  3D electroweak sphalerons on the lattice are used as test configurations for
definitions of various topological defects. In the maximally Abelian gauge they
are shown to contain a symmetric array of Abelian monopoles and anti-monopoles
connected by two kinds of Abelian vortex strings. Gauge-invariant lattice
definitions of the Nambu monopole and the Z-vortex string are formulated which
correspond to Abelian projection from the unitary gauge. The sphalerons contain
in their core just one (non-Abelian) Nambu monopole-anti-monopole pair
(connected by a Z-string) in an unstable saddle point bound state. This
provides an example for the monopole-pair unbinding mechanism expected to work
at the electroweak phase transition. The definitions of defects developed here
will be used in future studies of topological aspects of this transition.
