High-power electrical machines employed in mining and power generation are critical systems that require (long lifetime) high reliability and availability. That is why proper specification, design, operation and maintenance are needed together with condition monitoring and diagnostics. Concerning the application, it must be considered a comprehensive system design, including manufacturing, testing, installation, operation and maintenance, regarding dielectric strength of insulation, materials, structural and mechanical fixations for ensuring expected lifetime. One key issue is the insulation aging because of thermal, mechanical, electrical and chemical factors. Any detrimental effect is progressive and affects lifetime of the machine insulation. Partial discharges activity is one sign of aging of insulation, stating the needs of proper monitoring and diagnostics regarding the insulation condition. There are important experiences with bus-connected machines, but relative few documented experience regarding mining applications. This paper gives a review and discussion of the main topics and recommendations given by the industry practice and by standards with the objective of helping engineers to identify course of actions for ensuring better reliability, of high power electrical machines employed in utility and mining applications.