question about fire penetration support

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FAMO wrote:

I thought Penetration could not lower (max) resist below 0?

Penetration does not lower maximum resistance at all.

Penetration means the hit treats the (final) resistance value as being x lower than the actual value. Since resistance values can be lower than 0, it has no special interaction with 0 resistances, and will still treat them as x lower than that actual 0 value.