Dear Gents,

I am wondering which x-factor could be applicable in below case.

440v lv main switchgear's typical gap between conductors is 32mm and distance X factor is 1.473 according to IEEE1584 table 4.

If actual clearance distance between conductors is 25 mm from manufacturer information, which x-factor is applicable in this case? X-factor 1.641 is possible to apply on this case which is category of MCC and panels since it has same gap distance 25mm?

Your any advices would be welcomed.

It is switchgear so you use 1.473. There is no intentional correlation between the distance exponents and the default bus gaps. The distance exponent changes as the equipment enclosure/shape/design changes. The bus gap is...the bus gap. The table of typical values is just that, typical values. Check out the table for medium voltage switchgear for instance where bus gaps are given as a range of "13 to 102 mm" for typical switchgear!

Great thanks for your reply!

It is understood that distance exponent is not related with bus gaps and this is from design aspect of equipment.

It would be highly appreciated if let me know some background of distance exponent factor how it is determined.

As the rest of the equations in IEEE 1584-02: curve fitting to actual test results.
Nothing more, nothing less.

From looking at the data provided in pages 33-37 of the 2002 guide I believe the distance exponent reflect "both" the gap and the enclosure size. Analysis of the formula (and common sense) provided would tend to lead one to believe that the smaller the enclosure the more narrow and concentrated the heat volume projected towards the worker will be, and the less attenuated it will be by distance.

E = 4.184 * Cf * En * 5 * t * (610^exp/D^exp)
 
Would indicate that if the lower the exponent the greater the incident energy for D < 610mm. Note that if D is larger than 610 the effect is the opposite.
Enclosure= ----------Large---Small----Smaller----Large----Small----Smaller----No enclosure
x =---------------------1.473---1.641----2------------1.473----1.641---2------------2
G = --------------------32-------25-------13-----------32--------25------13----------10-40mm
D = --------------------610------610-----610---------455-------455------455--------910
Ratio 610^x/D^x = --1--------1-------1-----------1.54------1.62----1.80--------0.45

Since the formulas do not seem to clearly define the role of gap versus the role of enclosure size in the determination of the exponent factor if you need to perform a calculation for a specific gap and a specific enclosure size whose combination does not match the recommended values, the conservative thing to do may be to use the gap as is known to conservatively determine arcing current and use the exponent factor that most matches the enclosure for which the calculations are being made if the enclosure is smaller and if that yields a higher incident energy. In a situation where the situation on the ground does not match the recommended assumptions in the guide you should probably endeavor to be conservative in your calculations.

Interestingly the distance exponent for open air is 2, same as for a small enclosure. Maybe there is an underlying assumption that in open air the working distance is always greater than 610mm. In that case the exponent of 2 will yield less energy as show by the column above for 910mm = D at exponent of 2. that makes sense to me as the energy is dispersed in all directions.

Open to suggestions by others if they have a different interpretation of the guide. This is a nuance I have wondered about but I have not seen any formal discussion regarding this issue.

Thank you very much for your systematic advices ! It gives me great helps to have perception how I approach to interpret IEEE1584.

I thought the open air case is obvious.

If we assume that energy is released at a point and radiates outward then the energy flux (cal/cm^2) would be dependent on the area of a sphere equal to the working distance. Thus the exponent would be 2 since it is proportional to 4/3*pi*R^2.

If the enclosure is very small then clearly the shape of the enclosure is immaterial. Similarly for very large enclosures there is no difference. However for "medium" size enclosures especially where there are groups of equipment such as in switchgear or MCC's, the incident energy tends to be "shaped". Unfortunately and this has not been mentioned yet, as the working distance changes, the influence of the enclosure shape also changes. If the working distance increases far enough no matter the enclosure shape, it will act like an "open" condition. Similarly if it is decreased at some point, enclosure shape matters. At this time though these different factors are assumed to be constants and there is no way that I know of to adjust for enclosure shape beyond picking the exponent from the table of equipment provided.