Although the Gas Insulated Switchgear (GIS) reliability has been proven for more than forty years, some failures have been reported. CIGRE report 25/23-01 mentioned the failures caused by free particles and foreign bodies represent 20% of the total distribution of defect type in GIS failures. The presence of these particles results in a local field concentration which influences the insulation medium and can result in breakdown. Under AC voltage, particles can start “moving" or "jumping" into higher electric field regions. Breakdown may occur if stressing overvoltages, e.g. AC + VFTO, DC + VFTO and AC + LI would occur at the particle get close or attached to the HV conductor In thesis the influence of free moving particles on the breakdown strength in GIS under different voltage stresses are investigated. For this purpose Different GIS test setups have been used. Particles lengths ranging between 2 to 30 mm have been investigated at 4bar gas pressure. The results have shown that the particles size of 5 mm and longer can start jumping under AC voltage, while shorter particles were tend to show only a little movements The visual observations have shown that the particles were moving towards the HV electrode and solid insulators inside the GIS. Therefore the particles as small as 5 mm are considered dangerous to the insulation system under continuous operating AC voltage. A breakdown area have been distinguished based on the particles length under AC + LI. the particles of 15 mm length and longer are critical under AC + LI. The investigation under AC + VFTO has been performed with the maximum peak 2.1 p.u. At this level, the breakdown has been observed with the 30 mm particle at 90% of the distance gap. During the experiments partial discharge measurements have been performed by means of: • the conventional method (IEC 60270) • the UHF method • the acoustic method The PD results have shown that some defects can cause a PD level higher than 10pC. Such PD level gives a good possibility to detect a defect.