How should sample approval be specified for industrial heating elements?

Sample approval must be tied to the real duty rather than the catalogue label. First lock in rated wattage, sheath diameter and material, surface load and cold insulation resistance; then check mounting thread or flange hole spacing, terminal clearance, lead length and ingress protection, and verify the thermostat and thermal cutout set points and reliable reset. High-temperature, corrosive or vibrating service calls for an added heat-cycle, immersion or vibration test, with the temperature rise curve and leakage current recorded. Once approved, sign off the measured values, tolerances, test conditions and the controlled drawing together, and procure spares only to that signed revision; any change requires the supplier to resubmit samples for approval.