One of the first questions asked years ago, on the “Question of the Week� was about the conductor gap distance and whether people use the default values from IEEE 1584 of 25, 32, 102 and 153 mm. Almost all of the responses indicated the defaults were generally used.

Now, many years later, the same question is being asked to gauge whether people are attempting to provide more detailed in the analysis. This would require reviewing equipment drawings or taking measurements and there may be many possibilities within one piece of equipment.

So, once again:

Do you / your consultant use the standard bus gap distances from IEEE 1584-2002?

Yes, we use the standard gaps
Sometimes we have used something else (please describe)
We always use measured values
Not something I specify or am involved with

This is a great question to revisit. I think we have learned a number of things from the recent arc flash research - among other things, that bus gap and enclosure dimensions are important variables that should be accounted for to obtain more accurate results.

I think the real question is, "If I can not de-energize the equipment, how do I safely measure the bus gap?" I think we all know that the answer is.."You can not measure the bus gap safely when it is energized." Therefore, using the default bus gaps is the next best option. This is the same for when measuring the depth of a cubicle. You really can not do it safely except for using laser distance measuring devices.

This is my big concern about the upcoming revision of IEEE 1584. Have we made it too difficult and expensive to be applied in the field? Measuring the bus gaps and cubicle dimensions when the equipment is energized increases the hazards of collecting equipment data for arc flash studies. The only way to safely do this is to de-energize the equipment and I fear that not too many owners will want to do this.

SKM doesn't have pad mount transformer as an equipment option so I have manually input that data for the secondary before.

Use a plastic caliper, or use a photo to estimate it...take a picture of a ruler held about the same distance away in a safe spot NEXT to the equipment. Then it's just a matter of using a photo editor to measure pixel distances of both the actual bus gap and the ruler, and scale accordingly. I think there are even apps that do all the math for you.

To properly determine which gaps to measure, an assessment must be made of the physical arrangement of the switchboard to identify the locations at which an arc will terminate. Typically, this will be at the far end of busbars or bus droppers, at the incoming terminals of PDs or the outgoing cable terminals. This establishes a number of dimensions that must be used in the calculations.
Routinely, this information is not readily available particularly where dimensional drawings can not be accessed. It is not always clear where insulating barriers or insulated bus sections may be installed. Some designs may promote arc extinction.
The standard dimensions are typically used in the studies that I have seen, with actual measurements being used when confirming a fault result.
Where the operation of PD is assured by suitable detection methods, (eg flash detectors), and the switchgear is IAC, then the dimension may not be material to the outcome.
There are also those who dispute the science behind the IEEE 1584 calculation method and claim that the relation to electrode gap is weak.

IEEE 1584 empirical equation is quite literally just that...an empirical equation. No attempt is made to provide any possible scientific basis for it other than statistical analysis. In other words, it's a curve fit.

There is however plenty of scientific basis for arc length being almost linearly related to arc energy because except for at the end points, the arcing voltage is linear (up until it starts wandering a lot...above 6-12") with the bus gap. This has been proven multiple times in the theoretical and experimental literature under a variety of different experiments and conditions.

From my own personal experience there is a surprising amount of consistency in electrical gear designs, particularly when it comes to gaps, and the table-based results are generally pretty much spot on for very modularized equipment such as MCC's and switchboards but less so for more non-mass produced equipment such as transformer enclosures.