What are the rules for opening size, number of openings and minimum spacings between the openings per 188.8.131.52.3 and 184.108.40.206.4 of a fire enclosure? When is testing required? Does the standard allow for a mesh or grid to be provided to cover the openings?
(References are to IEC 62368-1:2014 (Ed. No. 2) except where otherwise noted.)
As stated in 220.127.116.11.3, top/side openings that are within the fire cone of Figure 41 are treated as top openings and shall not exceed 5 mm in any dimension or 1 mm in width regardless of the length. If the openings exceed those dimensions, the standard requires the needle flame test of Clause S.2 which shall be applied as explained in 18.104.22.168.3.
The standard does not rule out other options such as providing a mesh or barrier, the acceptability of which will need to be determined upon review of an actual sample. Additionally, the standard has no requirement specifying the maximum number of top/side openings that may be provided. There are also no requirements regarding the spacings between the openings for top/side opening. However, keep in mind that too many openings may affect the strength of the enclosure. The robustness of an enclosure safeguard will be assessed per subclause 4.4.4, by the mechanical strength tests of Annex T if deemed necessary. An enclosure functioning as an electrical and fire enclosure (safeguard) will need to meet all applicable performance requirements.
The standard defines bottom openings as any openings within the zone as shown in Figure 42 including intersection with side openings as noted in 22.214.171.124.4. In lieu of the flammability tests in Clause S.3, the standard allows several options for compliance with the requirements as explained in 126.96.36.199.4 a) through d).
As stated in 188.8.131.52.4 a), openings that do not exceed 3 mm in any dimension or 1 mm in width regardless of length are considered to comply. As stated in b), openings may go up to 6 mm in any dimension, or 2 mm in width if located under components that are rated minimum V-1, HF-1 or components that pass the needle flame test of IEC 60695-11-5. Alternatively, as explained in c) the standard also allows a metal mesh that does not exceed 2 mm by 2 mm of at least 0.45 mm diameter wire to be provided. One last option which only applies to metal enclosure is explained in 184.108.40.206.4 d) and detailed in table 34. Openings shall be dimensioned and spaced away per that table based on the thickness of the enclosure and shape of opening (circular or other). The standard does not specify any limits on the number of bottom openings if they are adequately spaced away per the table.
Please also note, there are additional requirements for openings in Annex P that are not fire related and can impact size of openings.
However, in IEC 62368-1 Ed. No. 3 extensive revisions were made to fire enclosure requirements that affect enclosure openings. First, in 220.127.116.11.1, Fire enclosure and fire barrier openings, see new Figure 40 – Determination of top, bottom and side openings. Extensive revision also was made to 18.104.22.168.5, Side openings and side opening properties, which can result in areas of the side enclosure having no restriction for fire considerations. For top openings subjected to performance testing, also note modifications to, Application of needle flame, in Annex S.2, Flammability test for fire enclosure and fire barrier integrity, including new Figure S.1. Also, in 22.214.171.124.4, Bottom openings and bottom opening properties, for openings below the PIS, the principles and requirements now are much closer to IEC 60950-1, including the reintroduction of the 5 degree downward projection principle. Finally, also in 126.96.36.199.4, original options c) and d) have been removed since this type of construction is considered to meet the criteria of the second part of option b).