The skin effect at DC is essentially non-existent.
Unlike alternating current (AC), which concentrates its current flow near the surface of a conductor (the "skin"), direct current (DC) distributes uniformly throughout the entire cross-sectional area of the conductor. This is because the frequency of DC is zero. The formula for skin depth is inversely proportional to the square root of frequency. As frequency approaches zero (as in DC), the skin depth approaches infinity.
Understanding Skin Depth
The skin depth is a measure of how deeply an AC current penetrates a conductor. A higher frequency leads to a smaller skin depth, meaning the current is concentrated closer to the surface. Conversely, a lower frequency allows for deeper penetration.
- DC (0 Hz): Skin depth is infinite. The current flows uniformly throughout the conductor's cross-section.
- AC (non-zero Hz): Skin depth is finite. Current density is highest near the surface and decreases exponentially with depth.
This uniform current distribution in DC conductors results in a lower resistance compared to the resistance at AC frequencies, where the effective cross-sectional area is reduced due to current concentration near the surface.
The provided reference states: "The skin depth is maximum if the frequency is zero—The DC has zero frequency so the skin depth is maximum and the total cross-section area of the conductor carries the current hence the DC resistance is low. The AC resistance is always higher than the DC resistance." This concisely summarizes the core concept.
