What earthing system is this?

[mechelec]

If the earth is derived from the neutral, in either of your examples, then both the neutral and the earth conductor at the cut-out will be at the same potential, and effectively be exactly the same as a service head PME connection. The earth bonded to steel frame is still effectively a neutral connection....


Not quite, Yes neutral and earth will be at the same potential under normal conditions but earth will be separate back to the main. For example take a neutral fault on the service on one supply in the steel framed building, (service and main PME) neutral becomes Open circuit so being PME the current flows through the neutral earth terminal, though the steel frame (bonded) and down the neutral of the other supplies. Not a good situation as overloading of the neutral and bonding conductors can result.
Now take the same idea and the services are TN-S back to the main which is PME'd in the service joint, open circuit fault on one service neutral will mean no current can flow in the steel frame as the only neutral earth connection is in the service joint on the main. Therefore only one service is affected.


(quote)
In a new PME cable installation, (for simplicity let's talk about a single phase main cable) the neutral is regularly rodded/grounded along it's length, from the sub-station. There is no common earth in the cable construction. So to give an installation an earth, the earth is derived from the neutral conductor at the service head. Any neutral fault will/should then only affect service heads beyond the fault. That is Not So with a converted TN-S cable, where it's sheath has been included in the N-E connections at joints and/or at the service head cut-outs. Acting as a common conductor, any neutral fault will be transmitted to installations that shouldn't be affected by the fault, the length of the cable in fact. Which is why TN-S sheaths should NOT be connected to a PME cut-out, and underground joints should not include the lead sheath in N-E rodded conversion connections...

TN-S cables have a connection between the neutral core and lead sheath at the substation and at joints along it's length. A neutral fault ie open circuit neutral core (very rare) will result in all neutral current flowing through the lead sheath which is of adequate CSA to carry this current. You would have to lose the lead sheath and neutral core for there to be major problems.
Neutral faults are thankfully rare and are usually caused by alu CNE cable when moisture gets in to the alu neutrals and corrodes them away.