CMH ballast keeps blowing fuses

I opened my tent the other day and one of my two 315w CMH were out. I took a look outside the tent and the ballast was off.
I checked the fuse and it was blown. I managed to find the same fuses locally so I put in and plugged into the surge bar and it blew right away again.
So I took it into my garage and out onto a different circuit that wasn't being used for anything. It blew immediately again.
Any idea what would cause this?
I bought both of these CMH at the same time about 3-4 months ago. I've contacted the place I bought them to hopefully get it replaced under warranty.

Most likely the ballast has a short in it. It's possible that a connection wasn't mated enough and has moved slightly and now is making contact with a bonded part. Usually you can open up the device and look for signs of electrical faults which come with carbon or splatter. If it hasn't destroyed something you could clean up the connection and re-seed it.

Thanks. I contacted my supplier and they are going to send out a replacement. I don't want to open this up yet...in case they want it back to send to their distributor for warranty.
But if they tell me to toss it...I might open it up just to see what happened

Just in case the short is in the cord you might try a different power cord on it, or if you have a multimeter you could save a fuse and just check that there is no short between any of the conductors.