FLT CREW FAILS TO NOTICE AN IMPROPERLY POSITIONED PRESSURIZATION OUTFLOW VALVE SELECTOR SWITCH WHICH RESULTS IN TEMPORARY CABIN ALT ABOVE 10000 FT. REPOSITIONING OF THE SWITCH SOLVES THE PROBLEM.

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ON THE INITIAL FLOW; I FAILED TO NOTE THE PRESSURIZATION SWITCH IN MANUAL. WE DID NOT CATCH THIS UNTIL WE WERE GETTING READY TO DO THE PRESSURIZATION CHKLIST AT APPROX 11000 FT. WE HEARD THE WARNING HORN BETWEEN 11000 FT AND 12000; STOPPED OUR CLB AND CHKED THE PRESSURIZATION PANEL. WE RAN THE AUTO FAIL CHKLIST. I CLOSED THE OUTFLOW MANUALLY. WE ASKED ZDC FOR PERMISSION TO STOP THE CLB AND DSND TO 10000 FT. WE CHKED THE SYSTEM IN ALTERNATE AND MANUAL. THERE WERE NO WARNING LIGHTS AT ANY TIME; ALTHOUGH THE GREEN MANUAL LIGHT WAS ON. WE SWITCHED BACK TO AUTO AND THE CABIN PRESSURIZED NORMALLY. ONCE WE WERE SURE THE PROBLEM HAD BEEN CAUSED BY MY FAILURE TO ENSURE THE PROPER POSITION OF THE SWITCH; THAT THE CABIN WAS NORMAL; AND THAT NO ONE IN THE CABIN HAD BEEN AFFECTED; WE ASKED ZDC FOR PERMISSION TO CONTINUE THE CLB AND PROCEEDED TO OUR DEST WITHOUT FURTHER INCIDENT. WE DID NOT DECLARE AN EMER OR ASK FOR PRIORITY HANDLING. WE MAINTAINED OUR ASSIGNED HDG THROUGHOUT THE TIME WE WERE ANALYZING THE PROBLEM. CONTRIBUTING FACTORS: THE ACFT HAD BEEN SET UP FOR DE-ICING FROM THE NIGHT PRIOR SO THE PRESSURIZATION SWITCH WAS LEFT IN MANUAL. I WAS ALSO SOMEWHAT DISTRACTED BY THE DIFFERENT LIGHT CONFIGURATION ON ACFT. (THE STROBE ON POSITION IS UP; WHICH IS THE NORMAL OFF POSITION.) EVEN WITH THIS DISTR; THE MAIN CAUSE; I BELIEVE; WAS MY FAILURE TO ENSURE THE SWITCH WAS IN THE PROPER POSITION DURING THE INITIAL COCKPIT SETUP. I WILL BE MORE VIGILANT TO DO A THOROUGH FLOW AND TO CHK THAT THE CABIN IS PRESSURIZING EARLIER IN THE FLT. I BELIEVE OUR CURRENT PROCS ARE ADEQUATE AND DO NOT NEED TO BE MODIFIED.