4-channel user/main station with 4A power supply. It fits in a standard 19-inch equipment rack and is one rack unit high. It can communicate with 4 intercom channels. Instantaneous auto reset (IAR), the newest technology in performance and safety, which uses a revolutionary new circuitry that dynamically monitors line fault conditions. Then, when the fault is removed, automatically brings individual power supply channels up. It includes listening with external powered speakers.