Discussion on “localizers, suppressors and experiments” (Creighton and Whittlesey), and “relay protective systems” (Elden), Boston, Mass., June 28, 1912. (see proceedings for July, 1912)

D. W. Roper: Referring to Fig. 1 of Mr. Elden´s paper where an outline of the Merz-Price system is shown, there is indicated a time limit overload relay in a generating station, in addition to the Merz-Price balance relay at both ends of the line. I do not quite understand the application of the overload relay at that place. As I understand the Merz-Price balance protection scheme, the object of it is to open up the switches on the two ends of a faulty section of cable so that this faulty section will be disconnected and allow the remaining sections to continue in operation. Later in the paper, it apparently indicates that the object of the overload relay is to take care of the trouble from a switch failure or short circuit on the station busbars, which are, as the diagram shows, obviously not protected by the Merz-Price balance protection system. If I am correct in my interpretation of these relay diagrams, I would like to learn how to set an overload relay so that it will not open in case of a cable breaking down, and have it open when the short-circuit occurs on the station bus-bars. If that scheme of discrimination can be carried out it certainly is very effective. The paper states that the operation of the Merz-Price system is a protection against faults in any part of a system, thereby permitting the operation of momentary and continuous overloads at the discretion of the operator. Apparently that wording, if I understand it correctly, refers to a system in which there is no synchronous apparatus, because synchronous apparatus would be thrown out of service by the heavy short circuits which that is apparently intended to cover. If that is the case I think it should be made more apparent because it is a very essential item in designing relay protection systems.