  We present the results of a careful analysis of associated absorption systems
toward NGC 5548. Most of the well resolved narrow components in the associated
system, defined by the Lyman alpha, C IV and N V profiles, show velocity
separation similar (to within 10~\kms) to the C IV doublet splitting. We
estimate the chance probability of occurrence of such pairs with velocity
separation equal to C IV doublet splitting to be $6\times10^{-3}$. Thus it is
more likely that most of the narrow components are line-locked with C IV
doublet splitting. This will mean that the radiative acceleration plays an
important role in the kinematics of the absorbing clouds. We build grids of
photoionization models and estimate the radiative acceleration due to all
possible bound-bound transitions. We show that the clouds producing absorption
have densities less than $10^9 cm^{-3}$, and are in the outer regions of the
broad emission line region (BLR). We note that the clouds which are line-locked
cannot produce appreciable optical depths of O VII and O VIII, and hence cannot
be responsible for the observed ionized edges, in the soft X-ray. We discuss
the implications of the presence of optically thin clouds in the outer regions
of the BLR to the models of broad emission lines.
