Fascinating! Makes me rethink the need for secondary breakers. Note that one cutout was open from the beginning. Can anyone tell what the connection was? Wye-wye or delta-wye?
Also the German transformer burning over the horizon was very eerie I couldn't help but think of War of the Worlds - it even had strange birdish sounds after it stopped.
The utility does not normally fuse to protect the transformer or the secondary conductors. Their primary fuses are intended to protect the primary distribution circuit, that is to prevent an incident like this from affecting other customers. I watched a similar thing at a hardware store fire. The service conductors and the 4" rigid conduit was vaporized for about 30' on the side of the building. The cable was sparking and burning in the underground conduit and the secondary cables were smoking in the transformer vault when the utility got to the fire scene. The conductors were 500 kcmil copper and the service was a 400 amp 120/240 wild leg delta. The transformer was a 750 kva. The primary fuses never operated and the transformer was not damaged.
Don
I know what you mean stevenal.
When I first saw it in another website, it was labeled as a "grounding" problem and it was so long that I was still actually watching it when I posted the link, so by the time I figured out what was really happening I had already posted it. I suppose it still was a "grounding" issue in that one and eventually all of the phases were going to ground!
At first I thought the PoCo worker was extremely nonchalant about this, but I suppose he has seen it a lot more than the rest of us do and understood what was happening. I'll give him the benefit of the doubt that he was just being careful. Knee-jerk reactions to dangerous situations are often worse than just watching and waiting.
Although RUS (REA) warns against tying the wye point to the neutral in a wye delta transformer bank some utilities still tie the primary wye point to the neutral. Although it is not evidence but only a suggestion, the one fuse open is common on a wye/delta bank with the wye point tied to neutral. This causes a lot of blown fuses. The line crews become frustrated with the frequent fuse blowing and start to over-size the fuses.
Normal fusing is 250% of full load current. I have seen wye/delta banks fused at 1000% and more. Although available short circuit current may be in the range of 2000% to 4000%, in the real world, primary voltage drops and secondary impedances can easily limit the short circuit current to a value that the 1000% fuse will support for some time. It appeared from the arcs that a primary recloser somewhere upstream had disconnected and was reclosing avery several seconds. That would give the fuse some cool-down time and allow it to hold in longer.
Another factor, if the primary neutral is floating, when one fuse blows the other two transformers go in single phase series line to line. They now have an average of 87% of rated voltage. This will reduce the available short circuit current and allow the fuses to hold in longer.
short story. The fuses should be 250% and the short circuit current should be 2000% to 4000%, more than enough to blow the fuses.
Out on the pole (in the real world), the fuses may be rated higher and the short circuit current may be less so that the fuse don't clear.
respectfully
