Why should copper be added to the insulation pad of generator bearing?

Today, I met the same problem. The bearing base at the excitation end of the generator is insulated from the ground by insulating board. One or more layers of metal sheets or a layer of copper foil are sandwiched between two insulating plates, which should be mainly used to measure the insulation between the bearing bush and the ground. Before starting or after stopping the generator, the insulation of the excitation end bearing bush to the ground should be measured to prevent the large shaft from forming a loop through the large shaft grounding carbon brush, bearing bush and its base due to induced voltage, so that the current can burn the bearing bush and journal through the bearing bush. Therefore, it is well known that heat insulation boards are installed. But how to simply measure the insulation between the bearing bush and the ground? If the vibration table contact pin is directly added to the bearing seat and the ground, insulation will definitely not work. Because the big shaft is grounded through the carbon brush; Even if the carbon brush is lifted, there is a bearing on the turbine side, and the bearing base on the turbine side is not insulated, or the rotor is lifted by other methods before measurement. After adding copper foil (copper foil is equivalent to the surface and bottom of the bearing seat), you can use copper foil as a pressing point (because copper foil is sandwiched between two insulating plates) to measure the insulation to the ground or the bearing seat and check whether the insulation to the ground of the bearing seat is qualified.