Why is there more than 20 volts when the multimeter is used to measure the voltage when there is no power in the socket?
This is induction electricity.
Impact: It will not affect ordinary devices and can be ignored. Special precision equipment may cause component damage and requires isolation.
Reason: Mutual inductance caused by parallel laying of other live power cables
The power socket is out of power, and there is a voltage of over 20 volts measured with a multimeter. This situation is usually caused by poor contact of the live wire of the AC power. Recently, Mr. Tony's power socket in our village experienced this situation.
Wiring of power sockets.
Mr. Tony is a small farmer who is skilled in farming, but he knows nothing about electricity. I measured the voltage of his home socket with a multimeter and it was only over 30 volts. Based on experience, if the voltage on the socket is too low to reach the rated value, it is usually due to a problem with the live wire contact. So I focused on checking the live wire in his house, searching and searching, and found that the screw connecting the air switch to the live wire was a bit loose. Tony told me that last month the live wire was about to fall out of the screw hole of the air switch, and he himself plugged the wire in. I tightened the screw with a screwdriver and measured that the voltage of the socket had risen to 100V, and the LED indicator light on the socket also lit up.
The LED indicator light on the socket is on.
Tighten the screws and the voltage rises to 100V, but it still cannot be used. I continued to search and found that there was an electrical tape wrapped around one of the wires on the courtyard wall. Tony told me that last year, many mice crawled along this wire to the house in the backyard where the food was stored to steal food. Later, he blocked the hole in that house, which may have been inaccessible to the mice. He was very angry and bit through the wire.
I checked and found that the interface of this 2.5-square aluminum wire has been severely oxidized, indicating a significant contact resistance, which has resulted in the socket voltage not reaching 220 volts. So the aluminum wire was cut and reconnected, and electrical tape was wrapped around it. Afterwards, the voltage of the socket was measured to reach 220 volts.
