What is it that causes the large transformers on wood power line poles to ignite and explode. Late last evening, a transformer caught fire, lighting afire the top 5 feet of a pole. There were a total of 8 large explosions, the 7th being the largest. Just prior to the big one, that piece of equipment went full blown glowing white cook off. It blew and lit things up for who knows how far. The Fire Department kept their distance. After the blow it was no longer on the pole. It was followed by a 8th explosion which was smaller and may have been a flood light below. I am told there is something like 15 gallons of oil inside them. Why do they cook off and blow up like this ?

We had one blow on Friday night. The light that came off it was white and made it look like day time. A few seconds later it exploded. This has happened several times in the 26 years we have lived in this house. In our case, it has always been the same thing, A Blue Heron. It has a very long wing span and when landing on the poles its wings touch both live wires causing a short.

Good question. We had one go off one night many years ago close to our bedroom, probably 40 yards away. It was so bright and loud that both my wife and I bolted upright in bed. No lightening involved, it just cooked off.

Jim

Yes, there is oil or kerosene in them that act's as a heat sink, if the transformer shorts out and sparks, it can ignite the kerosene and explode......one did this on the pole right behind my house a few years back.

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Originally posted by jimmy123x:
Yes, there is oil or kerosene in them that act's as a heat sink, if the transformer shorts out and sparks, it can ignite the kerosene and explode......one did this on the pole right behind my house a few years back.

It’s a non conductive oil. I don’t think you’ll find too many with kerosene in them.

I'm sure we have someone here from a utility company that can explain far better than I, but they can light off for a number of reasons. The ones we most commonly see are a broken conductor causing arcing, low oil, overloading, the occasional lightning strike etc... Most if not all of the explosions you see is arc flashing, not actual explosions like you'd normally think of with flammable liquids, etc.

And the transformers are filled with mineral oil, not kerosene.

We just let them burn because attempting to fight the fire would just make it worse. Either we'd get electrocuted in the process, or since oil floats on water, we'd cause the burning mineral oil to create a river of fire running down the road. It's rare to have an exposure problem with a burning pole, so we just let it burn itself out. And if there is an exposure problem we treat the exposure, not the burning pole/transformer.

Transformers contain mineral oil as a heat transfer fluid. When the oil level gets too low, the transformer will overheat and the oil will reach its flash point then Boom!

That's why many transformers prior to 1978 were filled with PCBs. Much more flame resistant.

Funny story. During a blizzard I was playing in the front yard with my ex and her son. I tell the ex, watch the lights are going to turn off on the house. She asked how I knew that. I told her once you've heard a transformer blow up you recognize the sound. Her son comes running over, "Oh, my God! I knew it! Transformers are for real! I laughed real good at that, then explained it was a different kind of transformer.

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Originally posted by David Lee:
What is it that causes the large transformers on wood power line poles to ignite and explode.

Sudden massive imbalances in the distribution grid causes sudden overheating that substantially exceeds their design criteria or overcomes their internal dialectic properties--which results in internal arcing.

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Originally posted by David Lee:
I am told there is something like 15 gallons of oil inside them. Why do they cook off and blow up like this ?

There is oil in them indeed. It's used to conduct the heat they generate to the outside to cool them. But when the transformer suddenly massively overheats or arcs internally, as above, the oil boils off or flashes to vapour, internal pressure raises too high for the container to, uh... contain, and *boom*.

Think of them as kind of like big electrical firecrackers

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Originally posted by ryan81986:
I'm sure we have someone here from a utility company that can explain far better than I, but they can light off for a number of reasons. The ones we most commonly see are a broken conductor causing arcing, low oil, overloading, the occasional lightning strike etc... Most if not all of the explosions you see is arc flashing, not actual explosions like you'd normally think of with flammable liquids, etc.

And the transformers are filled with mineral oil, not kerosene.

We just let them burn because attempting to fight the fire would just make it worse. Either we'd get electrocuted in the process, or since oil floats on water, we'd cause the burning mineral oil to create a river of fire running down the road. It's rare to have an exposure problem with a burning pole, so we just let it burn itself out. And if there is an exposure problem we treat the exposure, not the burning pole/transformer.

Thank you. That 8th boom went off right over top the utility truck. I thought with all the rain, the pole would go out, not so. The utility truck guy went up and was spraying the pole and put the flames out. AFAIK, the transformer wasn't needed as the 3 lines are now attached to insulators. You can see right through the top of the pole.

A fault (electrical connection between one or more of the 3 lines (phases) or from a phase to ground) will cause a transformer to explode if the protection circuits don't detect the fault and open the circuit breaker protecting those lines in time. External faults are the result of tree limbs or critters connecting two lines. Usually an insulation failure causes an internal fault. Also if the cooling oil leaks / boils off and gets too low, the insulation on the wiring inside the transformer will melt and an arc will form, conducting electricity, causing an internal fault.

There's a lot of energy running through even a pole mounted distribution transformer. The utility I worked for had one for each suburban block, stepping the voltage down from 7000 volts to 220 or 110. I saw one vaporize itself one time.

To the OP's question about the multiple explosions, capacitors are used to smooth out and control the line voltages. There are sometimes a small array of them near the transformer. If they get overloaded, they will fail in a pretty dramatic way as well.

I forget the voltages but the lower the wire on the distribution pole the lower the voltage. Bottom cables feeding transformers are 7500 volts if memory is correct.

normal cause when lots of them go is a huge voltage spike. like lightning. They don't have kerosene or the like but a mineral oil (since PCB's were outlawed). It's rare but sometimes they get hot enough the mineral oil burns, but that's not usual. As ryan81986 says the normal course of events is to just let whatever is burning burn. and if necessary deal with any secondary effects (also rare). look and sound spectacular but very benign in the overall scheme of things.