I have started sung this rule in my gam directly out of the FFG Warhammer RPG page 59. I'm a little surprised it wasn't included
MINIMUM WOUND RESULT
If the difference is zero or a negative number, the attack still inflicts one normal wound – the minimum result for a successful attack. If one or more critical damage effects are triggered when the difference is zero or a negative number, then the attack inflicts a number of normal wounds equal to the total number of critical damage effects generated.
Even if an effect is triggered that would convert normal damage into critical damage, if the only wounds inflicted are due to the minimum wound result, the wounds are all normal wounds.