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 Post subject: Getting Xfmr Oil Samples
PostPosted: Tue Dec 01, 2009 2:50 pm 
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We have two 15KV-480V cabinet xfmrs where the analysis determined the FPB to be 21FT and the IE to be 95Cal/cm2 @ 1-1/2Ft from the xfmr secondary. The oil sample tap is located about 2ft below the transformer secondary bushings. Future plan is to have this tap routed to the outside of the enclosure.

Is there any safe way to get oil samples from the transformer other than shutting the power off?

Thanks


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PostPosted: Tue Dec 01, 2009 3:12 pm 
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PAult wrote:
We have two 15KV-480V cabinet xfmrs where the analysis determined the FPB to be 21FT and the IE to be 95Cal/cm2 @ 1-1/2Ft from the xfmr secondary. The oil sample tap is located about 2ft below the transformer secondary bushings. Future plan is to have this tap routed to the outside of the enclosure.

Is there any safe way to get oil samples from the transformer other than shutting the power off?

Thanks


When I was doing these as a NETA tech I found this the hardest thing to deal with when the 2000 70E came out. Shut it down until you get the external tap installed. There are several companies with kits to do the external samples.

Funny how no one ever considered the arc flash hazard of this task the last 100 years.


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PostPosted: Wed Dec 02, 2009 8:46 am 
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How does oil sampling qualify as an interaction like to cause an arc at the bushings two feet away?


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PostPosted: Wed Dec 02, 2009 9:23 am 
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I would say that this is similar to "Perform infrared thermography and other non-contact inspections outside the Restricted Approach Boundary" for 600V switchboards in Table 130.7(C)(9) which has HRC 1. This implies that it is an interaction that might cause an arc flash hazard.


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PostPosted: Wed Dec 02, 2009 11:03 am 
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stevenal wrote:
How does oil sampling qualify as an interaction like to cause an arc at the bushings two feet away?


Oh yes, many times you need to wrestle the cables around to get your sampling equipment hooked up.


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PostPosted: Thu Dec 03, 2009 7:36 am 
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jghrist wrote:
I would say that this is similar to "Perform infrared thermography and other non-contact inspections outside the Restricted Approach Boundary" for 600V switchboards in Table 130.7(C)(9) which has HRC 1. This implies that it is an interaction that might cause an arc flash hazard.


Our electricans are trained to wear the proper Arc-flash PPE once the FPB is crossed and there is exposed energized conductors. For these xfmrs it would require them to wear 100Cal/cm2 clothing once the door was opened which is not advisable. If I used the table wouldn't I be ignoring the Hazard?

How are others doing this task?

Thanks for all the comments


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PostPosted: Thu Dec 03, 2009 8:45 am 
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Once you have done the calcs, you can't or should not back up and use the tables since you would be ignoring the hazard as you say. Why not schedule the sample when you can get some down time on the unit?

Is the 21 ft and 95 cal a result of available current, or clearing time?

If it is available fault duty, the blast just might move them out of the boundary before anything clears.


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PostPosted: Thu Dec 03, 2009 2:21 pm 
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acobb wrote:
Is the 21 ft and 95 cal a result of available current, or clearing time?


This is a result of clearing time. Clearing time is limited at 2.0Sec, Available fault is about 40KA on one of them. One thing I did notice is that the SKM software is using 25mm as the gap. I think the actual gap is closer to 6". I will see what this looks like with a larger gap.


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PostPosted: Fri Dec 04, 2009 1:15 pm 
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The 25 mm gap is the default gap for low voltage distribution equipment. The gap for your case presents an interesting question that presently does not have a good answer.

I was in the lab performing arc flash testing of a pad mounted transformer. The bushings / stabs were about 9 to 10 inches apart (diagonally) and the test was on the 480 volt side with 30 kA of fault current. I could initiate the arc flash with a trigger wire (18 guage) but the arc would not sustain itself across such a large gap.

Larger current or higher voltage may provide a different result but unfortunately, there is presently not enough data to support any kind of a conclusion so we are left using the existing information that we have.

Everytime we think we figure something out, we find a video or some other data that rasies more questions. As usual, stay tuned and I'll post any new developments here.

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Brainfiller.com


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PostPosted: Mon Dec 07, 2009 8:06 am 
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I ran the calc's on this xfmr with different gaps using SKM software. Here is the info:

Gap 25mm, IE 94 Cal/cm2 @ 18", FPB 21'-6"
Gap 153mm (6"), IE 45 Cal/cm2 @ 18", FPB 13'-7"
Gap 229mm (9"), IE 9.1 Cal/cm2 @ 18", FPB 4'-2" (Used Lee Equation)

Would it be safe to use EI and the FPB based on the actual gap or is there some reason why I should leave the gap at 25mm? I think the actual gap is between 6 and 12 inches.

Thanks


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PostPosted: Sat Jan 09, 2010 4:56 pm 
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6" bushing gap is pretty typical on the secondary side for this size xfmr. I would be more worried about electrocution if the oil tap is 2 feet from the bushing.


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PostPosted: Mon Jan 11, 2010 9:46 am 
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2 ft is greater than the restricted approach boundary and less than the limited approach boundary. Worker must be qualified.


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PostPosted: Mon Jan 11, 2010 1:48 pm 
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stevenal wrote:
How does oil sampling qualify as an interaction like to cause an arc at the bushings two feet away?


I know this guy working for an oil testing firm that was in San Francisco pulling a sample from a 3000KVA unit and the way the cable loop was done by a contractor to the busway that when the tester pushed the cables back it created what people standing away from him said he turn into a large ball of fire. He was glad to be wearing his full suit but still sore. Apprently, the cables had a worn spot due to the way the wiring was installed. I believe this went to ground but not sure of the final analysis. There was some legal stuff involved after that. Long story.

I was like you; normally the hazard risk to taking an oil sample would be not so complicated. But I suppose there are a lot of bad installations.


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