2. If the garage has used the house PME AND a rod has been installed then this simply means the rod will also be at 230V? Actually worse than without a rod.
If scenario 2 above happens, will the ground itself not become 230V? What is the potential difference actually between, the rod and the ?????. If the ground itself is at 230V how would you get a shock touching the rod if the rod is in the ground?
I need to stop the questions there
Actually I'd say you are asking the right questions, you don't need to stop asking them, just pause until you have good answers before asking more.
I'll try to answer the bit I've quoted, and try to break it down in to small logical points.
For the sake of argument I'll assume the broken PEN affects only the service cable to this one installation, I'll also a ssume that a number of purely resistive electrical loads are connected and switched on at the moment of the PEN fault occurring.
If we first consider the installation without any connections to the mass of earth then at that moment in time that the PEN breaks the MET rises up to line voltage, this is because it is connected to the line via the N-E link, neutral, load and then line conductor.
The earth bar at the garage CU will also rise up to line voltage.
Next consider what happens if the earth rod was a perfect (zero ohms) connection to earth. In this case the MET will not rise to line voltage, the full neutral current will flow via the earth rod back to the substation N point, the installation will continue to work as normal and the broken PEN may go unnoticed. However that 4mm CPC connecting the earth rod to the MET may not be able to carry the full neutral current and could result in further problems.
These are the two extreme ends of the scale and are unlikely to occur in my opinion.
In reality something in between will happen.
The earth rod has a resistance to Earth, with the broken PEN fault this resistance is put in series with the resistance of the loads in the installation. You could simplify this to a diagram of two resistors in series with the MET connected to the middle point between the two resistors.
This will behave in accordance with the laws of resistors in series, current will flow and voltage will drop across each resistor, it will form a potential divider.
If the resistance to Earth of the rod is exactly equal to the resistance of the connected load then the MET will sit at a voltage to earth exactly half the line voltage, and a current will flow that is half of the normal load current.
If the resistance to Earth of the earth electrode is 9 times the resistance of the electrical loads then the MET will be at 9/10ths of line vokatge above earth, and a current will flow which is 1/10th of the normal load current.
If the rod is 100 times the resistance of the loads (this is becoming a more realistic value) , the MET will be at 99% of line voltage and 1% of the normal current will flow.
Hopefully this goes some way to making sense (and hopefully my maths is right for the last part)
The earth rod doesn't raise the earth in general up to line voltage, the earth rod will rise up to the same voltage as the MET and this will 'taper off' in the earth around it back down to 0 volts, if you Google voltage gradients around earth rods you'll likely get pictures with lots of concentric circles around the earth rod showing this.
This is why the top of an earth rod should be below ground level, to avoid the vokatge gradients appearing on the surface. This is what I was taught at college and the text books had pictures of cows standing with their legs in two different circles of the voltage gradient from an earth rod sticking up above the ground surface and getting electrocuted.
