Skin Effect

When a conductor carries a steady or direct current, this current is uniformly distributed over the cross-section of the conductor. However , the current distribution is non-uniform is the conductor carries alternating current . The current density is higher at the surface then it's center . It decreases with greater depths in the conductor exponentially. Thus, the current is concentrated near the surface of the conductor , as shown in the figure beside. 

                                                                                                                                                                                                                                Due to this skin effect, the effective cross sectional area offered to the flow of current decreases which increases resistance. The increment of this resistance may be up to 30% , so the effective resistance for conductor is more for AC  than the loss when the value of DC is flowing through the conductor.   

The skin effect is predominant with the increase in the frequency . The skin depth is inversely proportional to the square root of the frequency.The frequency where the skin effect just start to limit the the effective cross sectional area of the conductor is called the crossover frequency.

                                                                                                                                        The skin effect is quite large in solid conductors , even at a frequency of 50 Hz . the skin effect depends on the following factors ----

           i. Nature of material 

          ii. Diameter of wire

         iii. Frequency of supply

        iv. Shape of wire

With the increases in diameter of wire , the skin effect increases. Similarly as frequency increases, the skin  effect increases. If we have stranded conductor rather than solid conductor diameter is less. It can be seen that when supply frequency is less than 50 Hz and conductor diameter is less than 1 cm skin effect is negligible. In large conductors at power frequencies the skin effect is a significant factor.