WRONG ALTIMETER SETTING SELECTED BY FO; PF; LEADS FLT INTO AN ALTDEV ALT OVERSHOT ON DSCNT IN AN A320. ALT XING RESTR NOT MET. CAPT FAILED TO XCHK ALTIMETERS.

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DSNDING FROM PHX-DTW ON POLAR 1 ARR; WE WERE CLRED TO CROSS GIBER INTXN AT FL230. THE FO WAS FLYING AND HAD SET THE ALT IN THE FCU ALT WINDOW AND SET THE RESTR IN THE FMGC. AT ABOUT FL250; I WENT OFF FREQ TO SAY GOOD BYE TO THE PAX. WHEN I CAME BACK ON FREQ; I NOTICED THAT THE AURAL AND VISUAL ALT ALERTER WERE GOING OFF. THE ALT INDICATED FL225. THE FO WAS ENGAGED IN CONVERSATION WITH ZOB. HE ASKED CLE 'WHAT DO YOU SHOW NOW?' 'FL225' CAME THE REPLY. THE FO THEN INCHED THE ACFT BACK UP TO FL230. THERE WAS NO TFC IN THE AREA AS FAR AS I COULD SEE ON THIS CLR NIGHT AND WE DID NOT RECEIVE ANY TCASII MESSAGES. I ASKED THE FO WHAT HAD HAPPENED. HE EXPLAINED THAT WHILE DSNDING ON AUTOPLT IN VNAV; HE RESET THE BAROMETRIC REF WINDOW TO THE LCL DTW ALTIMETER SETTING OF 30.42 IN PREPARATION FOR OUR DSCNT AND APCH CHKLIST. THE ACFT CAPTURED ALT BASED ON 30.42 INSTEAD OF 29.92 BEFORE HE RESET THE ALTIMETERS TO 29.92. OUR COMPANY POLICY AS SPELLED OUT THROUGH OUR FLC OPERATING MANUAL SAYS NOT TO RESET THE ALTIMETERS TILL PASSING THROUGH FL180. IT HAS BECOME A MATTER OF PLT TECHNIQUE TO RESET THE ALTIMETERS EARLY ON; THEN RESELECT 29.92 ON THE BAROMETRIC REF KNOB. THEN WHEN ACCOMPLISHING THE DSCNT AND APCH CHKLIST; ALL ONE HAS TO DO IS PUSH THE BAROMETRIC REF KNOB IN AND HE'LL HAVE THE CORRECT ALTIMETER SETTING. I'VE ASKED OUR LCL ALPA CENTRAL AIR SAFETY COMMITTEE TO GET THE WORD OUT TO EMPHASIZE NOT RESETTING THE ALTIMETERS UNTIL ACTUALLY PASSING THROUGH FL180.