This work models the performance of Mobile particles in compressed gas-insulated substations under an intentionally applied impulse voltage of reduced magnitude whereby the subsequent microdischarges can be used to detect the presence of particles and identify their approximate sizes. A comprehensive computer model is described which develops and interrelates the following : potential and field distributions on and around the particle, particle lifting and movement, timing of the initial microdischarge, and the complete pattern of the microdischarges as may be detected by an external monitoring circuit.