High resolution X-ray spectroscopy was used to evaluate the fractional population of non-thermal electrons generated when electromagnetic waves in the lower hybrid frequency range are launched in the ASDEX tokamak. In complete current drive discharges a typical value of 1% is found for this parameter, in agreement with a theoretical estimate. Furthermore, it is shown that the toroidal electric field is very effective in enhancing tail electron generation, as confimed by β measurements, and that the wave penetration to the plasma centre is inhibited when interaction of the waves with plasma ions is the dominant damping mechanism.