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 Post subject: Arc gaps and distance factor
PostPosted: Thu Oct 02, 2008 12:09 pm 
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Hi Everyone,

I'm in the process of analzying our 120/208 V and 277/480 V transformer and metering applications using IEEE 1584. The question I have is in regards to the arc gap and distance factor. I am assuming that padmounted transformers, CT cabinets, instrument meter sockets/test switches, and self contained metering are switchgear. The 'typical' arc gap is defaulted to 32 mm, and has an associated distance factor (x) of 1.473. I would like to change the arc gaps to their actual distances for the associated equipment I am analyzing since it has a big impact on the arc current. However, I am unsure if the distance factor associated with the equipment types would need to change based on changing the arc gap (I don't want to change the arc gap and get bogus results since I kept the same distance factor). One reason I ask is that the IEEE calculator has the arc gap inputs hidden which may imply that your not supposed to change them...Does anyone have any info/suggestions on this?

Also, has anyone done anything with regards to instrument metering and clearing times? I know the instrument wire will (depending on fault current) mostly likely melt quickly to clear the fault and would be curious to know how people are calculating this clearing time.

Thanks for the help!

Tim LeMere
WPSC


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PostPosted: Fri Oct 03, 2008 7:52 am 
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Joined: Wed Jun 04, 2008 9:17 am
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The distance factor is based on curve fitting with a particular gap and box size. According to IEEE Std 1584-2002, Section 9.10.4, the user can select any gap within a range shown in Table 6. For low-voltage applications, except for open air, there is no range. The guide says to select the equipment type that best matches your application.

The LV switchgear factor was based on a 32 mm gap and a 20" cubic box. The LV MCCs and panelboards factor was based on a 25 mm gap and a 12"x14"x7.5" box.

The gap size is a relatively small factor in the incident energy calculation. I think the best approach would be to use one of the specified typical gaps and distance factors, based on the best equipment type match. However, if the actual gap is outside the valid range of the empirical model (13-152 mm), I would not use the empirical model, but use the theoretically derive Lee method.

Appendix B says you can change the gap values in the Reference Tables tab of the spreadsheet to a value within the valid range. There will be no error indicated, however, if you enter a value outside this range.

According to Section 4.2, equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply. I would think that any instrument metering would fall below these limits.


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PostPosted: Fri Oct 03, 2008 1:03 pm 
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Thanks for the response. I will attempt to clarify my question a bit with a generic example:

Consider a 300 kVA, 24.9 kV/ 120/208V transformer, 12 amp high side kearney fuse, and a bolted fault current for a 3 phase fault on the 120/208 V side of 57 kA. Lets assume the phase spacing is actually 63.5 mm (2.5"). Below are the two scenerios:

1) Using the IEEE default arc gap of 32 mm, the arc current is 13.7 kA which results in a clearing time of .23 seconds. With a distance to the arc of 15" this results in an incident energy of 13.56 cal/cm2.

2) Using the actual arc gap of 63.5 mm and the same distance factor, the arc current is 9.63 kA which results in a clearing time of .52 seconds. With a distance to the arc of 15" this results in an incident energy of 22.7 cal/cm2.

While I realize the arc will most likely not sustain at 208 V with a 63.5 mm gap the above scenerio could be used with other voltages such as 480V. Also IEEE 1584 seems to imply this needs to be analzyed at 208V since the transformer is larger than 125 kVA. I am just curious as to which approach others are taking. Also I am wondering if others are assuming 208 V can not be sustained at these arc gaps.


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PostPosted: Tue Oct 07, 2008 8:39 pm 
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I'm struggling with the same issue, arc hazard analysis at the secondary of large distribution transformers. I hadn't really considered the effect on the increased fuse clearing time for lower arc currents with larger gaps.

The larger gaps are within the stated the valid range of the empirical model, but are not the gaps used in the low voltage tests.

Another problem is what is the actual gap? The low voltage bushings are spaced 90 mm center to center, but the actual gap would depend on the pad and bolt sizes used and how many terminal pads are used on each bushing. The worst cases for arc hazard would normally be the larger transformers, but these will be the ones with multiple pads per bushing, reducing the gap.

I think you could justify using the default arc gaps, considering that for low voltages, an 85% reduced arc current is used to cover variations of arc currents.

Do you have any thoughts on how to mitigate high incident energy levels? Requiring de-energization is tough when you have to open the secondary compartment of a padmount to access the bolts to open the primary compartment. Maybe a tool to loosen the penta-head bolts with a hotstick? Somehow, I can't see many linemen taking the trouble to loosen a bolt on a padmount using a hotstick.


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PostPosted: Tue Oct 14, 2008 4:01 am 
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For now, most people seem to be using gaps based on IEEE 1584 for 3 phase and the NESC gaps / methods for 1 phase. You are allowed to use actual gaps for the IEEE methods if they are in the ranges provided.

Some test data over the past few years that is not published indicates a 208V arc flash of lower current levels can not sustain itself and when the gap goes much beyond 1 inch. I listed some of the test data that PG&E presented at an IEEE 1584 meeting previously on the forum and also here> [url="http://www.brainfiller.com/documents/PGETestingbrainfillerposting.pdf"]Test Data[/url]

On the other side of this, a person sent a few photos of a worker a 6 cal. shirt that was pretty badly burned on the 208V secondary of a padmount where the fault current was higher. The conclusion? Right now it seems best to included the 208 side unless it is a very low fault current i.e. < 125 kVA transformer - some people are taking it down to 75 kVA to be conservative.

The X factor that was mentioned in this thread can be thought of as a variation on the exponent "squared" function. In general the incident energy drops as the inverse square of the distance from the arc flash. Except it was found it does not work exactly that way depending on the equipment and size of encolsure so the X values are adjusted exponents.

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PostPosted: Tue Oct 21, 2008 10:49 am 
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Thanks for the comments and the link to the test data. This data does help to confirm our approach for analyzing the secondary compartment on padmount transformers.

Based on our utility's experience, PG&E's test data, and on an NESC change proposal from subcommitte 8 (probably based partly on PG&E's test data), our game plan at this point will be to analyze the secondary compartment on padmount transfromers using a fast clearing time (like 5 cycles) for both 120/208V and 277/480V. Our phase spacings are actually 5" (phase to ground is 2.5"), so I believe that 5 cycles would be somewhat conservative. We will also be using the default arc gap in IEEE 1584 since I don't want to mess with the distance factor. Using this approach, our max clothing requirement for opening padmount transformers and/or testing voltage on the secondary should be around 6 cal/cm2.

For metering, the actual arc gaps are much closer to the default IEEE gaps. We will be looking to go with more instrument metering (vs self contained) with current limiting fuses on the instrument wire for the larger transformers.


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PostPosted: Mon Mar 16, 2009 11:53 am 
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Gap.

I recently ran into another situation regarding ├óÔéČ┼ôGap├óÔéČ distances it involves a 4800V system we specify 15KV gear to accommodate this installation however when the study was done the individual selected the system voltage at 4800V which is correct however this defaulted the gap distance to the 5Kv category, when using the 15Kv gear the gap increases and by going back and adjusting the Gap to the 15Kv gear specifications the arc-flash hazard in some cases increased another category.


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