What is the difference between characteristic impedance and surge impedance?
2025-08-12In practical terms, the terms "characteristic impedance" and "surge impedance" are often used interchangeably in the context of power transmission, but there is a technical distinction. Characteristic impedance (Zc) is the ratio of the voltage to the current of a single traveling wave propagating along an infinitely long transmission line. This value is determined by the physical properties of the line, such as its inductance (L), capacitance (C), resistance (R), and conductance (G). The formula for characteristic impedance is .
![](data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" width="400em" height="1.28em" viewBox="0 0 400000 1296" preserveAspectRatio="xMinYMin slice"><path d="M263,681c0.7,0,18,39.7,52,119 c34,79.3,68.167,158.7,102.5,238c34.3,79.3,51.8,119.3,52.5,120 c340,-704.7,510.7,-1060.3,512,-1067 l0 -0 c4.7,-7.3,11,-11,19,-11 H40000v40H1012.3 s-271.3,567,-271.3,567c-38.7,80.7,-84,175,-136,283c-52,108,-89.167,185.3,-111.5,232 c-22.3,46.7,-33.8,70.3,-34.5,71c-4.7,4.7,-12.3,7,-23,7s-12,-1,-12,-1 s-109,-253,-109,-253c-72.7,-168,-109.3,-252,-110,-252c-10.7,8,-22,16.7,-34,26 c-22,17.3,-33.3,26,-34,26s-26,-26,-26,-26s76,-59,76,-59s76,-60,76,-60z M1001 80h400000v40h-400000z"></path></svg>)
Surge impedance (Zs), on the other hand, is a specific case of characteristic impedance. It applies to a lossless transmission line, where resistance (R) and conductance (G) are considered to be zero. In this ideal scenario, the formula simplifies to . For long, high-voltage power lines, this lossless model is a very close approximation, which is why the terms are frequently used as synonyms.