Per M.4.3, a fire enclosure is required for secondary lithium batteries. In the case the equipment enclosure does not meet the criteria of 6.4.8, such as being rated HB, can a fire barrier fulfill the intent of the requirement?
More specifically you asked: Annex M of 62368 states "Secondary lithium batteries shall be provided with a fire enclosure according to 6.4.8. The fire enclosure may be that of the secondary lithium battery itself, of the cell or of a combination of cells or that of the equipment containing the secondary lithium battery. Equipment with batteries are exempt from the above requirement if the equipment uses a cell that complies with PS1." My question is, if the battery pack does not meet the criteria set out in 6.4.8 in regard to material flammability rating and the battery is considered a restive PIS, can a fire barrier be used between it and overall enclosure of the product and be in compliance? The battery would be less then 5mm distance from an enclosure using HB rated material. So basically, no overall fire enclosure either around the battery or the overall product. Seems to me the answer is "no" since annex M does not state anything about fire barriers.
We agree, the answer to your question is “No,” i.e., the scenario you describe, with an HB Enclosure (casing) combined with a Fire Barrier, does not meet the intent of M.4.3. However, we suggest you contact UL to discuss in greater detail since not all battery and battery pack constructions are exactly the same and compliance can depend on the details associated with a specific construction.
Let's think of the difference between fire enclosure and fire barrier. The definition of fire enclosure is given in 188.8.131.52, which is to prevent fire spread from within the enclosure to outside the enclosure. Although the term "fire barrier" is not defined in clause 3, it can be implied from 184.108.40.206 and other relevant subclauses that a fire barrier is to prevent fire spread from a PIS to a specific combustible material located in proximity to the PIS. In this context, the fire barrier is most likely to block fire propagation to one direction, but not necessarily in all directions. Thus, a fire barrier might not prevent the spread of fire from within the equipment (preventing fire spread in all directions) by surrounding the PIS and cell operating above PS1 with a fire enclosure. For this particular secondary lithium battery fire enclosure context, the purpose of the Fire Enclosure would be to prevent fire from a lithium battery and protection components from spreading outside the battery or equipment, instead of preventing fire spread to a specific direction toward specific combustible material(s) only.
Please also refer to the previous Q&A that discusses a related topic of fire enclosure for secondary lithium batteries, in which the on-going work of TC108 interpretation panel is mentioned. See: "For DC powered products with internal PS2 circuits and containing only resistive PIS, does this construction require a Fire Enclosure?" - https://62368-ul-solutions.com/engineers/for-dc-powered-products-with-internal-ps2-circuits-and-containing-only-resistive-pis,-does-this-construction-require-a-fire-enclosure/.