  The Sloan Digital Sky Survey (SDSS) discovered a few unusual quasars with a
characteristic break in the continuum around 3000 A that neither shows the
typical structure of broad absorption line (BAL) troughs nor is explained by
typical intrinsic dust reddening. We used the method of Kohonen self-organising
maps for a systematical search for quasars with such properties in the SDSS
spectra archive. We constructed a sample of 23 quasars classified as 3000 A
break quasars and two comparison samples of quasars with similar properties, to
some extent, but also showing typical BAL features. We computed
ensemble-averaged broad-band SEDs based on archival data from SDSS, GALEX,
2MASS, UKIDSS, and WISE. The SEDs were corrected for intrinsic dust absorption
by the comparison with the average SED of normal quasars. The de-reddened
arithmetic median composite SED of the 3000 A break quasars is found to be
indistinguishable from that of the unusual BAL quasars. We conclude that 3000 A
break quasars are most likely extreme versions of BAL quasars. Assuming that
the intrinsic SED of the continuum source is represented by the quasar
composite SED, the 3000 A break quasars tend to be intrinsically more luminous
than ordinary quasars. (Abstract modified to match the arXiv format.)
