What are drooping signals?
In single-wire transmission, heat causes the transmission wire to stretch or shrink, and this can result in an incorrect indication of the signal aspect. For instance, the most restrictive aspect may end up being below the horizontal in upper-quadrant signalling on a hot day – this is termed a drooping signal when the angle is more than 5 degrees. Wire adjusters are provided to compensate for temperature variations. The problem is minimized in double-wire transmission as there is positive movement of the wire in each direction and the wire remains in tension at all times.
