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

If you read this post
https://www.electrical-contractor.net/ubb/Forum1/HTML/005239.html

you know the company I work for has a large super market chain for a customer.

Here is another of their locations that had an outage.

I was not involved with this one, I don't know where I was while this was being repaired.













I am told the high voltage cut out did not open until the oil in the transformer was boiling and the primary's shorted.

Some of the guys on this repair got to see a security tape of the sparks blasting out of the pipe until the power failed.

Bob

What a long day that repair turned out to be. The temp generator was installed on Sunday. On Monday we started at 6:00 am and left for home at midnight with the store back on normal power.
The security tape iwire talks about was incredible. The sparks shot 75 feet from the conduits out to the street. The tape lasted 1 1/2 minutes before the power went out.

Wow!! Incredible pics!! I sure wouldn't have wanted to be in those houses across the street! (On the other hand, it would've made for an exciting light show.)

Luckily that padmount shorted and took the HV fuses before it exploded too. What a mess that would have been on top of this.

Any info on what caused the initial fault?

And any chance that video can be encoded and put online? Might make for a good safety discussion.

Wow!
That burnt up like it was tinfoil.
Must have been spectacular.

What caused the short circuit on these wires in the conduit/box?

We do not know the cause here, once our guys get there the cause is unimportant, they just start ripping the old out and replacing it.

The utility had to change the pad mount and our guys had to replace the conduit and conductors.

The conductors go a bit inside as well the electric room is the penthouse you can see in the photos, the conduits go in to the bottom of the gear in the penthouse.

Bob, I take it these are service conductors?

This looks like a perfect illustration of the reason that <1000 amp WYE services require GFPE!

Yes these where service conductors and yes once inside they hit a 1,600 amp main with GFP protection.

I think it is a good illustration of why the service disconnect is required to be located "nearest the point of entrance"

In this case there is I would guess 10' to 15' of service conductors inside before they reach the service disconnect.

It is a good thing the fault did not happen on the inside portion.

This service is 480Y/277 1,600 amp fed with 4 sets of 600 kcmil CU in 4" RMC.

In this case, wouldn't the GFPE be located in the building's switchgear? Since these appear to be the service conductors the GFPE wouldn't help. [Edit: Iwire's post went in as I was typing this ]

Something on the idea of a cable limiter that utilities use would do the trick. It would have been located at or in the padmount.

Originally I was going to remark that better fuse coordination by the utility would have limited the damage, but it's possible that depending on the primary connection the fault current seen by the primary wasn't high enough to open the fuses at the start of this fault. (Thinking delta/Wye).....
Please do not take this the wrong way, I mean no offence here, but if something like this happened to any installation or repair I was involved with, I would certainly want to know what caused it!

[This message has been edited by mxslick (edited 12-25-2004).]

[This message has been edited by mxslick (edited 12-25-2004).]

No offense taken.

The customer does not care what happened at first, they average about $1,000,000 per week gross at each location. That is a little under $150,000 per day. What they do care about is the power is restored immediately.

Once they are back on line then they want to know what happened, then any clues are in the dumpster or scrap yard.

Yes the generator we bring in allows them to open for business. The generator burns 75 to 100 gallons per hour or about $3,600 per day plus rental costs.

In a case as bad as this it really does not matter what happened all the damaged equipment is replaced.

As far as power company fuse coordination it is not possible to fuse the primary side of the transformer in a way that will protect one set of parallel conductors.

Individual cable limiters would be an option I guess, but I have never seen that done.

Bob

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.

Wow!,
This looks like a big job.
Just a silly little question, were the wires in the 3 conduits above the one that blew out, checked or tested to make sure that they weren't damaged from the heat?.

Hi Mike



All wires replaced.

The switchgear was checked by a manufacturers (GE) technician.

Bob

[This message has been edited by iwire (edited 12-27-2004).]

Mike, not a silly question at all. As Bob said all cables were replaced. Yes it was a big job. We started out with five guys at 6AM. Brought in 2 more at 10AM, and 3 more at 4PM. We all left at midnight with everything packed up. Except for the painters coming the next day you would never have known we were there.

I'll take a stab at the cause: Insulation damage on the conductors in the bottom conduit where they emerge through the bushing.

It looks like the bonding-bushing lugss on the other three conduits could be pressed into their respective conductors, too.

I try to "pre-bend" wires to keep them centered in, or at least not jammed against, the conduits from which they emerge.

I say it was the bullet hole in the conduit in picture #3. What ever blew it was vaporized early in sceme of things.

Bump...

Now THAT is a real eye opener to what can happen !!!That would have been something pretty spectacular to see... That would put some fireworks displays to shame!

Some years ago when I used to work at the supplier a building down the road had its main underground service short out and it also wiped out the padmount transformer ( Service was 400A 120/240V 1 phase, that consisted of 2 ducts containing 3/0 TWH in paralell)... One of the ducts shorted and burned the conductors bare from the transformer to the fault... Turns out when they were pulling out the damaged conductors, a chunk of broken fish tape was drawn out right where the fault appeared to have originated, and had been tangled in there since day one.. About 15 years if not a bit longer..

A.D

Bob, Could I have your permission to use your photo. Of course I would give credit.

DHester:
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