We demonstrate that frequency- and time-domain boundary integral equation f
ormulations involving the surface impedance boundary conditions (SIBC's) ca
n be transformed to invariant forms that depend only on the geometry of the
problem and do not contain temporary parameters of the held, Thus the inte
gral equations have to be solved only once for a given system of conductors
and then the results for any source field (steady state or transient) can
be easily obtained. We considered both low- and high-order SIBC's so that t
he formulations obtained can be applied to a wide range of skin effect prob
lems, A numerical example is included to illustrate the theory.