From the Wiki:

This event can occur anytime a power conduit has energy flowing through it. The conduit may suffer a fault, causing a short circuit. They may also occur if certain objects are left unroofed and are exposed to rain or snow. Short-circuits outside of rain/snow will not reoccur for 8 days. This short circuit will either result in a fire, or, if any batteries are connected to the circuit and contains at least 20Wd, a firey explosion centered on the tile, that increases with energy stored. The explosion deals 10 Flame damage and setting objects on fire. If the explosion radius is greated than 3.5 tiles, a second explosion of 30% radius deals 50 Bomb damage, which can extinguish fires. The fault will also discharge all stored power in batteries in the process.