Here's a thought for R-R control system designers:
If the IP shaft fails, as it seems wont to do, the N2 (IP) turbine accelerates damn fast, because all that gas stream energy is driving only the free turbine, and not the LP compressor.
Simultaneously, the LP compressor will DECELERATE quickly, because no driving torque is being supplied, but its blading is still trying to compress more air.
Now then - I'm guessing the N2 tachometer indication is derived at the cold end of the engine, as are N1 and N3. There is a normal relationship between N1, N2, and N3 that crews are familiar with. If N2 is suddenly out of whack, much slower than expected, an IP shaft uncoupling is a likely cause.
The big question: can this condition be detected quickly enough by the FADEC to shut off fuel before the IPT disassembles itself?