except I would guess the 'common' between two inverters is in this case the ESS and Mach4.. I would doubt that both working inverters would suddenly exhibit the exact same issue. Receiving communication issues between the ESS and Mach4 is not a good start and probably indicative to something else causing the problem. Could be noise, could be a connection, could be something else you may have running on the pc.. could be some issue that's causing the program (Mach4 internals) to hang somewhere which could cause communication errors. That's certainly a more difficult issue to troubleshoot. I've had this issue (communication) and it turned out to be a monitoring program that was basically doing nothing and sitting in the background. Took a while to diagnose but eliminating it corrected the problem.  The overspeed might be corrected by this. As a safeguard temporarily you might be able to set the max speed on the inverters lower so this won't destroy the bearings.

Hi,
how have you programmed the VFD's?. All VFD's have  parameter that determines the max frequency and therefore the max motor rpm.
Surely you have programmed the VFD's to some max and safe frequency such that even if the CNC controller/program were to runaway no damage can be done?

Craig

Have you looked for a wiring problem, in particular a dodgy ground connection?