About the electric pen

A search on Baidu yields tons of information on “the working principle of a voltage tester”: lol

Principle of a voltage tester: 1. Ohm’s law: I=U/R； 2. The formula for the total resistance in a series circuit: R = R1 + R2 ； 3. The structure and characteristics of a voltage tester. A. When a voltage tester is used to determine whether a conductor is live wire or neutral wire, the current flowing through the voltage tester (i.e., the current flowing through the human body) I is equal to the total voltage U applied across the voltage tester and the human body, divided by the total resistance R between the voltage tester and the human body. B. When testing the live wire, in an lighting circuit, there is a voltage of around U=220V between the live wire and the ground. The resistance of the human body is generally very low, usually only a few hundred to a few thousand ohms, while the resistance inside a voltage tester is typically several megohms. The current flowing through the voltage tester (and thus through the human body) is very small, usually less than 1 milliampere. Such a small current does no harm to the human body, and it causes the neon bulb in the voltage tester to glow. C. When testing the neutral wire, U=0 and I=0; in other words, no current flows through the neon bulb in the voltage tester, so naturally the bulb does not glow. In this way, we can determine whether it is the live wire or the neutral wire by checking whether the neon bulb lights up. D. The principle is exactly the same when measuring high-voltage lines; it’s just that the internal resistance of high-voltage voltage detectors is higher, and with the human body’s resistance in series, the current flowing through is usually less than one milliampere. For 35 kV and below ungrounded neutral systems, the zero-sequence current is measured. For 110 kV and above neutral point systems, the current from the phase wires to ground is measured.