ISO 23551-4:2018 specifies safety, constructional and performance requirements of valve-proving systems (VPS), intended for use with gas burners and gas-burning appliances. It also describes the test procedures for checking compliance with these requirements and provides information necessary for the purchaser and user.ISO 23551-4:2018 is applicable to all types of VPS which are used for the automatic detection of leakage in a gas burner section having at least two valves designed in accordance with ISO 23551-1 and which give a signal if the leakage of one of the valves exceeds the detection limit.ISO 23551-4:2018 is applicable to VPS with a maximum working pressure up to and including 500 kPa for use in systems using fuel gases.ISO 23551-4:2018 is not applicable to VPSs for use in explosive atmospheres.NOTE Provisions for production control are not part of the ISO 23551 series.