This part of ISO 23551 specifies safety, constructional, and performance requirements for thermoelectric flame supervision controls, energized by a thermocouple, intended for use with gas burners and gas-burning appliances, hereafter referred to as “controls”. This part of ISO 23551 applies to thermoelectric flame supervision controls for gas burners and gas-burning appliances of nominal connection size up to, and including DN 50, that can be used and tested independently of these appliances. These thermoelectric flame supervision controls are suitable for fuel gases, such as natural gas, manufactured gas, or liquefied petroleum gas (LPG) at inlet pressures up to and including 50 kPa. This International Standard covers type testing only. This International Standard is not applicable to a) the thermocouple and b) controls which use auxiliary energy (e.g. electrical energy supplied externally).