This week's question is another hypothetical one. The existing 2002 version of IEEE 1584 equations are valid from 208 volts to 15 kV. The new version of IEEE 1584 will likely maintain this same voltage range. There are limited methods for addressing incident energy above 15 kV. These include the NESC tables, Arc Pro (used to develop the NESC tables) the 1990's Duke Heat Flux Program and the 1980's Lee Equations.

Other than the Lee Equations which are very theoretical and ultra conservative (i.e. extremely large values at higher voltages) everything else is more of a "black box". - meaning no published equations or peer review.

Here is this week's question:
Would you use/need arc flash equations for systems > 15 kV?
-Yes
-No

This would be so helpful but too bad IEEE is not working on this. I have had some 23 kV and 34.5 kV and it is a bit disturbing that I don't have equations that I can track to see what is happening.

The company I work for at the moment does not have systems above 6kV.

Because of the new regulation that OSHA is putting into place regarding PPE and analysis on TD&T equipment, our company will need to perfrom analysis on our substations where our techs interact with 4.16kv-161kv equipment. I had a conversation with one of the software manufacturers that our company uses for arc flash analysis. They told me that their software automatically defers to the Ralph Lee equations when you enter a voltage over 15kv, and their calculations will be accurate.

Our only equipment at greater than 15kV is the high side of the main utility transformers (46kV). We would never work on those live, though, so I don't really have to have an accurate model. I have them in my system model, but the bus tied to the transformer secondary is the first point that really matters to me.

We are getting more requests from small utilities to perform arc flash studies on distribution systems. Unfortunately I do not see any other method of providing a study than the software method we have already and they are tedious and cumbersome at best or use an approximation method that is difficult to stand behind.

That is the problem. The "calculations" may be accurate based on using the Lee equations (i.e. correct math) which are the only equations (that I know of) for use above 15 kV, but because of the theoretical nature, they give unrealistic results at higher voltages. So, accurate math of theoretical equations.

There is quite a need for better equations above 15 kV which right now don't exist.

In response to this question and the last poll regarding single phase arc flash equation, any idea on when IEEE will pursue such?
At KSC, the EE's have performed tests for 15KV splices at the KEMA labs for SLG faults within a manhole. Needless to say, they received much greater than expected results. Previous arc flash incident energy equation to determine proper PPE was calculated using the Lee equation for three phase and then multiplying by 50%, as recommended by reps who work with IEEE. This was certainly not the case as test results were received at KEMA. One major issue that was realized is it appears the incident energies are better calculated by the available power at such faults. Meaning the available voltage does make a significant difference for single phase to ground faults. It does not solely rely on available current and trip times. Once the power is determined using the area under the curves (integration - using the wave form captures from your breakers/relays), the energy can be more accurately determined, but would like to get further proof by trials if IEEE is willing....

1. OSHA dida comparison of models above 15 kv. Their conclusion is to use Arc Pro.
2. Arc Pro is based on published, peer reviewed papers.
3. No available testing above 15 kv so no way to validate above that. However at the boundary there is good agreement with IEEE 1584 empirical model and thus test data. Duke heat flux consistently under predicts. Lee gets more and more overpredictying due to a model error of assuming voltage and current are interchangeable.

So if its good enough for OSHA to recommend Arc Pro and same with NESC, its good enough for me.