The paper briefly reviews methods for the centralized control of switching operations on distribution networks, by injection into the distribution system of currents which differ in frequency from the main supply. The supsrimposition of ultra-audio frequencies for limited and specific purposes, such as the protection or the remote switching of a transmission line, has not, so far, been employed to any large extent in this country, and is therefore beyond the scope of the paper.Several devices employed to respond to injection currents are briefly described; and the probable applications of this method of control are outlined. Particulars of the development of remote signalling over the network of a large electricity supply undertaking are given, together with reasons for the final adoption of the method selected. Particulars of the components of the equipment are given, and the power required is analysed for two frequencies.