Abstract: Prolonged contact and friction between the moving contact of the HGIS disconnector and the edge of the contact fingers can lead to the accumulation of foreign particles. Under the influence of external forces such as electromagnetic force and airflow, these contaminants may migrate to the surface of the moving-side shielding. During disconnector operation, either closing or opening, these particles jump or fall off, resulting in SF₆ gas gap discharges between the shielding electrodes and the enclosure. This study investigates the foreign-particle-induced discharge phenomenon observed in the contacts of 750 kV horizontal-type HGIS disconnectors. Through inspection tests, sample analysis, fault replication, and simulation modeling, the underlying fault mechanisms are thoroughly examined and validated. By analyzing the impact of internal contaminants on the electric field distribution around the contacts, the paper proposes targeted improvements in structural design, component selection, assembly procedures, contamination removal strategies, and on-site installation practices. The findings provide critical guidance for the full life-cycle management of HGIS equipment, including specification, procurement, manufacturing, acceptance testing, installation, and operation and maintenance. Furthermore, this paper offers valuable insights for enhancing equipment reliability and minimizing failure risks.