PLTS ON TRAINING FLT LOST ALL ELECTRICAL PWR WHILE ON TOP RESULTING IN LOST COM AND AN EMER DSCNT THROUGH A HOLE IN THE UNDERCAST TO VFR CONDITIONS.

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WE DEPARTED VFR AND; AS WE BEGAN TO CLB OUT; REALIZED IT WAS NECESSARY TO GET AN IFR POP-UP CLRNC TO GET ABOVE THE CLOUDS TO DO OUR AIRWORK (MULTI-ENG DEMONSTRATIONS). THE CTLR CLRED US DIRECT TO BUU VOR AND MAINTAIN 7000 FT. AFTER REACHING BUU AND LEVEL AT 7000 FT; WE WERE CLRED TO PRACTICE NW OF BUU -- VFR ON TOP. APPROX 5 MINS INTO THE FIRST DEMONSTRATION; WE LOST ALL ELECTRICAL PWR. IT WAS AT THIS TIME THAT I NOTICED MY STUDENT NEVER TURNED ON THE ALTERNATORS. AND I HAD FAILED TO CATCH THIS ON THE RUN-UP. WE SQUAWKED 7600 (EVEN THOUGH THE XPONDER WASN'T WORKING!) AND BEGAN TO HEAD W TO FIND SOME VFR WX. WE MAINTAINED 7000 FT AND PROMPTLY TURNED ON THE ALTERNATORS. ABOUT 3 MINS LATER WE FOUND A HOLE LARGE ENOUGH TO DSND THROUGH THE BROKEN LAYER. WE MADE AN EMER DSCNT OUT OF 7000 FT TO 2000 FT; AND LEVELED OFF IN VFR CONDITIONS DIRECTLY OVER E TROY ARPT. THERE WE CIRCLED FOR APPROX 15 MINS UNTIL THE ALTERNATORS CAME BACK ON LINE; AT WHICH TIME MKE PICKED US UP ON THEIR SCREEN SQUAWKING 7600. WE MADE CONTACT. GOT A NEW SQUAWK CODE AND RETURNED TO MKE FOR A FULL STOP LNDG. WE DECIDED TO CALL IT A DAY.