose connection to the negative post mount since I had the battery disconnected. But after further investigating I saw the Battery fuse in the fuse box was blown. Luckily I broke down opposite auto barn. So I went and got a couple spare fuses. But they blew the second contact was made. So I had the car towed home. That night pulled the alternator back out of the car to test if it was an installation error or the alternator causing the short (Car and I had a nice date for valentines). Testing the cable it all seemed fine. And as soon as I touch the main cable to the alternator that's were the short happens at the fuse. I measured the ohm resistance through the alternator positive to ground and to my understanding it should read 0.L? but it just reads 0.

So what would be a likely cause for an alternator to short? Faulty parts or dodgy installation? And what has me confused is why would it work fine for 20 minutes and then all of a sudden start shorting out? Any insight on this would be greatly appreciated.

Auto electrician is shut on weekends but I'm hoping after all the trouble and hassle this has caused me he will have the decency to fix the alternator since it's a new problem caused by him and not just say the rebuild wasn't guaranteed. As it's my understanding $250 is on the pricier side of a rebuild.