An arcover is the unwanted arcing of electricity from one component to another.
The resistance of an air gap is effectively infinite until it reaches its arcover threshold voltage; then it drops to near zero as the air in the gap ionizes. It returns to effectively infinite when the current through the gap is insufficient to keep the air ionized.
Use a megger or a insulation tester. Place one lead on any of the phases nd other lead the alternator boy. Observe the indicator. In megger it s good with infinity or it must be minimum 100M OHM, IF IT SHOWs 50 it could be coz of moisture in winding by heating the alternator the insulation resistance value may improve