Introduction on How to Check for Leakage with a Digital Multimeter
Set the digital multimeter to the AC-750V range. As shown in the attached diagram, touch one test lead to the metallic coating of the power socket, and touch the metallic part of the other test lead with your hand. The multimeter clearly shows a voltage value of 119V! This indicates that the metallic coating of the socket is indeed electrified. Upon inspection, it is found that the fixing screw of the socket has broken through the plastic sheath of the live wire and is in contact with the live core. This screw presses on the metallic coating, making the socket electrified. The internal resistance of the voltage range of the digital multimeter is mostly 10MΩ. The human body and the ground can be equivalent to a resistor (the series resistance of shoes and wooden flooring) in parallel with a capacitor (the distributed capacitance between the human body and the ground), as shown in the attached diagram. The voltage on the live wire passes through test lead 1, the multimeter, test lead 2, the human body, the DC resistance of the wooden board, and the distributed capacitance of the human body to the ground. The value displayed by the multimeter is across the internal resistance of the multimeter. Where UDY is the voltage of the live wire to the ground, RB is the internal resistance of the voltmeter, and Z is the total impedance between the human body and the ground. Obviously, the larger the internal resistance of the multimeter, the higher the sensitivity of the electrification test. It must be pointed out that for the above electrification test method, there must be current flowing through the human body for the multimeter to give an indication. Undoubtedly, this current should be small enough so that the human body cannot feel its presence. The digital multimeter has an internal resistance of 10MΩ. Even if standing barefoot on a wet ground, the current flowing through the human body is as follows. When the voltage of the live wire is 220V, the effective value of the current flowing through the human body is only 22μA. An analog multimeter generally cannot be used for electrification testing because the internal resistance of the voltage range of such a multimeter is much smaller than 10MΩ. Therefore, the sensitivity of the electrification test is very low. It is wrong to attempt to use a small voltage range to improve the sensitivity of the electrification test because the internal resistance of a small voltage range is usually also small (the internal resistance of the voltage range of an analog multimeter is the ohmic value per volt multiplied by the voltage range), and the indication sensitivity is hardly improved. Instead, there is a higher risk of electric shock accidents.
