标准里关于防火外壳不理解的point

qinglang

在60950的4.7.2.2 Parts not requiring a fire enclosure中有写
components in SECONDARY CIRCUITS:supplied by internal or external power sources that are limited to a maximum of 15 VA
(see 1.4.11) under normal operating conditions and after a single fault in the equipment
(see 1.4.14) and mounted on HB75 CLASS MATERIA,L if the thinnest significant thickness
of this material is < 3 mm, or HB40 CLASS MATERIAL, if the thinnest significant thickness of
this material is ≥ 3 mm;
这里说如果PCB板的厚度<3mm,它至少要用HB75 CLASS MATERIA，如果PCB板的厚度>3mm,它至少要用HB40CLASS MATERIA，75的等级不是比40的要低吗？板子越薄应该用较高的防火等级才是啊，怎么是相反的呢？
哪位高手能指点迷津啊？非常感谢。