Class I: Using basic insulation between primary and secondary against an electrical shock from dangerous voltage.
Besides, it is necessary to provide a safety ground that satisfies ground safety principles for resistance. When the basic insulation fails, the safety ground will receive the dangerous voltage.
Class II: Using double insulation or reinforced insulation between primary and secondary against an electrical shock from dangerous voltage. If one of the reinforced insulation and double insulation fails, Class II will still prevent the risk from electric shock without earth ground.