Mistakes happen, and there are plenty of opportunities to make them when performing an arc flash calculation study. The good news is commercially available arc flash software can help simplify the study process and perhaps even reduce errors. Software typically contains extensive libraries of data and can sometimes provide default data. However, the bad news is, if you don’t know what you are doing, the default data can provide very convincing wrong answers.
One mistake that seems innocent enough is to incorrectly identify the type of equipment in the model. Does it really matter whether a particular piece of equipment is designated as a panel or switchgear? Yes!
Most arc flash studies are based on the equations defined by IEEE 1584—IEEE Guide for Performing Arc Flash Hazard Calculations. Several equations use variables that are unique to the type of equipment. If the wrong equipment type is selected for the computer model, incorrect variables would also be used. Two of the more important equipment-dependent variables are the working distance and the distance X factor. Most arc flash software automatically defaults to the appropriate values shown in the table (on the opposite page) based on the equipment type.
Equipment type and working distance
The working distance is the distance from the point of the arc flash to a worker’s head and body, and it is used when calculating the prospective incident energy of an arc flash. IEEE 1584 further clarifies the definition by stating, “Typical working distance is the sum of the distance between the worker standing in front of the equipment, and from the front of the equipment to the potential arc source inside the equipment.”
Since the working distance includes distance inside the equipment, the equipment type is important. The table on the opposite page shows the working distance can range from 18 to 36 inches, depending on the equipment type.
A serious error can occur by not properly defining the equipment type. What if a calculation was being performed for a panel but the equipment was mistakenly defined as switchgear? A panel has a default working distance of 18 inches, but switchgear is listed as 24 inches. Using the greater distance would result in lower incident energy and a potentially dangerous conclusion.
In general, the incident energy varies as the inverse of the distance squared based on the following equation:
Ei2 = Ei1 × [(1 ÷ (D2 ÷ D1 )]2
Ei1 = incident energy 1 at distance 1
Ei2 = incident energy 2 at distance 2
D1 = distance 1
D2 = distance 2
If the incident energy in the example were calculated as 6 calories per square centimeter (cal/cm2) at the incorrect working distance of 24 inches, using the correct working distance of 18 inches would result in the higher value of 10.7 cal/cm2 as shown below:
10.7 cal/cm2 = 6 cal ÷ cm2 × [(1 ÷ (24 inches ÷ 18 inches)]2
Equipment type and distance X factor
When the current IEEE 1584 standard was developed, tests revealed that the decrease in incident energy is not only dependent on distance, but also on the type of equipment. Instead of the incident energy varying exclusively by the inverse distance squared, the squared term “2” can vary anywhere from 0.973 to 2.0, depending on the equipment enclosure and whether it is low or medium voltage. To allow for this variation, the equation uses a variable known as the distance X factor as a substitute for the term “2.” These values are shown in the table above.
The graph on page 50 illustrates the rate at which the incident energy decreases with distance from the arc source as a function of the distance X factor. If the arc flash is in air, the incident energy decreases quickly with distance as a squared function. If the arc flash occurs in a panel or switchgear, the distance X factor is smaller and the incident energy will decrease more slowly with distance.
A significant problem is that the distance X factor is not obvious when using most arc flash software. Yet, if the equipment type is not properly defined, the wrong distance X factor may be used, and the calculated incident energy would be incorrect. The distance X factor also affects the arc flash boundary, which is the distance from an arc flash where the incident energy drops to 1.2 cal/cm2.
This “garbage in, garbage out” abbreviation still applies. Although arc flash software can simplify the study process, not understanding important details, such as the equipment type, can result in the software producing very convincing, well-formatted wrong answers—answers that someone’s life may depend on.