After Xiao Fang's family plugged the refrigerator into the three hole socket on the wall, the refrigerator worked normally, but when she touched the refrigerator shell with a pencil, she found that the neon tube was glowing.
When the three-pin plug is inserted into three hole socket, the E pin of the three-pin plug is connected with the E pin of three hole socket, so that the metal shell is connected with the earth. In this way, if the electrical shell of the metal shell is charged, current will be introduced into the earth through the earth wire to prevent electric shock accidents. Now the shell is charged, and the ground wire in three hole socket is broken.
So the answer is: three hole socket's ground wire is broken.