ACFT #1 DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACFT #2. SYS ERROR.

Narrative

WHILE CTLING THE LONDON (LOZ) LO SECTOR OF ZID; AN OPERATIONAL ERROR OCCURRED. ACFT #1; ENRTE FROM CMH TO BNA AT FL180 AND ACFT #2; CLBING TO 17000 FT; OFF OF LEX ENRTE TO ATL. ACFT #1 WAS ISSUED A R TURN OF 15 DEGS (APPROX A 210 DEG HDG); TO JOIN THE ARR RTE INTO BNA. APPROX 1 MIN LATER; ACFT #2 WAS TURNED TO A HDG OF 170 DEGS. HAVING PUT A POLYGON (5 MI RING) ON ACFT #2 EARLIER IN THE SIT; I JUDGED THAT THE ACFT; ACFT #2 WOULD REMAIN LATERALLY SEPARATED FROM ACFT #1. AFTER DSNDING ACFT #1 APPROX 15 SECONDS AFTER ISSUING THE 170 DEG HDG TO ACFT #2; I DSNDED ACFT #1 TO 15000 FT; AND WENT ON AND TOOK CARE OF ANOTHER CLRNC TO ANOTHER ACFT ON THE FREQ. DURING ISSUANCE OF THAT CLRNC; ACFT #1 WAS OBSERVED JUST INSIDE THE POLYGON ON ACFT #2. (400 FT AND 4 MI AT CLOSEST POINT). MY OBSERVATION WAS THAT ACFT #2 INCREASED ITS AIRSPD AFTER IT LEVELED AT 17000 FT AND THE LATERAL SEPARATION I THOUGHT I HAD INSURED; WAS EATEN AWAY BY THE INCREASE IN SPD BY ACFT #2. NO FURTHER ACTION WAS TAKEN DUE TO THE ACFT DIVERGING BY APPROX 45 DEGS; AND THERE WAS NO FURTHER INCIDENT. ERROR DETECTION ACTIVATED; AND I ADVISED MY SUPVR IMMEDIATELY; THAT I MAY HAVE LOST STANDARD SEPARATION.