The problem of connecting a PME "earth" to the earth of a TT system is that when the PME "earth" is no longer earth potential ( due to a neutral fault ) the TT earth will be pulled up to the same potential. This could be a hazardously high voltage. There will be some current flowing into the TT ground rod which does not pass through any RCD or MCB and is therefor limited only by the impedance of the circuit. It is possible that the PME "earth" could be pulled up to 230 volts via equipment in the PME property if the break in the neutral is between house and street cable. Hence the TT ground rod would be at 230 volts above ground. A rod impedance of 100 ohms would result in up to 2 amps flowing Voltage gradients around the rod would be hazardously high. If the ground rod was a metallic service pipe with a very low impedance ( or other bonded pipes provided very low impedance routes to ground ) then the neutral current of the PME property would be returning to the sub-station via the TT system earthing arrangement. An electric shower in the PME property would send 40 amps through the earthing of the TT property, There would be no obvious indication of this potentially very serious fault.