This part of ISO 15500 specifies tests and requirements for the pressure relief valve (PRV), a compressed natural gas fuel system component intended for use on the types of motor vehicles defined in ISO 3833. This part of ISO 15500 is applicable to vehicles using natural gas in accordance with ISO 15403 (mono-fuel, bi-fuel or dual-fuel applications). It is not applicable to the following: a) liquefied natural gas (LNG) fuel system components located upstream of, and including, the vaporizer; b) fuel containers; c) stationary gas engines; d) container mounting hardware; e) electronic fuel management; f) refuelling receptacles. NOTE 1 It is recognized that miscellaneous components not specifically covered herein can be examined to meet the criteria of this part of ISO 15500 and tested according to the appropriate functional tests. NOTE 2 All references to pressure in this part of ISO 15500 are to be considered gauge pressures unless otherwise specified. NOTE 3 This part of ISO 15500 is based upon a service pressure for natural gas as a fuel of 20 MPa [200 bar1)] settled at 15 °C. Other service pressures can be accommodated by adjusting the pressure by the appropriate factor (ratio). For example, a 25 MPa (250 bar) service pressure system will require pressures to be multiplied by 1,25.