Appendix D: Bus Bar System
Appendix D was introduced in April 2020 version of UL 508A. It makes clear what was before a common practice but not formally correct. That means an industrial control panel manufacturer is obliged neither to use a certified bus bar system nor to perform short circuit test, provided it complies with TABLE G3.1 of UL 891 (named Table D3.1 in UL 508A).
That table is now included in the new appendix and that makes it formally possible that practice.
"FACE TO FACE" OR "EDGE TO EDGE" ARRANGEMENT
The table distinguishes between bus bars set as Face to Face or Edge to Edge.
For example, in case of bus bars set Edge to Edge, if the distance between insulators is not more than 533 mm, provided the size of the bar complies with table D3.1, and the rest of the architecture complies with SUPPLEMENT SB (all components having a SCCR equal to 50 kA), the panel can be declared with SCCR = 50 kA.