https://blog.gongquiz.com/2021/08/27/ks-c-iec-60364%ec%97%90%ec%84%9c-%ec%b6%a9%ec%a0%84%eb%b6%80-%ec%a0%84%ec%b2%b4%eb%a5%bc-%eb%8c%80%ec%a7%80%eb%a1%9c%eb%b6%80%ed%84%b0-%ec%a0%88%ec%97%b0%ec%8b%9c%ed%82%a4%ea%b1%b0%eb%82%98-%ed%95%9c/