Surly the path  of the rod back to the star point needs to be low enough to operate the protective device. 

Also, in regards to three phase generators, if one of the lines comes into contact with mass of earth ( True earth) how does the value of the electrode influence potential neutral inversion ? 

Although slightly of topic , this incident is still somewhat relevant. Recently, I became aware of a 630mm single neutral conductor from a transformer to a store being severed and stolen. Remarkably, the electrical supply remained uninterrupted, and the theft was only discovered upon review of CCTV footage. The Distribution Network Operator subsequently visited the site and isolated the supply. I am astounded by the expertise of these thieves, who specifically targeted the neutral conductor.

How about let’s say HO7RNF generator tails on the surface. What happens if they get cut and exposed copper touches the ground?

All depends on the soil resistance and the contact area. As a for instance, let's imagine that the soil is of a type where a 4' x 3/8" rod (so surface area about 30,000 mm²) gives a resistance of 200Ω - and say the gash in the cable exposed say 20x1 mm of conductor (20mm²) - then as a very simple approximation you might imagine that the fault would gave a resistance about 1500x higher than the rod (30,000/20) - so around 30kΩ.. (All very rough, if the soil eas dryer near the surface it could be a lot higher, of if in a puddle it might be lower) Anyhow 230V and 30kΩ would mean a fault current of less than 8mA - so nothing trips, not even a 30mA RCD,

Cue comments about choosing a cable that's suitable for the environment. RN sheathed is pretty robust and isn't easily cut into by the usual blunt trauma - but if sharp implements are a credible risk and other precautions aren't sufficient, then consider armoured (or at least braided) cables instead.

  - Andy.

or to put it another way, ADS works when you can have a fault of negligible impedance - i.e. a fault directly between metallic parts, so you can then know the overall earth fault loop impedance, and calculate on that basis. That's really the essence of "Indirect" shock protection (as it used to be called) - faults to exposed-conductive-parts. Other problems - e.g. due live conductors becoming accessible - is closer to what used to be called direct contact and there's little ADS can do for that. It's either a case of doing your best to make sure that basic insulation remains intact (e.g. by physically protecting the cable) and/or by additional protection (e.g. 30mA RCD) - but even then you don't expect the fault to disconnect immediately it occurs - just when someone is gets a large enough shock for it to be potentially fatal.

  - Andy.

AJJewsbury said:

Separation is only good for small systems

More importantly, systems in which:

1. There is, ideally, no more than one item of Class I equipment; and
2. There are no interconnections between protective bonding or functional bonding circuits in equipment by either direct link such as USB. HDMI, or via capacitive coupling

With regards 2. above, note that wired Ethernet requires a static discharge path to PE (which in some equipment is sent to line or neutral of the supply, which is not strictly in the Ethernet standards) via a suitable resistor/capacitor network.

The above is why IT or separated 'island mode' supplies are strongly recommended against for domestic systems in the IET Code of Practice for Electrical Energy Storage Systems, and are prohibited by MCS standard MIS 3012.

AMK said:

Surly the path  of the rod back to the star point needs to be low enough to operate the protective device. 

Which could be RCDs even in TN-S systems - see IET Code of Practice for Electrical Energy Storage Systems.

AMK said:

Thanks Andy. How about let’s say HO7RNF generator tails on the surface. What happens if they get cut and exposed copper touches the ground?

If that is the line conductor, and exposed-conductive-parts are connected to Neutral (the other line conductor) then all exposed-conductive-parts become live !

The only way to protect against this, without additional protection and suitable connection between N and Earth (earth electrode) is to have an RCD immediately after the N-PE link in the generator, that will operate if anyone touches an exposed-conductive-part of connected Class I equipment.

Chris Pearson said:

So why do we earth a generator please?

So we can control what happens in the various kinds of faults that can occur. If we don't earth the generator, but a live conductor becomes earthed by accident later, it makes a TN system anyway, but the results are less predictable than having line and neutral conductors with a specific, pre-defined, role.

The principles were established fairly early on in the development of electrical installations.

Graham, thank you.

I feel that I am getting a better understanding, but I shall stick to mains only. :-)