  Secure operation of electric power grids fundamentally relies on their
dynamical stability properties. For the third order model, a paradigmatic model
that captures voltage dynamics, three routes to instability are established in
the literature, a pure rotor angle instability, a pure voltage instability and
one instability induced by the interplay of both. Here we demonstrate that one
of these routes, the pure voltage instability, is inconsistent with Kirchhoff's
nodal law and thus nonphysical. We show that voltage collapse dynamics
nervertheless exist in the absence of any voltage instability.
