  The electric charge renormalization constant, as defined in the Thomson
limit, is expressed in terms of self-energies of the photon-Z-boson system in
an arbitrary R_\xi-gauge to all perturbative orders. The derivation as carried
out in the Standard Model holds in all spontaneously broken gauge theories with
the SU(2)_w \times U(1)_Y gauge group in the electroweak sector and is based on
the application of charge universality to a fake fermion with infinitesimal
weak hypercharge and vanishing weak isospin, which effectively decouples from
all other particles. Charge universality, for instance, follows from the known
universal form of the charge renormalization constant as derived within the
background-field formalism. Finally, we have generalized the described
procedure to gauge theories with gauge group U(1)_Y \times G with any Lie group
G, only assuming that electromagnetic gauge symmetry is unbroken and mixes with
U(1)_Y transformations in a non-trivial way.
