The Transmission Grid 'A' consists of a transmission grid installed in a full-height EMS module. This module is used to open and close an ac power circuit under normal conditions or to interrupt this circuit under fault or abnormal situations. It can also be used for the connection of EMS motor/generator sets and loads as well as the protective relaying equipment for transmission grid protection. The transmission grid is opened at intervals by eleven three-pole relays with normally open (NO) contacts. These relays can be operated by switches located on the module front panel or through the action of dc control relays or protective relays as part of a protection scheme. The module has a dc bus, energized from an external dc power supply, to distribute power to the relay coils.