Immersion Heater. Backersafe Immersion Heaters are manufactured to the latest Legislation of BSEN60335-2-73. Two specific clauses within this specification are aimed at eliminating the risk of boiling water. The most significant requirement is that the Immersion Heater be isolated from the mains supply, should the primary thermostat fail with the contacts closed, which would allow the water to reach a temperature in excess of 85°C.