A sensor for use in installing a plastic service pipe inside a steel service pipe connected to a main includes a flexible element which is fed through the steel pipe and through the plastic pipe. The element has at its remote end two light-emitting diodes and two photoconductive cells. The flexible element and the pipe are fed together into the steel pipe. The sensor indicates when the cells have entered the main because the intensity of the light reflected to the cells falls to a low value and gives an audible or visual signal. This also indicates that the end of the plastic pipe which carries a seal is positioned correctly and that sealant can be injected into the space between the steel pipe and the plastic pipe. After the sealant has set the sensor can be recovered by withdrawal through the noise-piece and through the pipe.