Protective sleeves prevent the resistance temperature detector and thermocouple from coming into direct contact with voltage. They feature characteristics including resistance to mechanical shocks such as vibration and impact, heat resistance, airtightness, etc. Protective sleeves are either metallic or non-metallic according to the material used, and the non-metallic types are further divided into resin and ceramic types. Metallic protective sleeves are mainly used as electrical conduits for non-PSE electrical appliances. Resin protective sleeves feature excellent corrosion resistance and are suitable for use with chemicals, and ceramic protective sleeves feature excellent heat resistance and are suitable for use with high-temperature thermocouples. Protective sleeves function differently according to both material and shape, such as flanged types and screw-in types, and must be selected according to operational environment and application.