Abstract: The safety structure of a water heater powered by both photovoltaic-supply and mains supply should consider the safety isolation between the photovoltaic-supply and the live parts of mains supply circuit when the photovoltaic-supply is safety extra-low voltage. Necessity and realizing method of linkage disconnection of double-circuit thermal cut-out of dual power supply water heater. Cautions in design of control circuit of dual power supply water heater. Dual power supply water heater’s working mode.

Key words: Dual power supply, Photovoltaic-supply, Thermal cut-out, Linkage disconnection, Safety isolation, Safety standards