A B737-500 MECHANIC WORKING THE HORIZ STAB HINGE PIN REPLACEMENT JOB CARD MISTAKENLY ACTIVATES THE STANDBY HYD PUMP CIRCUIT BREAKER. PUMP CAME ON AND EXTENDED THE INBOARD LEADING EDGE KRUEGER FLAPS INTO THE OPENED THRUST REV COWLINGS.

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WHILE WORKING THE HORIZ STABILIZER HINGE PIN REPLACEMENT AIRWORTHINESS DIRECTIVE; I WENT TO THE COCKPIT TO RESET CIRCUIT BREAKERS AND RUN THE STABILIZER THROUGH FULL TRAVEL; AS OUTLINED ON THE AIRWORTHINESS DIRECTIVE. THE LIGHTING IN THE COCKPIT WAS VERY DIM. I RESET A STANDBY HYD CIRCUIT BREAKER THINKING I SAW STABILIZER TRIM WRITTEN ABOVE IT. THE STANDBY HYD PUMP CAME ON AND DROVE THE KRUEGER FLAPS INTO THE INBOARD THRUST REVERSER COWLS. THE STANDBY HYD PUMP KICKED ON BECAUSE SOMEONE LEFT THE ALTERNATE FLAP SWITCH IN THE ARM POS. THE ALTERNATE FLAP SWITCH IS A RED GUARDED SWITCH AND DID NOT HAVE A TAG OR STREAMER HANGING FROM IT AS AN ALERT THAT IT WAS IN THE ARM POS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED BOTH ENGINES HAD THEIR INBOARD AND OUTBOARD REVERSER COWLS OPENED. THE FLAP HANDLE WAS SET AT THE 40 DEGREE POSITION. HE JUST NEVER NOTICED THE NORMALLY RED-GUARDED 'ARM' SWITCH FOR THE ALTERNATE FLAP DRIVE WAS IN THE 'ARM' MODE IN THE DIMLY LIT COCKPIT. THE INBOARD REVERSER COWLS WERE DAMAGED.