How should dry-run protection be specified for commercial heating elements?

Size dry-run protection from the sheath-temperature rise once the medium is lost. Base the allowable wall temperature on watt density, rated power, tube diameter, surface load and heat dissipation, then match sheath material and terminal-box IP rating to site water quality and corrosion conditions. Interlock the low-level switch, main temperature control and an independent over-temperature cut-out; a dry condition must trip the heater immediately without automatic reset, preferably manual reset. During acceptance, test low-level, full dry-run and control-failure states, recording trip temperature, response time, reset mode, mounting geometry and electrical parameters, and confirm the sensor sits below the minimum liquid level.