Refers to 5 categories of insulating material for resistance to acid, oil, ozone and combined resistance at low temperatures.Cross References:ISO 2592:1973ISO 2977:1974ISO 3104:1976 IEC 60050-121:1978IEC 60050-151:1978IEC 60050-601:1985IEC 60060-1:1973IEC 60212:1971IEC 60410:1973IEC 60417IEC 60707:1981CENELEC HD 437 S1:1984EN 60417CENELEC HD 441 S1:1983ENV 50196:1995