This standard specifies the insulation requirements and corresponding insulation tests for specified windings and their terminals. This standard also recommends the air gap between the live part of the oil immersed power transformer bushing and the grounding body (Chapter 16). Guidelines in this regard can be found in IEC 60071. The insulation level and insulation tests specified in Chapters 4, 5, 6, and 7 of this standard are only applicable to internal insulation. Although it is reasonable to use the rated withstand voltage specified for the internal insulation of transformers as the reference for external insulation, this approach may not be in line with all practical situations. The failure of non self restoring internal insulation is catastrophic and usually leads to the long-term shutdown of the transformer. However, when external flashover occurs, it may only cause a brief interruption of operation without causing permanent damage. Therefore, in order to improve safety, users can specify that the test voltage value of the internal insulation of the transformer is higher than that of the external insulation of other equipment in the system. When there is such a difference, the outer insulation gap distance of the transformer must be adjusted to fully withstand the test requirements of the inner insulation