The current transformer ratio should be selected according to the primary load calculation current IC. The standard ratio of the primary side rated current of the current transformer (such as 20, 30, 40, 50, 75, 100, 150, 2×a/C) and other specifications, the secondary side rated current is usually 1A or 5A. Among them, 2×a/C means that the same product has two current ratios, which can be realized by changing the connection mode of the product. When connected in series, the current ratio is a/c and when connected in parallel, the current ratio is 2×a/C. Under normal circumstances, the selection of the conversion ratio of the current transformer for measurement should be such that the primary rated current I1n is not less than the load current in the line (that is, the calculation IC). If the calculated current of the load in the line is 350A, the current transformer ratio should be 400/5. In order to ensure the accuracy requirements of the current transformer for protection, the transformation ratio can be selected larger.