Abstract: Fires caused by photovoltaic (PV) systems occur from time to time. The special structure of PV systems, difficulties in background data analysis and diverse fault modes bring significant challenges to fire investigation. Starting from a fire case involving a rooftop PV system of a factory building, this paper identifies the fire origin and cause through on-site trace analysis and physical evidence collection, and further verifies the accident cause via a 1：1 scale simulation test. The study reveals that fires triggered by arcing due to poor contact at single-phase connection terminals of PV systems exhibit the following characteristics: arc marks are formed at the contact points of single-phase terminals, and abnormalities occur in the background operation data, such as fault alarm information related to voltage, current, and leakage current.

Key words: photovoltaic (PV) system, poor contact, arc mark, background data, fire investigation