Hello

What is the optimal X factor for disconnect switches. I assumed it is 2 because that’s the default value that ETAP gives you. I'm I correct for assuming this? Furthermore, It falls under the low voltage category (0.208-1), so it can be the value for MCC's and panels which is 1.473. Which value is optimal for Arc Flash calculations?

Thanks

IEEE 1584 suggests 2 for open air arcs, 1.641 for Low Voltage distribution equipment and 1.473 for Low Voltage power/switchgear equipment. 1.473 would be worst case since the incident energy would not decay as quickly as using 2 however most would probably use 1.641 since switches may be considered Low Voltage distribution. You will have to decide based on this, how to proceed.

Disclaimer >>> This is based on my thoughts and may or may not reflect the views of IEEE 1584 or the IEC TC78 of which I am Secretary and International Chairman respectively.

The "X" factor has to do with the the shapes of the enclosures that were used for the lab testing used to develop IEEE 1584. If you are sufficiently far from the arc, it appears as a point source of thermal radiation. It radiates out in all directions equally so the thermal flux at a given location will be proportional to the surface area of a sphere with a radius of the same distance. In other words, X=2.0. If the arc occurs inside an enclosure though it will be reflected off the walls and as a result it comes out more as a cone. IEEE 1584 used the following size enclosures for testing:
Small: 12Wx14Hx7.5D
Medium: 20Wx20Hx20D
Large: 45Wx30Hx30D

In each case there are 3 bus bars pointed straight down in a vertical arrangement with one side of the box open. The busbar distance was varied and several measurements were made at different distances in front of the box in order to estimate the "X" factor.

The inherent engineering problem here is how the end user can determine the correct "X" factor. IEEE 1584 comes with an "equipment table" to help you do that. It works great if you have similar equipment (panelboards, MCC's, 5 kV switchgear, 480 V switchgear), but becomes problematic with equipment outside these common types. That's where you have to make an educated guess knowing that the original test work is based on. A cutout or any other kind of open air disconnect would clearly be X=2. Most fused safety disconnects would clearly be a case of "shallow box" dimensions (same as panelboard) but others may be more similar to the "cubic" box or even the large box.