When an electrical device M is damaged and the casing becomes short-circuited, a short-circuit current Id flows through the grounding electrode. This current creates a grounding voltage Ud = Id × Rd at the grounding point, which is commonly referred to as the fault voltage. The potential distribution curve corresponding to this situation is shown as Curve C in Figure 2. Typically, the impedance of the grounding wire is negligible, so the potential on device M is essentially Ud. If a person is located within the divergent zone, the potential at their feet, Uφ, will be higher than zero. When they touch device M, the contact voltage Ut = Ud − Uφ is created. As the person stands on the ground, the resistance of one foot (including shoes, socks, and ground resistance) is Rp. When they touch M, both legs are in parallel, resulting in a combined resistance of Rp/2. Under the influence of Ut, this resistance is in series with the body's resistance RB. The current flowing through the body is IB = Ut / (RB + Rp/2), and the voltage the body experiences is Ut = IB × RB = Ut × RB / (RB + Rp/2). When the insulation of the electrical device is compromised, the contact voltage between the hand touching the device and the foot on the ground depends on the distance between the device M and the grounding point. As shown in Figure 2, when M is closer to the grounding pole, Uφ increases, reducing Ut. Conversely, when the person is outside the divergent zone, Uφ = 0, and Ut reaches its maximum value, Ut = Ud × RB / (RB + Rp/2). The position of the person within the divergent zone affects Uφ, so the contact voltage is usually calculated assuming the person is standing 0.8 meters away horizontally from the device and 1.8 meters vertically from the point of contact. If the device is outside the divergent zone, these distances do not need to be considered during the calculation of Ut.
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