将导体壳放入外电场中，导体会在表面产生感应电荷，并达到静电平衡状态，导体壳腔内的电场处处为零，这就是

静电屏蔽效应.然而，如果外电场极强，或者导体内部的自由电荷太少，以至于感应电场不能完全抵消外电场，则静电屏蔽效应

将失效，这就是静电屏蔽的上限问题.本文从静电屏蔽的原理出发，将导体壳简化为一对平行金属平板的模型，定量的讨论了

这一问题.通过计算我们发现，由于金属内存在大量的自由电子，在非极端问题中，宏观的导体装置都远远不会遇到静电屏蔽

的上限问题.

The conductor shell in external electric field will produce inductive charge on its outer surface，and

reach the electrostatic balance. The electric field in its cavity is zero everywhere，this is so called electrostatic shielding. However，if the external electric field is extremely strong，or the number of free electrons in the conductor is

too small，so that the inductive field cannot offset the external field completely. This is so called the upper limit of

electrostatic shielding. In this paper，we simplify this problem as a toy model in which the conductor shell，is illustrated as a pair of parallel metal plates. Through calculation quantitatively，we have found that due to the large amount of free electrons in the metal，any macroscopic conductor devices are far from encountering this upper limit

in non-extreme question.