How should life testing be specified for industrial heating elements?

Specify life testing against the real duty first: sheath diameter and material, surface load and watt density, medium type and flow rate, switching or on-off frequency, protection rating and mounting constraints such as insertion depth, support spacing and vibration. Continuous energisation alone proves little, so the supplier must state test temperature, cycle count, sample size and failure criteria like leakage, insulation resistance drop or power decay, with the control method and thermocouple placement recorded. Accept against measured wattage, insulation resistance and sheath temperature curves; any deviation beyond the agreed tolerance is a reject.