PG 34 of the Manual:
The attack has to deal at least one point of damage to the Wizard in order to trigger and use the shield, and this
damage is counted after armor has already taken effect. If the attack deals no damage to the Wizard (for example, all damage is absorbed by armor, or the attack is blocked by a Defense), the shield is not used.
So if the roll has both Normal and Critical Damage your armor will absorb the normal damage, up to the amount of armor, after that up to the next three points of damage (including critical) are absorbed by the Voltaric Shield.