This International Standard describes the requirements for resistance temperature detectors (RTDs) suitable for nuclear power plant (NPP) services. The requirements of RTDs include design, materials, manufacturing, testing, calibration, procurement, and inspection. In nuclear application, both “direct-immersion” and “thermowell-mounted” RTD are commonly used; however, this standard does not exclude any other design of RTD which may be required for certain special applications in various types of reactors. RTDs can be supplied in different internal constructions, which depend on the manufacture, qualifications, and applications. For RTD being used in an NPP, the design and structure of the RTD should consider the environmental conditions in which the detector is being used under normal operating and under design basis accident conditions, as well as the qualification tests specified by the user 1. The use of a flexible mineral-insulated (MI) cable between the RTD and the connector is commonly adopted, and the user may also adopt any other construction. A variation of this design may include a rigid exterior sheath over the MI cable between the RTD and the connector, these being welded to each other. The scope of this standard does not cover the design, material selection, and construction of the thermowell, the guide tube, the extension cable, and the temperature transmitter or bridge which may be associated with the RTD.