How should insulation resistance be specified for commercial heating elements?

Specify insulation resistance for commercial heating elements from the actual duty rather than catalogue values. State the DC test voltage, commonly 500 V DC, or 1000 V DC for damp, condensing or higher-voltage service, plus the stabilization time, such as a one-minute reading, and the minimum acceptable value, for example 50 MΩ or the supplier standard, then record it on the drawing and acceptance sheet. Selection should also consider rated power, sheath diameter and length, sheath material, watt density, sealing method, protection class, terminal type, mounting orientation, and exposure to moisture, washdown or process fluids. Where a thermostat or thermal cut-out is fitted, note how its trip temperature and reset method affect insulation. Require batch test records, and re-test stock stored in humid conditions before installation, since unused elements can still absorb moisture and read low.