I had a situation where I had 240 3ph voltage and needed 480 3ph voltage. I thought I could take a transformer that I had and back feed it or take a step down transformer and make it a step up transformer. I took a 480-208/120 transformer and installed it with the 240 volt side on the low side of the transformer. I then took all the taps and put these on the lowest side or tap 7 since the voltage was 30 volts higher than the transfromer was rated. I then check the voltage with a Simpson 260, the voltage was right at 490 volts on the primary side of the transfomer. The transformer spraked a small amount on the grounding lug of the case. It hummed fairly loud, I checked the voltage again and things seemed like they may be ok. Then the ground sparked and burned into. I had 2/3 w/Ground SDT cable coming from a 100 amp fused disconnect to the transformer. The transformer was a 300 Kva. Then the grouynd sparked in the 100 amp disconnect so I turned it off quickly. The ground wire which was a #6 had burned completly into. No fuses had blown. What happened here? I am at a loss. I have to find a proper transformer to run about 30 horsepower at 480 volts. ANy help would be great.