HV Earthing

Hi. I am learning about High Voltage earthing regulations. I was told that the HV earth and LV earth were to be kept 9m apart. I understand that under fault conditions a current through the HV earth would result in a pool of potential and so the LV earth must be kept a certain distance apart. If to say the LV earth was not within a safe distance away how exactly does this create a hazard, and can you explain to me in detail what happens?

Thanks