L30 Controller receiving OJT experienced operational error when aircraft being vectored for a visual approach entered a MVA area below the appropriate altitude; reporter claiming facility visual turn on procedure was contributory.

Narrative

While training on Satellite Sector at Las Vegas TRACON at XA55Z; I had an MVA violation. The sequence of events are as follows: 1) Aircraft X was given a 270 degree heading with a descent to 6;000 FT already given. 2) I pointed out the airport to Aircraft X at 3 o'clock position and 12 miles. 3) Aircraft X reported airport in sight; instead of clearing Aircraft at this time for the visual approach. I advised Aircraft X to expect 1 more turn to final to comply with a local procedure here at L30 where we shall give a 30 degree intercept to final when clearing aircraft for a visual approach. 4) I then became distraction with an aircraft departing a satellite airport and after that coordination was done; I observed Aircraft X going through final and into a higher MVA area. Cause: I truly believe the cause for this deviation is the local requirement of all visual approach clearances require a 30 degree intercept rule. I will agree there are times when a 30 degree turn is useful when clearing aircraft on visual approaches; but a broad rule that all visual approaches will require a 30 degree turn I truly feel caused my deviation.