Why is there a voltage of more than 20 volts measured by a multimeter when there is no electricity in the socket?
Influence: It will not have an impact on ordinary equipment, and it can be ignored. For particularly precise equipment, it may lead to component damage, and isolation is required.
Cause: It is caused by the mutual inductance with other live power cables laid in parallel.
When there is no electricity in the power socket but a multimeter measures a voltage of more than 20 volts, this situation is generally caused by poor contact of the live wire of the alternating current. Some time ago, this situation occurred with the power socket in Mr. Tony's house in our village.
I used a multimeter to measure the voltage of the socket in his house, and it was only a little over 30 volts. According to experience, if the voltage on the socket is too low to reach the rated value, there is usually a problem with the contact of the live wire. So I focused on checking the live wire in his house. After searching, I found that the screw connecting the live wire to the air switch was a bit loose. Tony told me that last month, the live wire was almost falling out of the screw hole of the air switch, and he stuffed the wire back in by himself. I used a screwdriver to tighten the screw, and the measured voltage of the socket increased to 100V, and the LED indicator light on the socket also turned on.
After tightening the screw, the voltage increased to 100V, but it still couldn't be used. I continued to search and found that there was a section of the wire on the courtyard wall wrapped with electrical tape. Tony told me that last year, a lot of mice climbed along this wire into the room where the grain was stored in the backyard to steal the grain. Later, he blocked the holes in that room. Maybe because the mice couldn't get in, they were so angry that they bit through the wire.
I took a look and found that the interface of this 2.5-square-millimeter aluminum wire was severely oxidized. In this case, there must be a relatively large contact resistance, which results in the socket voltage not reaching 220 volts. So I cut off the aluminum wire, reconnected it, and wrapped it well with electrical tape. After that, the measured voltage of the socket reached 220 volts.
