Insulation temperature standards for industrial-control coils

FOR many years, American Standard C19 (Underwriters Laboratories, Inc. issue of 1938 and National Electrical Manufacturers' Association industrial-control standards publication 40–59, April 1940) for industrial-control apparatus has specified an allowable temperature rise for industrial-control coils with class-A insulation of 85 degrees centigrade by resistance. Although this is 25 degrees centigrade higher than the value of 60 degrees centigrade rise by resistance established for rotating machinery with class-A insulation, service experience with control coils has been thoroughly satisfactory. It is estimated from field experience that not more than five per cent of all coil failures have been caused by temperature, despite the frequent use of above-normal voltages. The author believes, therefore, that the present control standard has proved to be conservative with respect to temperature-rise limits.