Abstract: An electrical fault detection system includes an underground transformer unit having an enclosure and an electrical busbar element extending from the enclosure, and an acoustic sensor apparatus operatively coupled to an external structure of the enclosure or the electrical busbar element. The acoustic sensor apparatus is structured to: (i) detect an acoustic signal within the enclosure, (ii) analyze the detected acoustic signal and determine whether the detected acoustic signal is indicative of an electrical fault within the enclosure using an event time correlation (ETC) algorithm, and (iii) responsive to determining that the detected acoustic signal is indicative of an electrical fault, output a message indicating that a fault has been detected.