If I'm interpreting this correctly, each machine is earthed separately by its own 4 ft. rod.
The least amount of work would probably be to install an RCD at the generator output to provide earth-fault protection to the whole lot.
Personally, I'd rather go with running earth conductors to each machine along with the phases and neutrals (thus creating a TN-S system), although obivously that's going to involve a lot more work re-running cables.
