The UK-4 is designed for visual control of 12 V supply voltage in 4-wire fire alarm loop, and also disablement of terminal resistor in signal circuits of the loop during power outages. The UK-4 provides galvanic uncoupling of power circuits and signal circuits using a transistor photon-coupled pair. The FAULT signal is issued in AC/DC signal circuits of fire alarm loop depending on the way of connection of signal circuits to output terminals.