Of you're talking about armor.50% or higher and it has full protection anything below 50% loses protection.
The more an armor is deteriorated, the more chance the wearer has a chance to receive "partial bypass" on critical as well as non-critical hits. Chance to be bypassed for each bullet applies when condition goes under 50% and is calculated by formula (50-condition)*2%. (e.g. 0% bypass chance at 50% condition, 50% chance at 25% condition, 100% chance at 0% condition). "Partial bypass" has no effect on traditional critical hit bypass. Deterioration of armor does not change its DT or DR.
wiki page...