  In this paper we study the effective lepton flavor violating vertex of an
electroweak $Z$ gauge boson and two charged leptons with different flavor,
$l_k$ and $l_m$, that is generated to one-loop in low scale seesaw models with
right handed neutrinos whose masses are heavier than the electroweak scale. We
first compute the form factor describing this vertex by using the mass
insertion approximation, where the flavor non-diagonal entries of the neutrino
Yukawa coupling matrix are the unique origin, to one-loop level, of lepton
flavor changing processes with charged leptons in the external legs. Then, by
considering the proper large right handed neutrino mass expansion of the form
factor, we derive a formula for the $Z l_k l_m$ effective vertex which is very
simple and useful for fast phenomenological estimates. In the last part of this
work we focus on the phenomenological applications of this vertex for simple
and accurate estimates of the $Z \to l_k {\bar l}_m$ decay rates. Concretely,
this vertex will allow us to conclude easily on the maximum allowed decay rates
by present data in the inverse seesaw model. The found rates are promising, at
the reach of future lepton colliders.
