Which of the following is not a common cause of single-phasing?

Harmonic distortion is characterized by the presence of frequencies in the electrical system that are multiples of the fundamental frequency. It can lead to various power quality issues, such as overheating and equipment malfunction, but it does not directly cause single-phasing. Single-phasing typically occurs when one phase of a three-phase power system becomes inoperative while the other two phases remain functional.

In contrast, conditions such as blown fuses, lightning strikes, and mechanical failures can directly interrupt the continuity of a single phase, leading to a situation known as single-phasing. Blown fuses can disconnect a phase entirely, while lightning strikes can damage a power line, and mechanical failures in switching gear can lead to phase outages.

Thus, the identification of harmonic distortion as not being a common cause of single-phasing emphasizes its nature as an issue related to power quality rather than direct operational failure in the supply of three-phase power.