The relay is there so that I can run the majority of the alarm circuit at 24V rather than the 5V the fault input on the PMDX is pulled up to, my understanding was that would provide more resilience to noise.

On the PMDX the fault and e-stop inputs are sampled together which to my mind means double the risk of noise causing a false e-stop (the manual doesn't say but I'm assuming they are OR'ed together).