MISC -- DELAYED LATE CONFLICT ALERT WARNING BTWN CGA X AND CPR Y.

Narrative

I WAS WORKING THE LCL CTL POS IN THE TWR AND I HAD AN EMS HELI INBOUND TO THE ARPT VIA THE LAKE SHORELINE. THE EMS HELI WAS CGA X ON AN ESTABLISHED INBOUND RTE TO THEIR LNDG SITE ON THE W SIDE OF THE ARPT. CPR Y; A CESSNA 340; WAS INBOUND FROM THE N ON A VISUAL APCH TO RWY 24 VIA A R BASE LEG. CGA X WAS AT 1200 FT MSL AND CPR Y WAS DSNDING FROM 2800 FT MSL. I EXCHANGED TFC TO BOTH ACFT. I HAD BOTH ACFT IN SIGHT AND WAS PROVIDING VISUAL SEPARATION. THE HELI SIGHTED THE C340 AT 2 NM; THE C340 WAS AT 1400 FT MSL DSNDING. THE HELI CROSSED THE C340'S 12 O'CLOCK POS AT LESS THAN 1 MI DISTANCE. THE AURAL ALARM OF THE ARTS IIA CONFLICT ALERT NEVER SOUNDED. WHEN THE ACFT WERE AT A DISTANCE OF MORE THAN 1 MI ON DIVERGING HDGS; THE AURAL ALARM SOUNDED. THE ACFT WERE MOVING AWAY FROM ONE ANOTHER BEFORE THE ALARM SOUNDED. THIS UNFORTUNATE INCIDENT OCCURS ON A REGULAR BASIS. THE CONFLICT ALERT FEATURE IS SUPPOSED TO AID THE CTLR IN PREVENTING CONFLICTS COLLISIONS: HOWEVER; DOES NOT FUNCTION IN THIS MANNER.