  Studies of the crab cavities at KEKB revealed that the RF phase could shift
by up to 50o within ~50 us during a quench; while the cavity voltage is still
at approximately 75% of its nominal amplitude. If such a failure were to occur
on the HL-LHC crab cavities, it is likely that the machine would sustain
substantial damage to the beam line and surrounding infrastructure due to
uncontrolled beam loss before the machine protection system could dump the
beam. We have developed a low-level RF system model, including detuning
mechanisms and beam loading, and use this to simulate the behaviour of a crab
cavity during a quench, modeling the low-level RF system, detuning mechanisms
and beam loading. We supplement this with measurement data of the actual RF
response of the proof of principle Double-Quarter Wave Crab Cravity during a
quench. Extrapolating these measurements to the HL-LHC, we show that Lorentz
Force detuning is the dominant effect leading to phase shifts in the crab
cavity during quenches; rather than pressure detuning which is expected to be
dominant for the KEKB crab cavities. The total frequency shift for the HL-LHC
crab cavities during quenches is expected to be about 460 Hz, leading to a
phase shift of no more than 3o. The results of the quench model are read into a
particle tracking simulation, SixTrack, and used to determine the effect of
quenches on the HL-LHC beam. The quench model has been benchmarked against the
KEKB experimental measurements. In this paper we present the results of the
simulations on a crab cavity failure for HL-LHC as well as for the SPS and show
that beam loss is negligible when using a realistic low-level RF response.
