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If an external power supply output to a product is power limited in accordance to 62368-1, could the product’s polymeric enclosure be flame rated HB?

Power-limited is a undefined term in IEC 62368-1, although what probably is meant is that the power supply supplying the product has an output limit associated with PS2 (< 100 W), or complies with the Limited Power Source (LPS) criteria, which allows for varying levels of power (not exceeding 100 VA) based on specific voltage, type of overcurrent protection, etc. Although circuits / devices supplied by PS2 or LPS do not require a fire enclosure, combustible parts, including casings, associated with the device / product in most cases are required to be separated from Potential Ignition Sources (PIS) by the 13 mm / 50 mm fire cone (Figs 37 & 38). Combustible parts that are not, may need to be V-1 or V-0 material (or equivalent testing per Annex S), depending on the actual separation distance. If the proper PIS separation distances are maintained, an HB rated casing may be fine. If not, a casing material with a V-1 or V-0 rating typically would be required.  Both Arcing PIS and Resistive PIS are defined in the Standard, but the steady state 15 W limit (after 30 s) associated with Resistive PIS typically has the most impact.

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