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?

So similar to a typical earthed generator configuration 

AMK said:

So similar to a typical earthed generator configuration 

Well ... yes, but I can measure a loop impedance with a rotary generator (although noted the results can be variable with the wrong test instrument). With an inverter, the loop impedance/prospective fault current measurement is meaningless because the rms voltage is kept pretty constant during the test by the inverter.

It might mean, for an inverter, that Ze ≈ 0 for currents below the current limit, and Ze →∞ [or at least a very large number] for currents above the current limit, after a particular time (which may vary from manufacturer to manufacturer).

Thank you, Graham. Could you please confirm if it is appropriate to treat a PV system in the same manner as a standard generator with respect to earthing arrangements and related considerations? The value of the earth electrode should be low enough to allow sufficient fault current to flow in the event of a fault to earth; such as May arise from insulation damage on a cable.

AMK said:

Thank you, Graham. Could you please confirm if it is appropriate to treat a PV system in the same manner as a standard generator with respect to earthing arrangements and related considerations?

Yes, in so far as there is currently no distinction in BS 7671, AND Regulations 551.1.1 and 551.1.2 pull in "static converters" ...

AMK said:

The value of the earth electrode should be low enough to allow sufficient fault current to flow in the event of a fault to earth

Well, which 'earth' are you talking about/

'Earth' (capital E) is the 'general mass of the earth':

BUT, BS 7671 considers 'single fault conditions' which would be L to PE ... but not L to 'Earth' unless we are talking about additional protection? However, ADS is not 'additional protection' in general ...

AMK said:

The value of the earth electrode should be low enough to allow sufficient fault current to flow in the event of a fault to earth; such as May arise from insulation damage on a cable.

According to?

You see ... as far as BS 7671 is concerned, an insulated and sheathed cable can't have a single fault to Earth.

But, an armoured cable, or a cable with a cpc, can have a single fault to PE ... which in a TN-S or TN-C-S system, does not include Earth in the fault path ?

Hi Graham, yes I was referring to protection against shock by ADS. And the value of the electrode resistance should be low enough to cause the protective device on the output of the Genny to operate within 0.1-5 seconds. In most circumstances an RCD should be used to protect the distribution cable.

TN-S arrangement 

But in a TN-S arrangement, the electrode simply references the N to Earth, it doesn't form part of the earth fault loop. Consider that a DNO's electrode may be between 1 and 20 Ohms, yet Ze for a TN supply is usually < 0.3Ω.

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.

  - Andy.

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) 

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) 

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.

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.

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.

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.