Over eight years of development, and broad industrial applications, Industrial Research Ltd (IRL) has implemented fast non-invasive portable on-line partial discharge (PD) measurement for insulation assessment of various power devices.1-4 The portable on-line PD measuring technique provides an efficient means to achieve cost-effective insulation diagnosis for the increasing large number of aged power devices widespread the systems. The technique allows industries to protect their main assets by implementing predictive and condition-based maintenance. Decisions as to whether to repair or replace million dollar assets can be made according to insulation assessments based on sound scientific grounds. This paper illustrates how the portable on-line PD measuring technique has been broadly applied for large machines, cables and especially aged indoor metalclad switchboard insulation assessment. The rationales, economic benefits and findings of the extensive on-site insulation assessment are given and discussed. The advantages of the portable device and the non-invasive PD sensor positions to couple PD signal for different equipment are shown in many on-site applications.