ECN Forum Forums Photo Discussion Forums Photos for General Discussion 4" RMC vs Utility Current

Sorry that I wasn't clear. What I mean is, imagine this fault occuring on the load side of the disconnect, without GFPE.

At a fire a few years ago, I watched as 30' of 4" rigid and 3 500kcmil cables vaporized. This was on a 120/240 volt single phase servive. There was much less of the conduit left than what is shown in Bob's pictures. It was a great fireworks show for about 25 minutes. The 750 kva transformer in an underground vault got hot and the cables at the transformer were smoking, but the primary fuses never opened and the transformer was not damaged.
Don

Bob:

Thanks for the info. With those grosses, I'd feel the same way! And I'm sure the genny costs seem like nothing against the potential losses.

Interesting that it is true the idea of investigating the cause has to take a back seat to restoration [not trying to be smarta**]of service. I have always had a facination and curiousity of all things electrical and if I had the necessary background would like to start an "investigation" service to pinpoint causes of incidents like this one. I wonder if such a company exists?

And as for cable limiters, I wonder if they'd work on paralled conductors?

CTwireman: Excellent point. Now who would want to be the one to open the service disconnect with this kind of fault happening? 480y/277 @ 1,600 amp rated...As a rough stab I'd guesstimate the fault current to be in excess of 50,000 amps that close to the transformer!

Happy Holidays gentlemen!!

Tony

Tony,
I've never installed cable limiters, but I thought that their main use was on both ends of all of the parallel conductors.
Don

WOW! Bob how much of that is going to be saved? I imagine that the buss work in the gear those conductors were parraleled to may have got pretty hot too. As to the cause, I would be checking EVERYTHING before it went back on line.

I'd love to see that security video....

Don, cable limiters are required to be installed on three or more sets of service conductors on our downtown network . We install them on our end of the service lateral and the electrical contractor is required to install them on the load end of the service lateral. The interesting thing is that we do require them to be installed where the service is off the downtown network. The situation that Bob has would not have been protected by cable limiters unless the customer wanted them.

The reason we require cable limiters on our downtown network is the loss of a building where one set of several parallel sets of conductors faulted. The current from the remaining parallel sets fed the fault by going through the service bus and back to the fault. This burnt up the service equipment and the building as well. We got to pay for part of the building since we didn’t do anything to prevent this situation from happening. I suspect we will start requiring cable limiters on outlying services as soon as we have to buy a portion of one.

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Charlie Eldridge, Indianapolis, Utility Power Guy

http://www.bussmann.com/library/bifs/1042.pdf

Question--
Why is 3 conductors per phase the magic number?

Redsy, you ask a good question and I would like to know the answer also. I know that we have the same requirement.

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Charlie Eldridge, Indianapolis, Utility Power Guy

How does the customer know if a cable has faulted, and the cable limiters for it have opened? If there are say 2 other cables that are still in service supplying the load. Everything would look to be normal, but for one phase only has 2 functioning cables vs 3 functioning cables for the other two phases. You may have eddy currents if there are three conduits, each with 3 phase cables, and one of the conduits has a dead phase. How would the customer know that there is a problem that needs to be fixed? If he chooses to ignore it, well that's another issue....

We have a team that checks for blown cable limiters on our network to see if we have had any fail. We have not had any service laterals fail because of lost cable limiters.