I'm entering this discussion with some trepidation as a retired aero engineer (who many years ago did work at Derby for a few years) but:
Engine Protection Systems
Introduction
The Protection System is incorporated into the EEC and provides hardware to perform the following functions:
LP & IP Rotor Overspeed protection
LP Turbine Overspeed protection
Thrust Control Malfunction (TCM) protection
LP & IP Rotor Overspeed System
The EEC monitors the LP & IP compressor shaft speeds (N1 & N2). If the measured values are above the defined limits, overspeed of the engine is detected and the engine is automatically shut down.
Would this have helped in this case? The IP shaft would have been winding down as the IP turbine end separated, so no overspeed would have been detected.
Turbine Overspeed System (TOS)
The EEC compares the LP compressor speed with the LP Turbine speed. If the speed difference is more than the limit, it is an indication of a shaft breakage and the engine is automatically shutdown.
Again, this wouldn't take into account the overspeed of the IP turbine, would it?