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?

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) 

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?

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?

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.

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.

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. :-)