I just wanted to share a crazy experience that happened the other day. Without going into all of the details of the job, I’ll do my best to keep it simple. I am the foreman of the crew. We were reconfiguring the three phase feed on an existing line. We checked rotation on a couple of open banks before de-energizing the existing feed so we would be able to verify correct rotation once the job was complete. We couldn’t open main disconnects at the customers business, so to cover our ass’s we isolated the secondary side by removing the buss from the transformers hot legs on the open wye-delta banks and left the neutral connected.
Our plan was to verify rotation on the open banks before re-connecting. Well, the journeyman and apprentice went up to check rotation on the transformer bank before re-connecting the service. After the rotation meter wouldn’t work, they decided to check voltage, and immediately blew up my secondary volt meter. They had over 1000 volts! Thank god they followed the rules and were wearing their rubber gloves!! They then decided to check voltage on each individual transformer (which was showed good voltage 120/240) but when they went from tub to tub they were getting crazy high voltage 800 volts.
How could each individual transformer test good voltage but when going tub to tub have such high voltage? It took me a minute to realize that in my 17 years, I had never tested a bank that wasn’t already bussed out. I asked the guys to disconnect the service wire and re-connect the buss work between the transformers.
Sure enough, connecting the transformers together brought the voltage back to what was expected 120/240/208, and we were able to re-connect. I never knew that if you measured voltage between the unconnected bushings that you would get high voltage, and I’m still confused on how it would be that high with 120/240 transformers.
The big take away from this is the importance of following the safety rules. It would have been easy to assume 120/240 expected voltage on the re-connect. I know a lot of lineman that wouldn’t have worn rubber gloves to check that secondary. Thankfully, my guys didn’t assume anything and were in their rubbers when the over-voltage occurred. It just goes to show how having good safety habits and following procedure can save your ass. Never make assumptions, and always listen to your gut. Complacency kills!