How should high-temperature insulation be specified for industrial heating elements?

Specify high-temperature insulation by actual duty rather than catalogue labels. First confirm cold insulation resistance, dielectric withstand, and the minimum hot insulation resistance at operating temperature, then relate sheath temperature limits to diameter, sheath material, watt density, power, mounting, and control method. Thermal cycling accelerates ageing, so for continuous high-temperature service require the supplier to provide hot insulation test data and cycle-life evidence. At acceptance, record cold and hot readings, test conditions, tolerances, and check them against the approved drawing.