UNIT-AP Arc Fault Protection Device is intended for the use with 6-110 kV switchgear and is used for detecting short-circuits, accompanied by an open electric arc and issuing a signal to shut off the emergency section without time delay for the operational elimination of the arc fault. UNIT-AP includes a microprocessor arc protection unit as well as the sensors-flexible fiber-optic cords of small diameter, resistant to electromagnetic interference and sensitive to pulses of radiation in the optical wavelength range. Due to the sensitivity of the entire length, the sensors do not lose the ability to detect the arc on condition of severe contaminination and even the sensor breakage.