  A measurement of inclusive semileptonic branching fractions of b hadrons
produced in Z decays is presented. An enriched Z -> bbbar sample is obtained
with a lifetime flavour-tagging technique. The leptonic events are then
selected from this sample, and classified according to their origin, which is
determined by comparing the distribution of several kinematic variables using
artificial neural network techniques. Using 3.6 million multihadronic events
collected with the OPAL detector at energies near the Z resonance, the values
BR(b->lX) =(10.83 +- 0.10(stat.) +- 0.20(syst.) +0.20 -0.13(model)) %
BR(b->c->lX) = (8.40 +- 0.16(stat.) +- 0.21(syst.) +0.33 -0.29 (model)) % are
measured, where b denotes all weakly decaying b hadrons and l represents either
e or mu. The second error includes all experimental systematic uncertainties
whereas the last error is due to uncertainties in modelling of the lepton
momentum spectrum in semileptonic decays and b quark fragmentation. The average
fraction of the beam energy carried by the weakly decaying b hadron, xe, is
measured to be <xe> = 0.709 +- 0.003(stat.) +- 0.003 (syst.) +- 0.013(model)
where the modelling error is dominated by the choice of b fragmentation model.
The agreement between data and various semileptonic decay models and
fragmentation functions is also investigated.
