In the last decade, compressed Gas Insulated Substations (GIS) and compressed Gas Insulated Bus Duct (GIBD) have been already developed with using various SF6-gas mixtures in order to reduce the disadvantages of using pure SF6- gas. The presence of contaminating particles inside gas insulated bus duct is one of the things that affect on the insulation design. In this paper, the Finite Elements Method (FEM) is used to evaluate the electric field distribution on and around multicontaminating wire particles with presence of different shapes of insulating spacers such as di-post and tri-post spacer inside gas insulated bus duct. With di-post spacer, the effect of two protrusions rested on the coating of earthed enclosure of GIBD on the electric field values is studied. The effect of length and radius of one protrusion at constant length and radius for another protrusion and also with constant spacing from insulating spacer on the electric field values are studied. With tripost spacer, the effect of three stick particles at different height from earthed enclosure on the electric field values is presented. Breakdown voltage calculations for various SF6-gas mixtures around multi-contaminating wire particles and protrusions with various shapes of spacers are carried out. The effect of gas pressure for all cases which mentioned above on the breakdown voltage is studied. The optimum gas mixture which gives higher dielectric strength with lower cost is determined. Finally, the percentage of reduction in breakdown voltage due to presence of two ground protrusions with different cases is carried out.