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 Post subject: Electrode Configuration Assumption for MV Overhead Feeders
PostPosted: Mon Dec 17, 2018 12:28 pm 

Joined: Mon Dec 17, 2018 9:33 am
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With the release of the 2018 standard I’ve been comparing some of our old results for overhead medium voltage distribution feeders that were calculated using the now superseded version of 1584 to results obtained from the 2018 revision. One issue we’re struggling with is the correct assumption for electrode configuration on an overhead distribution feeder. The phase conductors are horizontal in air, but the examples provided in the 2018 standard use this configuration when the electrodes are pointing out of the equipment at the worker (i.e. the LV spades on a padmounted transformer) such that the arc would be directed from the tips of the electrodes towards a worker standing in front of them. If a lineman is up in a bucket performing live work with rubber gloves adjacent to a phase conductor that isn’t “pointing” at him/her would it be better to assume vertical electrodes in open air? The horizontal case results in a much higher incident energy value so I’ve been assuming that configuration as a worst case, but want to make sure we’re not being overly conservative when we begin using the new standard. Anyone give any thought on how to apply the new standard electrode configurations to scenarios like this?

As an example, below are some assumptions we have typically used for assessing incident energy where a utility lineman is working a 13.8 kV feeder live from a bucket using rubber gloves:
Fault type = 1LG (our utility clients would normally use conductor cover ups to reduce the likelihood of multi-phase arcs).
Arc gap = 2” (OSHA standard value for a 15kV class circuit with a single conductor in air).
Work distance = 15” (OSHA standard value for rubber glove work).
Grounding = solidly grounded 4-wire feeders.

With the assumptions above I’m getting the following values for a location with bolted fault current equal to 10kA:
Old standard: Arcing Current = 9.71kA, Clearing time = 0.311s, Incident Energy = 6.62 cal/cm2
New standard (HOA): Voc = 7.97kV, Arcing Current = 8.88 kA, Clearing time = 0.328s, Incident Energy = 21.98 cal/cm2
New Standard (VOA): Voc = 7.97kV, Arcing Current = 8.58 kA, Clearing time = 0.336s, Incident Energy = 8.06 cal/cm2

Based on the big differences in the results above you can probably tell why I’m wondering whether HOA is the appropriate electrode configuration for this scenario. I suppose we could also switch to using arcpro for the 1PH arc flash analysis since 1584 acknowledges it is going to be conservative for 1PH analysis anyways.

Appreciate any feedback on how others are planning to approach electrode configuration assumptions.

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 Post subject: Re: Electrode Configuration Assumption for MV Overhead Feede
PostPosted: Thu Jan 17, 2019 4:22 pm 
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ArcPro might be a lot more appropriate. It assumes VOA but it's also single phase and simply uses multipliers to estimate other cases. Also IEEE 1584 if I recall correctly only has test data up to around 12.5 kV so they are extrapolating out to 15 kV.

One inherent problem with what you describe though is arc propagation. See this video of what it typically looks like:

If the arc occurs at say the bushings on top of a pole mounted apparatus (recloser, transformer, etc.) then IEEE 1584 applies. But in the middle of a line or simply working on pin insulators and the like, there's nothing anchoring the arc to that location. The arc is magnetically propelled down the line usually at very high speed as the above video shows. Research at JHU gave an arc propagation speed of about 400 to 600 feet per second. So you'd have to be I guess in a bucket or climbing a pole on a dead end and standing opposite the dead end for HOA to ever apply for overhead lines. I could see some cases of bus bars and substation apparatus where this would come into play but again even in tubular busbar open air arrangements this is going to be rare compared to indoor/enclosed horizontal configurations. It's only at the apparatus that the major concerns should ensue, and in those conditions VOA certainly applies since you're dealing with jumpers connected at or near the dead end arrangements or mid-span with saddles or line clamps. So you may want to re-evaluate your risk assessment first before moving on to hazard assessment which will most likely land you squarely in a VOA arrangement as the most appropriate model.

Also it seems odd that the differences between the old and new model are that dramatically different, almost 20% higher. Are you sure all the assumptions are the same? As a guess probably the biggest driver in the calculated value is going to be the gap distance.

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