When using overhead cables, the voltage to ground is not entirely applied to the cable insulation; it is largely borne by the air. Therefore, overhead cables have higher insulation performance, significantly better than ordinary paper-insulated cables and cross-linked cables. This is because the lead sheath of paper-insulated cables and the shielding layer of cross-linked cables are usually grounded, so the voltage passing through the cable is entirely applied to the main insulation. In overhead cables, however, the voltage is shared by the main insulation and the air.
Therefore, relevant standards in various countries stipulate that the insulation thickness of overhead cables should be thinner than that of ordinary underground cables. Generally, in Europe, America, and Japan, the insulation thickness of overhead cables is 10-20% thinner than that of ordinary cables. Furthermore, the insulation quality of overhead cables does not significantly deteriorate under long-term voltage exposure, unlike buried cables which can develop water treeing over long-term operation. When water treeing reaches saturation, the cable breakdown voltage drops by 1/3 to 1/2, while overhead cables generally do not develop water treeing. Therefore, overall, overhead cables have superior insulation performance.
