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 Post subject: 3 phase overhead distribution line; single phase arc flash?Posted: Thu Jan 13, 2022 8:20 am

Joined: Thu Jan 13, 2022 8:12 am
Posts: 1
OSHA 1910 Appendix E Table 3 permits single phase arc flash calculations if it can be demonstrated that multiple phases are separated sufficiently. In the case of 15kV+ arc flash studies, ARCPRO is permitted for single phase arc flash studies without having to use a multiplier/conversion factor.

When you put these two together, how does one justify that a 3phase overhead 15kV+ line would likely only result in a single phase arc flash? Is the innate conductor spacing sufficient for this?

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 Post subject: Re: 3 phase overhead distribution line; single phase arc flaPosted: Mon Jan 17, 2022 9:12 am
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1637
Location: Scottsdale, Arizona
For distribution and transmission lines, the spacing between phases is considered too great for an arc to initiate. Good question!

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 Post subject: Re: 3 phase overhead distribution line; single phase arc flaPosted: Tue Jan 18, 2022 8:36 am
 Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 579
I understood the new rules were intended to encourage covering the unworked phases.

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 Post subject: Re: 3 phase overhead distribution line; single phase arc flaPosted: Mon Jan 24, 2022 10:15 am

Joined: Mon Oct 29, 2018 9:10 am
Posts: 3
arkanos wrote:
OSHA 1910 Appendix E Table 3 permits single phase arc flash calculations if it can be demonstrated that multiple phases are separated sufficiently. In the case of 15kV+ arc flash studies, ARCPRO is permitted for single phase arc flash studies without having to use a multiplier/conversion factor.

When you put these two together, how does one justify that a 3phase overhead 15kV+ line would likely only result in a single phase arc flash? Is the innate conductor spacing sufficient for this?

Some make the assumption that if coverups "blankets" are used on the other 2 hot phases, then a 1ph arc will occur. With coverups not used, it is possible multiphase arcs can occur on oh distribution. I suggest reading EPRI Papers, they have tested many real-world utility equipment for scenarios such as this, arcs up to 10 inches and greater may occur on oh distribution. Furthermore, the 2-inch gaps used by ArcPro are much too short for real-world conditions, so you may want to beware of OSHA's Tables that reflect a short, 2inch arc at 15-25kV.
EPRI's lab testing energy results differ quite significantly from industry standards. I believe a new Table may be provided in the next NESC that depicts EPRI's testing results for utility Pad mounted Switchgear and Transformers. Energies 2-3 times larger than what is seen in IEEE Standards resulted from testing on real-world utility equipment. Evidently, NESC subcommittee decided to side with EPRI's results for creation of this new Table for Padmounted equipment and Transformers from 1-35 kV. So do you use NESC tables for padmounted equipment and Switchgear over 1kV, IEEE1584-2018 Standard, or suggested OSHA multipliers to apply to Table 410-2 of the NESC? Good luck with your decision making.

Search for these EPRI papers on IEEE Scholar site:
"Medium-Voltage Arc Flash in Open Air and Padmounted Equipment"
"Comparison of IEEE 1584-2018 Predictions with Tests on Real-World Equipment"
"Medium-Voltage Arc Flash in Switchgear and Live-Front Transformers"

Greg Shirek

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