Open Air Switching over 15kV

I am trying to perform a systemwide arc flash analysis for open air switching. For the voltages under 15kV, it was easy enough to calculate the level of current that was the dividing line between HRC 2 and 3 and figure out where our existing switching jackets and gear were adequate and where we needed to look deeper or specify more PPE. For the voltages over 15kV, however, the Lee equations give extremely high incident energy for conservatively chosen clearing times (in order to try to make systemwide assesments). I am wondering what other utilities have done for these higher voltage, open air switching studies... or what you have done policy-wise for PPE for this switching.

Thanks!

First: If you are a utility and are operating utility equipment, the HRC categories do not apply. It's all about the ATPV ratings. Picking at straws, but does keep one from mixing standards.

Next: We work for about 25 municipal utilities in the southeast and we have used Arcpro up to its limits for outdoor, open air equipment system wide. IE values are not over 5 or 6 cal. Yours may be different though.

For switching, have you considered that the line folks will be using a stick and will be 6 to 8 feet from the work? If they are on the ground operating a group operated switch, they will likely be 30 to 40 feet plus from the switch.

Let me know your thoughts.

Alan

By the HRC categories not applying, what functional difference are you implying? For instance, if we determine a potential IE of 17cal/sq_cm for a certain piece of equipment and subsequently categorize it HRC 3, that will require PPE to have a minimum ATPV of 25cal/sq_cm if my understanding is correct. Are you saying that we would just require field personnel to wear PPE with a minimum ATPV at or above the highest potential IE exposure (in this example 17cal/sq_cm) and specify all the other PPE necessary – e.g. gloves, hood, etc?

As far as the assumed working distances, I had been using a minimum of four to six feet based on the use of a switching stick, and I’m waiting on feedback from the lead of our line crew to narrow down reasonable switching distance assumptions for the various voltage classes.

On ArcPro… do you know if it also uses the Lee method for open air over 15kV?

Thank you so much for the help!

You are correct. If you do the analysis and determine it is 17 cal, then you only need 17 cal or better PPE. Also HRC is a 70E classification.

The reason I brought up the distance is because the IE decreases as the inverse of the square of the distance increase. Hot stick makes a huge difference. If you double the working distance, your 17 cal will be 4+ cal.

Don't believe Arcpro uses the Lee method, pretty sure their method was uniquely derived from the physics of the arc, but maybe someone else knows for sure. Do know that it was used to create the NESC tables.

IMHO, if you are conducting outdoor open air utility studies, you should get Arcpro.

Alan

Another point is that, if you are a utility, NESC AF tables are for a SLG and most software programs I believe default to 3phase fault. This may change your IE.

wbd brings up a great point that I failed to mention!

Utility Calculations

You fall under the NESC-2007 rather than NFPA 70E in this work as noted by Alan.

1. Consider working distance especially if this is done with a hotstick/shotgun stick or by remote device.
2. Using ArcPro is the most accurate calc. It DOES NOT default to LEE as all others do (except SKM which now uses a look up of the NESC tables). NOTE: The NESC tables (the updated ones on IEEE site) are from ArcPro calculations BUT they use VERY conservative arc gap distances. IF you run ArcPro you might get different results.
3. Seek out some expert info on using ArcPro. It is really useful but only one tool. You will want IEEE 1584 and an integrated software system like SKM, EasyPower, EDSA, ETAP,MillSoft, Asplundh etc.

Hugh,

Did you mean to say non-conservative arc distances? Small gaps lead to less conservative IE values.

Higher Voltages and Arc Flash Calculation Problems

No the issue is one of formula. The "Ralph Lee" equation was a totally theoretical equation he developed before anyone really thought about arc flash (no criticism intended). It unfortunately wasn't an arc flash model. The equation makes arc flash energy a direct function of voltage. That isn't a problem at 480V but as the voltage increases this gets out of whack with the real energy. IEEE 1584 had almost no data in the last version (2002) in the area of medium voltage and just defaulted to the RL equation perpetuating the error.

ArcPro doesn't do this. It has limitations but works well for utility conductors and for some other applications with factors. If the overly conservative numbers work for your issues it is fine. If not there are other means.