BEFORE THE FLT, THE STUDENT HELICOPTER PLT RECEIVED A WX BRIEFING, WHICH WAS FORECAST TO BE MARGINAL. HE ELECTED TO FLY, THOUGH INSTRUCTED NOT TO GO IF WX WOULD BE A FACTOR. THE FLT HAD BEEN PLANNED FOR A ROUTE TO BLYTHE, BUT AFTER TAKEOFF, THE PLT PROCEEDED TO PAUMA VALLEY & MADE AN UNAUTHORIZED STOP. A PASSENGER BOARDED THE HELICOPTER & THE PLT TOOK OFF AGAIN. SUBSEQUENTLY, THE HELICOPTER COLLIDED WITH A POWER LINE AS THE PLT WAS FLYING ABOUT 300 FT ABOVE AN INTERSTATE HIGHWAY. IT THEN CRASHED TO THE GROUND & WAS EXTENSIVELY DAMAGED. A WITNESS RPRTD THE CLOUDS WERE LOW, BUT THERE WAS NO FOG. ANOTHER WITNESS RPRTD MIST WAS FALLING. ABOUT 14 MI SW AT OCEANSIDE, THE WX WAS, IN PART: PARTIALLY OBSCURED, 1000 FT OVERCAST, VISIBILITY 5 MILES, WIND FROM 160 DEG AT 4 KTS. NO PREIMPACT MECHANICAL PROBLEM WAS EVIDENT. THE PLT HELD A SINGLE ENGINE LAND AIRPLANE RATING AND WAS RECEIVING FLIGHT INSTRUCTION IN HELICOPTERS.