L1011 FLC EXPERIENCED A LOSS OF CABIN PRESSURE AT FL390. AN EMER DSCNT WAS INITIATED TO 14000 FT WHERE CTL WAS REGAINED.

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DURING CRUISE AT FL390 THE 'ESSENTIAL STANDBY PWR' LIGHT ON THE PLT'S CAUTION WARNING PANEL ILLUMINATED. NO OTHER INDICATIONS. 30 SECONDS LATER; THE 'DC STANDBY BUSS' FAILURE LIGHT ON THE FE'S PANEL ILLUMINATED. ALL OTHER BUSSES -- AC AND DC -- WERE PWRED. THIS IS A MOST UNUSUAL SIT SINCE THE 'DC STANDBY BUSS' IS THE LAST BUSS TO LOSE PWR AND IS PWRED SEVERAL DIFFERENT WAYS. AS THE CAPT AND FE WORKED THE PROB THE CABIN BEGAN TO RISE; SLOWLY AT FIRST AND THEN UNCTLABLY. THE PRESSURIZATION SYS WAS COMPLETELY UNCTLABLE IN ALL 3 MODES; AND ALL SUBSYSTEMS WERE NOT WORKING AS WELL. AT THIS POINT; WE DONNED OUR OXYGEN MASKS AND INITIATED AN EMER DSCNT; NOTIFIED ATC AND DECLARED AN EMER; COMMUNICATED WITH THE SENIOR FLT ATTENDANT AND MADE A PA TO THE PAX. THE CAPT AND FE CONTINUED TO WORK THE PROBS AS I EXECUTED THE EMER DSCNT. AS I LEVELED AT 14000 FT; THE FE REGAINED CTL OF THE PRESSURIZATION SYS; ONCE THAT WAS STABILIZED WE DECIDED TO REMAIN AT 14000 FT. AFTER CONTACTING DISPATCH; WE DECIDED THAT WITH THE FUEL REMAINING IT WAS SAFE TO CONTINUE TO THE DEST. CONSIDERING THE NATURE OF THE FLT; DIVERTING TO CAIRO WAS AN UNATTRACTIVE OPTION. AFTER SECURING ALL CHKLISTS THE CAPT; SENT THE FE BACK TO THE CABIN TO EVALUATE THE SIT. FORTUNATELY; THERE WERE NO INJURIES AND THE PAX WERE IN GOOD SPIRITS. THE FLT CONTINUED TO THE DEST UNEVENTFULLY. WE KNOW WHAT HAPPENED; JUST NOT WHY. FOR THE 'STANDBY DC BUSS' TO FAIL ON ITS OWN AND HAVE THE OUTFLOW VALVES DRIVE OPEN WITH A PRESSURIZATION FAILURE DEFIES LOGIC; BUT IT HAPPENED. THANKS TO GOOD TRAINING; THIS EVENT WENT SMOOTHLY. EVERYONE KNEW THEIR ROLES AND PERFORMED WITHOUT A GLITCH. GOOD COM WAS MAINTAINED BTWN THE CABIN AND FLT DECK WHICH HELPED IN KEEPING THE PAX INFORMED.