Abstract- A method is presented for detecting and identifying the shape and size of a contaminating metallic particle in gas insulated system (GIS). A fixed particle–initiated negative corona in air insulated co-axial cylindrical configuration is investigated at a voltage slightly above the corona onset level. The characteristics of the negative corona pulses (Trichel pulses) are calculated by mathematical modeling the process taking place during the negative corona discharge. An experimental set-up is built up to measure the Trichel pulse characteristics and to check the accuracy of the present calculation. The calculated values of corona Trichel pulse amplitudes and repetition rates agree well with the measured values. So, the Trichel pulse characteristics are calculated for different particle sizes and shapes. These data of pulses are used as a bias for designing and training the artificial neural network technique (ANN). For earlier detection of insulation defect, the Trichel pulse characteristics are measured and then given as an input data to the trained ANN. Then, we could identify the particle shape and size.