I am trying to square this up with a public supply, in which the LV end has either one or two conductors at earth potential, which is ensured by earthing at the transformer.

So is the output of the inverter any different from the output from a public transformer. Granted the cables are short, but they are also short if you live adjacent to a transformer.

If the PCE has an internal impedance of 5 Ω to 6 Ω, then ignoring whether this is Ze or not, how can any Zs be satisfactory?

I am only considering a generator as the primary supply here. 

Understood - I was just comparing it with a DNO supply, since they're both the same TN arrangement and work in the same way (electrode not part of the earth fault loop).

  -  Andy.

Interesting. So is the value of resistance of the electrode not to be considered in a typical earthed generator configuration ? I was under the impression that the value should be low enough to allow ADS in the event of insulation damage on the distribution cable to true earth (Ground) 

AJJewsbury said:

Faults direct to the general mass of the earth aren't quantifiable, as there will inevitably be a very significant resistance at the point of the fault - as unlike metals soil, even wet soil, has a considerable resistance - so the fault loop could easily be many kΩ so even an RCD can't guarantee immediate disconnection.

I was imagining a mobile generator on standard rubber tyres (like a car). If you touched a live conductor when standing beside the generator, you would not get a shock because there is no circuit.

However, the situation becomes different if the generator (along with the chassis, etc.) is earthed. So why do we earth a generator please?

So is the value of resistance of the electrode not to be considered in a typical earthed generator configuration ?

Yes, it's to be considered - as I recall BS 7430 recommends max 20Ω (although many seem to be on the opinion that significantly higher values can be satisfactory in some circumstances) - what I'm saying that's got little to do with Zs or normal ADS though in a TN system.

The problem with faults to true Earth is the resistance of the fault - i.e. the resistance of the soil that's in contact with the exposed live conductor - we just don't know what it's likely to be - it's likely to be many kΩ - so even if you could calculate something, the numbers are unlikely to work even for RCDs. That's why buried cables with any chance of damage need to have a concentric c.p.c. (e.g. armour).

   - Andy.

I was imagining a mobile generator on standard rubber tyres (like a car). If you touched a live conductor when standing beside the generator, you would not get a shock because there is no circuit.

Yup - protection by separation. Often used with small generators (e.g. ≤ 3kVA) especially when feeding a small system with a limited number of Class I items.

However, the situation becomes different if the generator (along with the chassis, etc.) is earthed. So why do we earth a generator please?

Separation is only good for small systems - larger systems have a couple of disadvantages for a separated system. Firstly long cable runs can mean the system goes get referenced to Earth - via stray capacitance (or even imperfect insulation) to Earth (BS 7671 set a limit of 500m of cable in total, and 100,000 Vm) . Secondly while separated system are only OK on 1st fault - as there's no disconnection on 1st fault, a 2nd fault can then occur which can be fatal - and larger systems will typically have more faults than smaller ones (all else being equal). There's also an EMC issue that some equipment with filters etc, like the PE to be connected to the same system as the live conductors, so they can actually divert the unwanted signals somewhere.

So all in all, larger systems tend to go down the TN route instead.

   - Andy.

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?

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.

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.