My carrier SS furnace blows the inline @<hidden> fuse to the limit switch and then blows cold air continuosly . Is my low voltage supply faulty or do I have a wire grounding out somewhere.

When does the fuse blow? Immediately after turning on power to the unit, or after some operation is initiated? Try disconnecting all thermostat wires from the circuit board to determine if the short is within the furnace or elsewhere.

The fuse blew after several hours of operation. It was in the middle of the night. I replaced the fuse and now I have no power to the thermostat.

Is it a digital thermostat, and the display isn't coming up? If you have a voltmeter, did you check for 24v from the "R" circuit wire to any of the other wires?

No it is a regular not digital thermostat but I changed it anyway and still have the problem of blowing the fuse. I have now tested all the wiring and the thermostat and am wondering about the circuit board with the 2 relays built in. Could a relay with bad points throw enough of a amperage fluctuation to blow the fuse. The circuit is back working with a new fuse so tonight will see if it continues to be a problem with the new thermostat.