Narrative:

[A320] checked on to my frequency descending to FL340 enroute to cyyz. I continued its descent to FL330. Normally these aircraft would be descended to FL280 but there was traffic. I had an aircraft that was stopped at FL300 and was requesting FL340. I climbed him to FL320 so I could start the [A320] down; and they were going to be traffic. I went on to other duties as I had about 8 other aircraft on frequency. I did not think that the targets were going to merge and that the aircraft would not get within a thousand feet of each other. A few minutes later [A320] advised me that he was responding to an RA even though he said he was unsure why the TCAS alert had changed to an RA. He went on to say his vertical rate was only 300 feet in the descent and in responding to the RA they climbed 200 feet. I informed my supervisor as this was occurring. The aircraft had already passed by the time he told me of the RA event; so I did not tell him of the traffic since he had stated he [had] seen it; and continued with the descent into cyyz. The other aircraft never stated anything and I continued his climb and told him to contact the next sector. I changed the [A320] to our next sector. There he stated there may have been a passenger hurt while he was responding to the RA. I later found out that there was an injury and a medical [situation] was declared in toronto airspace.my only recommendation would be for the airlines to check on their TCAS equipment because this scenario seems to be happening on a normal basis. I myself just had another RA event a few months ago where I knew aircraft would merge and informed both aircraft exactly what the other was doing and called the traffic multiple times. Yet there was still an RA that went off and [the] pilot had to respond.

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Original NASA ASRS Text

Title: ZOB Controller reported an A320 responded to an RA during descent; even though standard separation was being used between aircraft.

Narrative: [A320] checked on to my frequency descending to FL340 enroute to CYYZ. I continued its descent to FL330. Normally these aircraft would be descended to FL280 but there was traffic. I had an aircraft that was stopped at FL300 and was requesting FL340. I climbed him to FL320 so I could start the [A320] down; and they were going to be traffic. I went on to other duties as I had about 8 other aircraft on frequency. I did not think that the targets were going to merge and that the aircraft would not get within a thousand feet of each other. A few minutes later [A320] advised me that he was responding to an RA even though he said he was unsure why the TCAS alert had changed to an RA. He went on to say his vertical rate was only 300 feet in the descent and in responding to the RA they climbed 200 feet. I informed my supervisor as this was occurring. The aircraft had already passed by the time he told me of the RA event; so I did not tell him of the traffic since he had stated he [had] seen it; and continued with the descent into CYYZ. The other aircraft never stated anything and I continued his climb and told him to contact the next sector. I changed the [A320] to our next sector. There he stated there may have been a passenger hurt while he was responding to the RA. I later found out that there was an injury and a medical [situation] was declared in Toronto Airspace.My only recommendation would be for the airlines to check on their TCAS equipment because this scenario seems to be happening on a normal basis. I myself just had another RA event a few months ago where I knew aircraft would merge and informed both aircraft exactly what the other was doing and called the traffic multiple times. Yet there was still an RA that went off and [the] pilot had to respond.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.