Does Anyone Know What A High-Impedance Fault Is?

Protection engineers have wrestled with the challenge of clearing earth faults for over one hundred years. On four-wire multi-grounded systems, like those used in much of North America, earth faults frequently produce currents with magnitudes lower than inrush transients from single-phase connected loads, making detection even more challenging. Even on three wire systems, however, detecting and clearing earth faults rapidly and cost-effectively remains a major challenge.

In recent decades, the term “high-impedance fault” gained widespread usage. Despite being commonly used, the term is often used imprecisely and frequently creates confusion (often unknowingly!) in discussions among practitioners. When used colloquially, protection engineers usually appear to mean something like an energized downed conductor or possibly a tree contacting a line. More generally, the term appears to mean something like, “faults that are hard to detect” – a vibes-based definition not out of line with a 1996 report produced by the IEEE Power System Relaying Committee which defined the term as faults which, “do not produce enough fault current to be detectable by conventional overcurrent relays or fuses.” Compounding this challenge is that so-called “high impedance faults” are not a homogeneous class of events, but rather a diverse set of conditions which share some characteristics in common but diverge drastically in others. While researchers and practitioners would generally agree with the previous statement, it does not stop them from casually lumping all such events together.

This paper draws on over forty years of research experience on operational distribution circuits, conducted by the Power System Automation Laboratory at Texas A&M University, with actual examples of events which might reasonably be called "high-impedance faults," to argue that more precise language around the types of events protection engineers are interested in detecting and clearing – rather than resorting to a catch-all, poorly defined category – is an important step toward effective solutions.