Transformer Tank Grounds.
A few years ago a potential fatal accident could have happened. Just lucky I guess. We built a single phase 7.2 kv primary extension with 25 kva tran on d.e. pole. We built the line as always and then hung the tran. Made all connection on pot and was ready to check voltage. The line was heated up and voltage checked. Voltage checked ok and job was completed. Well a few weeks later we had a no light call on this same customer. Trouble man went out to check out call and found a fuse down on tran. Trouble man used extendo stick to close up and got power back on. He didn’t bother to check voltage, just figured it was squirrel, bird, or who knows what. Sometimes you never find cause. Trouble man left. Well about two or three weeks, maybe a month later we had another trouble call same customer. Different trouble man this time. He got to location and started checking out call. The first thing he found was the fuse blown on tran and the pole was smoking. It was smoking at the bottom transformer bolt. The pole had been on fire but had gone out. There was a big hole in pole around tran bolt. After checking out what might have caused problem trouble man found that the case ground of transformer had not been made up. It was still rolled up and had never been connected. The transformer strap ground (strap from neutral bushing to case) was also gone. It didn’t have one for some unknown reason. Trouble man made tank grounds up and problem was solved. Hope everyone knows that the case of a single bushing tran. is the other end on the primary winding. Other words the case of tran had 7.2 volts on it ever since we hung the tran. Guess the new pole had enough ground to carry the load on pot for a while. Plus the customer had driven ground at house to carry house.