More stories about high-power electrical stuff...

Mostly Cajun works with very large electrical systems. This technology is completely different from your normal house wiring. House-wiring rules do not apply here. Every so often, he writes up a story of an interesting client -- today's is a classic: bq. Somebody messed up� This is why we do some of the things we do. bq. 4:30 pm a couple of days ago. I�d made it out of the office and was happily contemplating a brisk workout in my recliner when my cellphone rang. I immediately looked at the display. It said �restricted� which meant it probably wasn�t going to be good news. I answered it and my suspicions were verified. It was a client. And it wasn�t a social call. It was an emergency. So I turned around and headed to his plant. Cajun sets up the story a bit, talks about the client's plant which runs on 13,800 volts (told 'ya it's not house wiring) and gets to the part that failed: bq. That brings us to the second problem: About a year ago the load tap changer mechanism was �refurbished". This means they replaced contacts that wear and erode during normal operation. However (there�s always a however in these sad stories) they did not do the prudent checkout after completion of the work to make sure that the mechanical work did not disturb the settings of those limits. bq. So problem one added to omission two and came up �disaster". The voltage coming into the substation went too high. The controller told the load tap changer to step down. When the tap changer mechanism got to its bottom limit, instead of its electrical drive motor stopping because it hit the limit, the mechanism jammed and the motor burned a fuse. Now the motor would not run. bq. So when the utility voltage got back down to normal, the output voltage of the transformer went waaaay low, actually down to 13300 volts instead of the 13800 we wanted. The client found out about this when he tried to start a large (and critical) motor. When the motor tried to start, it sucked the system voltage down even further, resulting in a few other motors dropping offline. That�s when I got the call. Interesting stuff -- how one small problem can cascade and cause all sorts of major issues...