FAILURE TO OBTAIN GREEN GEAR SAFE LIGHTS TRACED TO OPEN AIR/GROUND RELAY CIRCUIT BREAKER BY B737-400 FLIGHT CREW.

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AUTOBRAKE DISARM LIGHT REMAINED ILLUMINATED AFTER SELECTION OF AUTOBRAKES 2 AT CRUISE FOR LNDG AT ZZZ. COMPLETED AUTO-BRAKE DISARM QRH RESULTING IN 'OFF.' MADE MAINT LOG ENTRY AND SENT ACARS MESSAGE TO MAINT PRIOR TO INITIATING DSCNT. NOSE GEAR RED LIGHT ILLUMINATED INTERMITTENTLY IN DSCNT. WE DISCUSSED OPTIONS AND CHOSE TO EXTEND GEAR EARLY ON HIGH DOWNWIND NEAR ZZZ AS A SYSTEM CHK AND SAFETY PRECAUTION. NO GREEN LIGHTS. SOUND AND FLT CHARACTERISTICS SEEMED TO INDICATE A GEAR DOWN CONDITION BUT THIS COULD NOT BE CONFIRMED. RAN THE MANUAL GEAR EXTENSION QRH. NO GREEN LIGHTS! DECLARED AN EMER AND REQUESTED A LOW PASS FOR A VISUAL GEAR CHK BY THE TWR. THEY SEE 3 GEAR. CHKED COCKPIT NOSE GEAR PERISCOPE; VERIFIED NOSE GEAR DOWN AND LOCKED; WENT TO CABIN TO DO SAME. UPON RETURN TO COCKPIT; CONVERSATION WITH CAPT AND MAINT REVEALED AN OPEN CIRCUIT BREAKER C18 AIR/GND RELAY. AGREED TO CLOSE BREAKER; GEAR INDICATIONS RETURNED TO NORMAL; 3 GREEN LIGHTS CONFIRMED GEAR DOWN. LANDED UNEVENTFULLY. ENTERED TRIPPED BREAKER IN MAINT LOG; HAD NUMEROUS CONVERSATIONS WITH MAINT; STATION AND EMER PERSONNEL.