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

High-temperature rating for industrial heating elements is set by the maximum sheath temperature actually reached, not by the medium or ambient figure. Define normal and peak duty, medium and atmosphere first, then choose sheath alloy, tube diameter and watt density accordingly — higher temperatures demand lower surface loading. Installation geometry, protection class and control accuracy also cap the limit and must be checked item by item. Ask the supplier for the design basis and measured data at that temperature, and verify sheath temperature and insulation resistance on site before acceptance.