WDB ACFT DEPARTS WITH INCOMPLETE SAFETY BRIEFINGS.

Narrative

OUR ACFT WAS DISPATCHED WITH SOME FEATURES OF THE PA SYS INOP PER MEL. AFTER TAXIING TO THE END OF THE RWY; AND AFTER FINISHING THE BEFORE TKOF CHKLIST; WE WERE BEING CLRED FOR TKOF AS THE PURSER ENTERED THE COCKPIT TO INFORM US THE PRE-TKOF PAX BRIEFING HAD NOT BEEN COMPLETED. BECAUSE OF THE ARPT CONFIGN; AND OUR POS AT THE RWY; AND ACFT BEHIND US; AS WELL AS A 'SLOT' TIME; I ELECTED TO TKOF. I DIRECTED THE PURSER TO COMPLETE THE BRIEFING AS SOON AS WE WERE AIRBORNE. AN ELECTION TO TAXI CLR WOULD HAVE RESULTED IN A LONG TAXION THE RWY AND A WAIT FOR GEAR AND BRAKE COOLING PRIOR TO TKOF; AS WELL AS OBTAINING A NEW SLOT TIME. AFTER TKOF; I WAS INFORMED THERE WAS A GREAT DEAL OF CONFUSION BTWN FRONT AND REAR CABIN SECTIONS AND; IN AT LEAST THE CASE OF 1 SECTION; NO BRIEFING WAS DONE AT ALL. WE LATER DETERMINED THAT AT LEAST 1 PA SWITCH WAS LEFT DISARMED -- ONE THAT IS IN AN OBSCURE LOCATION AND NORMALLY NOT DISTURBED. THIS; WITH THE TECHNICAL ANOMALIES WITH THE PA; AS WELL AS APPARENT LACK OF COORD WITH THE PURSER AND THE FLT ATTENDANTS CREATED A CONFUSING SIT FOR THEM. WE WERE UNAWARE OF THIS IN THE COCKPIT UNTIL THE LAST SECONDS AND; IN FACT; WERE UNAWARE OF THE SCOPE OF THE PROB. PRIOR TO DEP; WE DID ASK THE MAINT FOREMAN (AN AMERICAN) TO SHOW THE FLT ATTENDANTS HOW TO OPERATE THE SYS -- WE ASSUMED THIS HAPPENED. I DO NOT WANT TO SUBJECT A HVY WDB TO EXCESSIVE TAXI DISTANCES NEAR OR OVER LIMITATIONS. I DID NOT WANT TO BLOCK A RWY WHILE I TAXIED A LONG DISTANCE TO THE FIRST AVAILABLE TURNOFF. IF I HAD BEEN AWARE OF THE CONFUSION BELOW; I WOULD NOT HAVE TAKEN OFF UNTIL ALL WAS SETTLED REGARDLESS OF HAVING TO RETURN TO THE 'BLOCKS.' ALL BRIEFINGS WERE COMPLETE SHORTLY AFTER TKOF. SUPPLEMENTAL INFO FROM ACN 245961: IN RETROSPECT; THE PURSER DID NOT PREPARE ADEQUATELY FOR HANDLING THE PROB WITH THE PA SYS AND CAUGHT THE CAPT BY SURPRISE RIGHT BEFORE TKOF. WE WERE FACING A TKOF SLOT AND WERE OVERLY FOCUSED ON MEETING OUR TKOF SLOT. THE PURSER'S FAILURE TO NOTIFY THE CAPT IN A TIMELY MANNER AND OUR 'GO' MENTALITY LED TO US TAKING OFF WITHOUT FULLY COMPLETING THE PAX SAFETY BRIEFING.