The first of the two previous papers discussed the general principles of electrical control of dangerous machines and the second their particular application to interlocked guards. In this paper are examined various methods of remote supervisory control developed for large power systems but also used for other industrial processes, such as the distribution of gas and railway signalling.The paper is not concerned with details of construction, and is not intended to advocate any particular system or arrangement, but it analyses the underlying principles and assumptions of a number of different methods and attempts to deduce general requirements which all systems should fulfil.In introducing this subject, the author would point out that where opinions are expressed they are his own and do not necessarily represent the considered policy of his department. They are subject to modification in the light of further information or experience.Some suggestions and comments which are deliberately controversial or go beyond current practice have been included with the object of promoting discussion.In his choice of graphical symbols the author has not followed B.S. 530: 1948 exactly, but has used symbols which he hopes will be easily recognized by power engineers, to whom the paper is primarily addressed.