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#119389 - 12/24/04 12:06 PM 4" RMC vs Utility Current
iwire Offline
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Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
If you read this post
http://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
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Bob Badger
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Massachusetts
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#119390 - 12/24/04 01:42 PM Re: 4" RMC vs Utility Current
capt al Offline
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Registered: 06/20/04
Posts: 206
Loc: Norton, Ma USA
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.
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#119391 - 12/24/04 02:47 PM Re: 4" RMC vs Utility Current
mxslick Offline
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Registered: 10/24/04
Posts: 803
Loc: Atomic City, ID USA
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.
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#119392 - 12/24/04 06:11 PM Re: 4" RMC vs Utility Current
electure Offline


Member
Registered: 12/24/00
Posts: 4259
Loc: Fullerton, CA USA
Wow!
That burnt up like it was tinfoil.
Must have been spectacular.
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#119393 - 12/25/04 06:30 AM Re: 4" RMC vs Utility Current
:andy: Offline
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Registered: 10/18/03
Posts: 289
Loc: Germany
What caused the short circuit on these wires in the conduit/box?
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#119394 - 12/25/04 06:51 AM Re: 4" RMC vs Utility Current
iwire Offline
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Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
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.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#119395 - 12/25/04 08:08 AM Re: 4" RMC vs Utility Current
CTwireman Offline
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Registered: 02/07/02
Posts: 840
Loc: Connecticut, USA
Quote:
I am told the high voltage cut out did not open until the oil in the transformer was boiling and the primary's shorted.


Bob, I take it these are service conductors?

This looks like a perfect illustration of the reason that <1000 amp WYE services require GFPE!
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#119396 - 12/25/04 12:03 PM Re: 4" RMC vs Utility Current
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
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.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#119397 - 12/25/04 12:08 PM Re: 4" RMC vs Utility Current
mxslick Offline
Member
Registered: 10/24/04
Posts: 803
Loc: Atomic City, ID USA
Quote:
This looks like a perfect illustration of the reason that <1000 amp WYE services require GFPE!


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).....
Quote:
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.
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).]
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#119398 - 12/25/04 12:44 PM Re: 4" RMC vs Utility Current
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
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
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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