This paper proposes a method to predict and control noise voltage caused by electrostatic discharge (ESD) to electronic equipment. The relationship of grounding system configurations for a typical set of equipment to ESD immunity has been derived using a mechanism of ground potential variations. The equivalent circuit representing ground elements as lumped constants enables us to predict the transient ground potential differences between PCB (Printed Circuit Board) ground planes connected via signal cables and induced noise voltage at the receiving end. The calculation shows that the contribution of ground potential differences to noise voltage is comparable to that of the electromagnetic coupling between the discharge current on the enclosure and the circuit loops. The calculation also shows some characteristic results, such as; the induced noise voltage is remarkably dependent on the unbalance in ground cable lengths and on the impedance of ground conductors connecting PCBs, especially when the equipment uses a single-point grounding system. These characteristics were confirmed by measurements of induced ground potential differences, noise voltage and immunity levels. Thus the proposed method is shown to be very effective to analyze the dependency of grounding conditions on ESD immunity and to improve ESD immunity in equipment design.