On January 17, 1997 at 100% power, there was a scram on turbine trip for spurious actuation of a relay Buchholz of main transformer. All safety systems called upon performed as expected, operators checked that control rods were fully inserted into the core. Hhowever, by reviewing post trip computer records, plus performing control rod drop times, it was noticed that: a) all control rods entered the dashpot (34 steps above core bottom, compared to total length of 228 steps) within the time established by Technical Specifications (TS), however: b) 3 out of 48 control rods suffered delays from step 6 or 12 to bottom. Justification: Table II: Severity classification criteria for degradation of defence-in-depth with an initiator, initiator: scram on turbine trip. Frequency: high/expected. Safety function:reactor Protection System. Availability: within operational limits and conditions. Rating: INES level 1.