De-Energized Lines Can Still Start Fires. Understanding The Risks and The Effects of Grounding

The fire risk associated with energized transmission and distribution lines is well known, and protection tripping to de-energize a line is commonly viewed as the method that eliminates the electrical energy that can cause combustion. In recent years some utilities have been proactively de-energizing lines during extreme wind events to prevent the possibility of sparking catastrophic wildfires. This is often referred to as Public Safety Power Shutoff (PSPS). It is common to refer to a line as de-energized once the voltage sources have been removed by opening their respective breakers, but due to the proximity of adjacent energized lines these “de-energized” lines can carry appreciable voltages that can present both a shock hazard and a fire hazard. The risk associated with catastrophic wildfires caused by utility infrastructure is of paramount importance to many utilities, and the purpose of this paper is to bring an awareness of the wildfire risk associated with de-energized lines. At the author’s utility many cases of broken hardware, downed conductors, and vegetation contact have been documented on lines during these high wind related PSPS events. Due to the concern over possible ignitions caused by de-energized lines a proposal was made to intentionally ground lines cleared for PSPS events or long duration clearances. Studies were conducted and field measurements made on cleared lines to quantify the relative risks of intentionally grounding a line versus leaving it ungrounded in the event that hardware failure or vegetation contact results in a path of current flow to ground that can spark a fire. Studies have shown that as little as 10 mA of current can ignite a fire. The unique electrical properties of the two distinct mechanisms by which de-energized lines can carry voltage will be discussed. The first is the voltage developed by the electric field dropped across the mutual capacitance between the energized line, de-energized line, and earth, and its magnitude is related to the voltage level of the energized line. The second is the voltage induced in the de-energized line by the magnetic field associated with the current flowing in the energized line. The risks associated with grounding a de-energized line versus leaving it ungrounded will be discussed, along with the results of actual field testing on de-energized lines and EMTP simulations. The often unappreciated paradox between Faraday’s Law, which states that the voltage around a closed loop is equal to the time changing magnetic flux inside the loop, and Kirchoff’s Voltage Law, which states that the voltage around a closed loop sums to zero, will be discussed, with the analysis of induced voltages on de-energized lines serving as an excellent example to reconcile this paradox.