Depending on its application, equipment is classified in insulation groups Ao , A , B , C, D relating to reduction in the performance of insulation caused by environmental influences, e. g. dust, dirt, humidity, condensation, ageing and aggressive ambient particles. In addition, these groups are determined both by the effects of damage produced by failure of the performance of an insulation material and also flashover voltages.
Generally, classification of equipment into the various insulation groups will be made in the appropriate VDE specifications by the relevant VDE commission.