Editing Characteristic impedance (section)

== Surge impedance loading ==
In [[electric power transmission]], the characteristic impedance of a transmission line is expressed in terms of the '''surge impedance loading''' ('''SIL'''), or natural loading, being the power loading at which [[reactive power]] is neither produced nor absorbed:
<math display="block">\mathit{SIL} = \frac{{V_\mathrm{LL}}^2}{Z_0}</math>
in which <math>V_\mathrm{LL}</math> is the [[Root mean square AC voltage|RMS]] line-to-line [[voltage]] in [[volts]].

Loaded below its SIL, the voltage at the load will be greater than the system voltage. Above it, the load voltage is depressed. The [[Ferranti effect]] describes the voltage gain towards the remote end of a very lightly loaded (or open ended) transmission line. [[Underground cable]]s normally have a very low characteristic impedance, resulting in an SIL that is typically in excess of the thermal limit of the cable.