If no lightning rod is installed, when a building is struck by lightning, the whole building will be in a strong electric field of tens of thousands of volts/meters, even hundreds of thousands of volts/meters, in which all objects, whether conductors or insulators, will be sensed. It should be charged (polarized charged) and produce displacement current in conductor or electronic circuit. If the outer shell of electronic equipment is not grounded, because of the existence of displacement current, it will break down those electronic devices which connect to grounded circuit and have larger conductor area (i.e. larger capacitance of the line body), especially those with lower voltage withstanding. Because the generation of displacement current is equivalent to that of capacitance being charged and discharged, the larger the capacitance is, the larger the charge and discharging current is, and the circuit in electronic equipment is equivalent to distributed capacitance. The longer the circuit or the larger the area of conductor, the larger the distributed capacitance is; and the larger the displacement current is generated when induction in electric field.
If the outer shell of the equipment is not grounded, it is impossible for the outer shell to be equipotential to the earth, and the potential difference between the two is very large.
The device acts as an isolated capacitor and the earth acts as another isolated capacitor, and the capacitors are charged and discharged in series (that is, the earth acts as another isolated capacitor).
Electrostatic induction (ESI) can easily damage electronic components during charging and discharging.