Large Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 29
Workload
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Human Factors; Procedure; Environment - Non Weather Related
Workload and traffic volume was extremely high. In addition to the breakup there were numerous crossing situations regarding aircraft flying southbound to (specified) airport and other airports vs. outbound aircraft climbing; eastbound aircraft vs. westbound inbounds. Many aircraft making altitude change requests; route requests and making initial check-ins on two different frequencies. Volume of aircraft was such that I could not keep up with keeping data blocks separated as many aircraft converged. Aircraft Y was part of a flight I had broken up for in-trail spacing to hand off to another sector. Aircraft X was handed off to me from (Specified) Departure; proceeding direct an intersection and climbing. I had stopped Aircraft X's climb to FL290 since the (Specified) inbounds would start asking for lower. I became preoccupied with other situations and radio transmissions to other aircraft that when the proximity of Aircraft Y and Aircraft X to each other was noticed. The aircraft were just about passing each other's left side with Aircraft X climbing out of FL250. The auto conflict alert alarm never sounded and aircraft passed with 'green between' on a 180-200 mile scale on my radar display.Take account of all traffic when using interim altitude; stopping Aircraft X at FL250 would have kept him below Aircraft Y. Do better job of scanning and projecting potential conflicts. Ask the other sets off eyes watching scope to query me and remind me about impending situations early.
Enroute Controller stated workload and traffic volume was extremely high. Reporter did not notice close proximity of two aircraft until they were side by side losing required separation. Reporter stated conflict alarm never activated.