Reasonably Designing the Insulation Grounding Structure of the Greentel DOME Fiber Closure: Insulating spacers conventionally used in fiber optic splice closures typically consist of two spacers. The upper washer is a grooved washer and the lower washer is a protruding washer. Since the fixing bolt is connected with the reinforcing core of the optical cable, the upper and lower insulating spacers can be combined to form the insulation between the metal component of the fiber optic splice closure and the cable reinforcing core . In the compressive strength test, a 15 kV DC voltage is applied between the fixing bolt and the metal part. Most of the materials used for insulating gaskets are high-molecular polymers. The dielectric strength of these materials is relatively high, typically 500 to 1000 megavolts/meter. Therefore, theoretically, only 0.1 mm of polymer-based insulation between two electrodes provides 15 kV protection. However, in fact, the insulating sheet used in the fiber optic splice closure cannot be complete. Since the structure is determined by the fact that it must be superposed by two parts, there must be a gap, in which case the gap is inevitable. Air is present, so at the same time the dielectric strength of the air and possible air and creeping discharges must be taken into account. However, since the air passage existing in the gap is not a straight line, but is a polygonal line or a curved line, the distance between the shortest air passages between the two poles is the most dangerous distance. Therefore, the tested insulation performance when there is a polymer insulating material is actually the shortest distance test of the gap pipe in the two pieces of insulating sheet. According to the standard, fiber optic splice closure in front of the voltage resistance strength test must be in 1.5 meters deep water soak for 24 hours, so if the seal is not good, lead to water vapor infiltration, will inevitably affect the voltage resistance. However, the probability of this situation is relatively small compared with the above mentioned situations, so it is appropriate to pay attention to it.