When a current through the medium, an electric field exists as well as a potential which varies in space. Suppose that there is a break in a high - voltage transmission line and the free end of a wire of length L is lying on the ground. An electric current flows through the regions of soil adjoining the conductor. If a man happens to be walking near by a potential difference, which is called the step voltage appears between the points where his feet touch the ground, Consequently, an electric current whose strength depends on this potential difference flows through the man. Let us calculate the step voltage. Since the conductor is quite long, we assume that the current flows from it to the ground in a direction perpendicular to the conductor. The equipotential surfaces are the surfaces of semi-cylinders whose axes coincide with the conductor. Suppose that the man is walking in a direction perpendicular to the conductor with a step of length 'b' the distance between the conductor and the foot closer to it being d. Assuming that the current flows uniformly from the conductor over the semi cylindrical region we obtain the following expression for the current density at a distance from the conductor: j=i/(pirL) In this case , the field strength along the radii perpendicular to the conductor is E_r=j/sigma=l/(pirLsigma) Consequently , the step voltage is V_(st)=int_d^(d+b) Edr =1/(pisigmaL)ln((d+b)/d) For example , If l=500A , d=1 m , b=65 cm and L=30 m we find that V_(st)=270 V. Much higher voltages may appear under other conditions and other shapes of conductors. When a part of a high- voltage transmission line falls on the ground, it creates a hazard