How should leakage current be specified for industrial heating elements?

Specify leakage current from real operating conditions, not a catalogue figure. Define voltage, medium, sheath diameter and surface load, then agree cold and hot insulation resistance, test voltage and the leakage limit. In multi-element assemblies, measure each element individually; a total-circuit reading hides defective units. Judge results against temperature, humidity and insulation temperature class. Never raise a protective device threshold to mask insulation weakness. State test conditions and pass/fail rules in the specification, and verify deliveries under those same conditions.