Will penetration have an effect on enemies at Zero or Negative resistance?

i've been fiddling with POB and looking for answers but i keep getting results from 9 years ago and conflicting replies can someone confirm if it mechanically works?

if an enemy is at 0 or -ve resistance will penetration do anything?

In PoE1 penetration is not capped. It meanst if you hit an enemy with 0 resistance for 100 damage, and you have 30% penetration, your hit will treat the resistance as -30% and deal 130 damage (50% pen - 150 damage and so on). More indepth explanation can be found on the official wiki (poewiki(dot)net).
However if you have a source of "your hits ignore enemy elemental resistance(s)" then penetration does nothing.

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PyschoWolf#1976 wrote:

i've been fiddling with POB and looking for answers but i keep getting results from 9 years ago and conflicting replies can someone confirm if it mechanically works?

if an enemy is at 0 or -ve resistance will penetration do anything?

Penetration cant go into negative and it's calculated after every other mods applied to the target.

Mob res 40% and you have 40% penetration it will be full damage.

Mob res 40% and you apply ele weakness for -30% res and hit the mob with 40% penetration you still hit for full damage but 30% remaining penetration are lost because it cant go negative.

40% res - 30% from curse/debuff = 10% resistance left so you only need 10% penetration for full damage.

If you can get the mob into negative resistance with curse and exposure then penetration is completely useless

If you use leopold's applause penetration can go down to -50% resistance.