Not meant to be a trick question, but as it turns out, it actually is!  It depends on if your workplace is a "general industry" or "construction" worksite.  Now stick with me as I explain the difference...

29 CFR 1926.155(l) defines a safety can and requires a flash arresting screen as one of the criteria for approval.  29 CFR 1910.106(a)(29) contains the same definition of a safety can, except that defintion does NOT require a flame arrestor screen.  Both 29 CFR 1926.155(a) and 1910.106(a)(35) do REQUIRE APPROVAL by Underwriters' Laboratories, Inc., (UL) or the Factory Mutual Engineering Corp., (FM). In addition, 1926.155(a) also accepts approval by Federal agencies such as the Bureau of Mines or the US Coast Guard.

Now here is where it gets tricky... FM requires flame arrestor screens in safety can spouts in order to receive thier "approval", but UL does NOT require flame arrestor screens in safety cans for their approval. Moreover, the NFPA Standard No. 30, Flammable Liquids, recognizes approval of UL or FM. Also, 25 CFR 1926.155(l) and 29 CFR 1926.155(a) are inconsistent because UL approval does NOT require a flame arrestor screen. On the other hand, 1910.106(a)(29) and 1910.106(a)(35) are inconsistent because FM approval DOES REQUIRE a flame arrestor screen.

Some believe that flame arrestor screens in the pouring and filling openings of safety cans provide little additional protection to the user. Moreover, the General Industry standards were promulgated under Section 6(a) of the Act and Part 1926 standards were promulgated in accordance with the Administrative Procedures Act (public hearings). Also, the Bureau of Labor Standards elected to provide for the additional coverage by requiring flame arrestors.

Bottom Line... I have always required flame arrestors in safety cans at my facilities, even so that they were specifically listed on the audit checklist.  There is just not enough relaible data to convience me they are not needed and they certainly can not hurt!!!