Electrical Vault Fire

[SKatzer]

Has anyone seen or can explain the following: Allegedly wiring from a high voltage electrical feed installed under a street overheated and allegedly caught fire. The wiring is approximately 60 years old and essentially has a paper based insulation encased in a lead conduit.

Allegedly the overheating of the wiring caused enough pressure to build up in the underground area to blow 150 lb manhole covers approximately 20 feet in the air. Supposedly, methane and natural gas are ruled out. I am not familiar with the old wiring and was wondering if any combustible type materials were used on wiring some 60 years ago. Someone told me the paper based insulation was soaked in oil back then but I have not heard of this. Any help is appreciated.

Thanks

 

[charlie]

We have a lot of that type of cable, both medium voltage and low voltage. The cable you describe is PILC, paper insulated, lead covered. It is paper that is soaked in oil and is very good. in fact, we still use it since our duct lines and man holes in the downtown area are close to steam lines and experience high temperatures. We have PILC that has been in service for almost 100 years and it is still in good shape.

The explosion was due to the high amount of fault current since, I assume, the fault was in a networked area in your downtown. The available fault current is so high because of low impedance transformers and multiple transformers connected together in a network. The gasses that raised the man hole covers are the result of large amounts of vaporized copper and lead that have expanded many more times than steam and are more lethal.

by the way, welcome to the forum.

 

[SKatzer]

Thanks for your input. Let me ask what is the reasoning behind soaking the paper in oil and is it a particular type of oil that was used?
Also, you mentioned vaporized copper and lead that expanded. After the manhole cover explosion, there was another explosion in an electrical room about 200 feet away. The conduits run underground through the same vault and into the electric room so I can see how the smoke can get into this area. What I was surprised about is that there was no fire in the room only damage from the blast. All other wiring in the room looks fine. Any thoughts?