  This seventh paper of the MUse RAdio Loud Emission lines Snapshot (MURALES)
project presents the results of the observations obtained with the VLT/MUSE
integral field spectrograph of 3C radio sources and discusses the optical
spectral properties of the nuclei of 26 objects with 0.3<z<0.82 (median
redshift 0.51). At these redshifts the H$\alpha$ and [NII] emission lines are
not covered by optical spectra and alternative diagnostic diagrams are needed
to separate the different spectroscopic sub-classes. We derive a robust
spectroscopic classification into high and low excitation galaxies (HEGs and
LEGs) by only using ratios of emission lines in the rest frame UV and blue
portion of the spectra. A key result is that FRII/LEGs are found also at the
highest level of radio power (up to L$_{178} \sim 2\times 10^{35}$ erg/s/Hz),
among the most luminous radio sources in the Universe. Furthermore, their
fraction within the FRII RGs population does not strongly depend on radio
luminosity. This suggests that the jet properties in powerful FRII radio
sources do not depend on the accretion mode and on the structure of the
accretion disk as expected if the jet launching process is due to the
extraction of the rotational energy of the supermassive black hole. The
alternative possibility of recurrent transitions between a LEG and a HEG phase
is disfavored based on the variation timescales of the various AGN components.
