Clearances of reinforced insulation shall be not less than those specified for basic insulation in Table 16, using the next higher step for rated impulse voltage as a reference.Compliance is checked by measurement. For double insulation, when there is no intermediate conductive part between the basic insulation and supplementary insulation, clearances are measured between live parts and the accessible surface, and the insulation system is treated as reinforced insulation as shown in Figure 11.