IEC 60601 11.2.2.1 Risk of Fire in an Oxygen Rich Environment, Source of Ignition

#1
Hello,

I have a question about the definition for a source of ignition, specifically clause 11.2.2.1 a), items 1-5.

I have typically seen electrical designers claim that because a circuit is energy-limited to the graphs in Figures 35-37, there is no source of ignition. However, those figures only apply to criteria #5, and the clause says that there is a source if ANY of 1-5 exist. Looking further in the rationale/guidance section, the standard says "in most cases the limitation in item 4) to 300 deg C is more restrictive than these. For most small components like decoupling capacitors, or where the failure of a component causes the maximum possible power to be drawn from the source, it is necessary to limit the power to about 1W."

This indeed is far more restrictive than the ~15W limit in Figures 35.

I interpret the rest of the guidance in this section to mean that the designer must prove that a component won't reach 300 deg, and this is where I struggle. The standard suggests that it only takes 1W for a small component to reach 300 deg. This sounds reasonable but I also wonder if it's reasonable to expect that a fuse can open more quickly than the time it would take for the failed component to reach 300 degrees. So going down this path of analysis, I need to have reliable estimates for (a) the thermal resistance of the failed component and (2) thermal mass/inertia of the failed component. I don't find these two parameters readily documented.

I'd like to know others' thoughts on this approach (analysis with thermal resistance and thermal mass). If it is reasonable to say that a fuse can be faster than the temperature rise, what are some estimates for thermal resistance and thermal mass? If it is not reasonable, and I cannot limit the the power to ~1W, does potting help (by keeping dust away from the hot part)?

Thanks.
 
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