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The piercing type current transformer operates on a principle similar to that of other current transformers; therefore, its secondary side still cannot be open-circuited. Here I will explain the principle of current transformation in current transformers. In fact, the principle behind current transformers is similar to that of voltage transformation in voltage transformers. The voltage ratio resulting from high voltage in a voltage transformer is equal to the number of turns in the high-voltage winding divided by the number of turns in the low-voltage winding; in other words, the voltage ratio equals the turn ratio. The primary side of the current transformer has very few turns, usually just one wire, while the secondary side has many turns. Based on the principles of voltage transformers, if a low voltage is induced on the primary side, then an extremely high voltage will be induced on the secondary side if that side is open-circuited. However, the voltage on the secondary side must not exceed 2000 V; therefore, it is absolutely not permissible for the secondary side to be open-circuited. From another perspective, since both the primary and secondary sides are connected in series to form a circuit, the voltage induced on the primary side is equal to the resistance of the primary side multiplied by the current flowing through it. The current on the secondary side is equal to the voltage induced on the secondary side divided by the resistance of the secondary side. When a large current flows through the primary side, the voltage induced on the primary side increases because it is equal to the resistance of the primary side multiplied by the current flowing through it. As the primary voltage increases, the voltage induced on the secondary side also increases. And since the current on the secondary side is equal to the voltage divided by the resistance of the secondary side, the current induced on the secondary side as well increases. As can be seen from the above, the factor that determines the primary side is current; therefore, changes in the primary voltage have no effect on the secondary current. As long as the insulation capacity of the primary side can withstand such voltage levels, it is possible to measure currents of any voltage level