This paper presents an energy quality evaluation of an open pit mine distribution system. A number of operational disruptions affecting mine shovels have been related to energy quality. Previous investigations have highlighted the large presence of harmonics as the main factor responsible for these problems. The aim of this paper is to produce an energy quality baseline and a precise diagnosis of the source and nature of the energy quality issues based on standards and the susceptibility of different technologies used by the shovels at the site. The random nature of the load composed by multiple shovels and drills requires computer simulation to predict worst-case scenarios. For this purpose a voltage regulation and harmonic distortion study was conducted with the model parameters tuned by measurements taken in mobile substations of the mine energy loop. A major aim of this study is to establish the relationship between energy quality and energy efficiency.