LTT HAS CARGO FIRE WARNING ACTIVATE WHILE ON FINAL.

Narrative

DURING THE EXPRESSWAY VISUAL APCH TO RWY 31 AT LGA; THE CARGO SMOKE WARNING FLICKERED TWICE. ON FINAL APCH AT APPROX 800 FT; THE CARGO SMOKE WARNING CAME ON AGAIN; ONLY THIS TIME IT STAYED ON. I IMMEDIATELY FIRED THE CARGO FIRE EXTINGUISHER. I THEN NOTIFIED ATC THAT WE WERE REQUESTING THE FIRE AND RESCUE BECAUSE WE HAD A POTENTIAL CARGO FIRE. I THEN NOTIFIED THE FLT ATTENDANT AS TO THE SITUATION. I TOLD HER THERE WAS NO TIME TO EXPLAIN; BUT THAT I WOULD ORDER AN EVAC THROUGH THE FORWARD DOORS (2) ONCE WE HAD LANDED AND COME TO A COMPLETE STOP AND TO AWAIT MY INSTRUCTIONS. WE LANDED ON RWY 31 AND EXITED AT HIGHSPD TXWY 'T.' THE FLC PERFORMED THE EMER EVAC CHKLIST AND THE CARGO SMOKE CHKLIST AND ORDERED THE EVAC. ACFT WAS EVACED QUICKLY; ORDERLY WITHOUT ANY INJURIES RPTED. THE FIRE DEPT SECURED THE AIRPLANE AND OPENED THE CARGO COMPARTMENT AND FOUND NO EVIDENCE OF FIRE. UPON INVESTIGATION BY CREW; WE NOTED THAT THE CARGO NET HAD BEEN HUNG ON THE METAL CAGE THAT PROTECTS THE CARGO SMOKE DETECTOR. THIS CAUSED THE CAGE TO PRESS AGAINST THE ELECTRICAL PLUG AND CAUSE A FALSE ALARM. ALSO FOUND WAS THAT THE CARGO FIRE EXTINGUISHER NEVER FIRED. MAINT LATER FOUND THAT THE BOTTLE HAD A DEFECTIVE SQUIB.