I replaced a 600 watt transformer. 2 lines-3- 50watt bulbs on each line not over loaded. Does a bad bulb ever cause a dead short.Did you overload the transformers? How many watts are they rated for and how much load did you place on them? Never exceed 80% of the transformer capacity in bulb wattage or you could be asking for trouble. Remember, you also lose "wattage" in wire resistance, so if you are maxing out the transformers you are probably way over capacity which would cause a thermal overload and in turn make the internal fuse or breaker to trip.