The isolating switch of some solid insulated ring main units (SIRMUs) features an air break. The term "air break" is used instead of "air insulation" due to its particularity in SIRMUs. While the insulating medium between the breaks is air, the insulating medium between the moving and static contacts is solid. Minute leakage currents may exist inside and along the surface of the insulating material, leading some to exaggerate it as a "false break." A method to avoid such a "false break" is to install a grounding electrode between the moving and static contacts, but this is often unachievable due to constraints in structure, space, and cost.
For an air isolating switch to achieve reliable isolation functionality, two requirements must be met: Firstly, the insulation withstand voltage test must comply with standard requirements, and the air clearance between the breaks must be adapted to the cavity environment (pollution level). Secondly, the overall insulation performance must be reliable, and the creepage distance along the insulation surface must also be compatible with the cavity environment.
Similar to the cavity design of the insulated pull rod in vacuum interrupters, the cavity design of air isolating switches requires careful consideration. The cavity should be effectively sealed to avoid the impact of atmospheric conditions and foreign objects. Open cavities with upward-facing openings are not recommended as they hinder air convection and make foreign object removal difficult. Hermetically sealed cavities filled with dry air or nitrogen can adapt to harsher environmental conditions.
This article is compiled and edited by DAYA Electric Group Co., Ltd. (https://www.hygraphics.com). Reproduction is permitted with proper attribution.