According to your situation, I think this is due to the quality of the earthing in the factory.
Basically, it should be zero.
So, there may has the high earthing resistance in between the earthing connection.
You need to check if there is so many high resistance wire in the path for earthing. What you can do is to check the earthing resistance by another equipment , named earthing tester, it can show if the equipment is connected to the "ideal" earthing. If the earthing resistance show very large, you need to add many yellow-green wire parallel to the existing wire so as to reduce the resistance of the wires.
hope this can help you. |