AJJewsbury said:

Although in the OP's suggested case, the inverter output would be permanently separated from the grid supply so no switching involved. 

Electrical separation alone doesn't prevent back-feeds to the grid.

A means of isolating the live conductors of the DNO supply is necessary to conform to BS 7671 and possibly also to comply with ESQCR. This could be manual, rather than automatic, but in any case should meet the requirements of BS 7671 for a device providing isolation.

gkenyon said:

Electrical separation alone doesn't prevent back-feeds to the grid.

I'm obviously missing something here - how does a separated system back feed into the grid?  Unless someone is daft enough to plug in a "suicide lead" to power the rest of the house from the solar system.

Electrical separation alone doesn't prevent back-feeds to the grid.

Separated in the general sense that there's no connection between the two systems at all (rather the separation in the sense of section 413) - perhaps "distinct" or "independent" might have been a better choice of words.  As I understood, the OP's original suggestion was for PV to feed a battery to feed an inverter which fed a number of fixed socket outlets - a completely off-grid system, with no connection with the grid supplied stuff (other than perhaps a shared earthing/bonding arrangement). It's neither a switched alternative nor parallel source.

   - Andy.

Simon Barker said:

I'm obviously missing something here - how does a separated system back feed into the grid? 

A transformer, such as an isolating transformer, can be used to provide "electrical separation", or even "galvanic separation".

But a transformer can work "backwards" as well as "forwards", whereas an isolator disconnects the circuit completely within the stated impulse withstand range.

Basically, "disconnection" or "isolation" is not always directly related to "separation", when we are talking about sources of supply on both sides of the "separation".

Confusingly, an "isolating transformer" provides separation, but not always "isolation" - the term "isolating" in this case is of course very confusing.

AJJewsbury said:

As I understood, the OP's original suggestion was for PV to feed a battery to feed an inverter which fed a number of fixed socket outlets - a completely off-grid system,

Worth having a look at products on the market ... and their arrangements between grid vs battery vs "backup". We can't assume ... whilst it appears to be "grid independent" in reality, if it's on grid some of the time, and that charges the battery, it's pertinent to consider how it's "isolated" from the grid when working in "power backup" mode.

AJJewsbury said:

Separated in the general sense that there's no connection between the two systems at all (rather the separation in the sense of section 413) - perhaps "distinct" or "independent" might have been a better choice of words. 

The choice of words is, I'd agree somewhat arbitrary to a general techincal discussion, but not perhaps when we are talking about conforming to standards and legislation.However, there's a huge difference between "isolation" and "separation", see earlier post regarding isolating transformers.

Perhaps, though, the actual inverter arrangement is all-important, and that is precisely what the OP has not provided input on !

Dear all  Wish there was wat to thank everyone for contributing and I guess close this thread .  So hope this suffices:

Not familiar with solar electrics Was relying on a supplier but with me locally laying the groundwork  Now we have more constraints 

1.I stipulated would not tolerate a volatile lithium battery in my house  ( therefore would site inverter etc in garage at rear of my garden)

2.Proposed underground cabling . Supplier ( Victron kit)  refuses any guarantees if I do that 

3. Supplier now states I will need to supply separate earth electrode 

Not prepared to disfigure may garden with exposed wires nailed to fences , Mind boggles at some "Brunel tunnelling  for earth electrode  

Is it also most of you express disquiet  in one form or another Not all understood. 

So I give up : 

PS Still think the idea has great  meriit for smaller domestic affordable solar back up    Believe power starvation ix coming . I cannot see the merit of anything grid tie . Seems in USA back up generators are not novel. Though solar rather than petrol perhaps is.

With thanks to you all 

Ms O 

Anne Otis said:

PS Still think the idea has great  meriit for smaller domestic affordable solar back up 

The UK does permit "backup" ... but the right product needs to be selected. There are products on the market that can be installed in a conformant manner.

So I give up : 

That's a shame

.Proposed underground cabling . Supplier ( Victron kit)  refuses any guarantees if I do that 

That sounds peculiar - electrically underground cable isn't that different to say clipped to a fence.  I wonder if something has been lost in translation - or is the underground cable for the DC solar side?

3. Supplier now states I will need to supply separate earth electrode 

That bit makes sense at least.

Mind boggles at some "Brunel tunnelling  for earth electrode  

In most situations, hitting in a 4' long 3/8" diameter rod into the ground with a hammer suffices. Traditionally a few inches were left above ground to make the connection, but the preference these days is to sink it into a small pit with a lid over - still you're only talking about digging up less than 1 sq ft to a depth of a few inches - hardly IKB's territory and not a lot compared with digging the trench for an underground cable. (For my detached garage, which I wished to TT, I used one of the access chambers for the underground supply cable to house the electrode - two birds with one stone as it were)

 Seems in USA back up generators are not novel.

Not entirely unknown this side of the pond either - the subject comes up i the Forum fairly regularly. .Usually as a switched alternative though (so supplying all (or most) of the normal house wiring, rather than independent dedicated circuits) but it's all horses for courses. The entirely off-grid community has a presence here as well.

rather than petrol perhaps

Most true backup generators tend to be diesel rather than petrol - as petrol "goes off" after a relatively short time (in a matter of weeks or a few months, the lighter more volatile components of petrol evaporate, and the remainder doesn't work as well) - just as an aside.

   - Andy.

your local earth electrode will not be on the fence, just underground ;-) all you will have on the surface once it is all over is a small access cover at the top of the rod or rod. Neater than this one, it is about 15cm by 15cm the rod is 4ft (1.2m)long, and the diameter of your thumb,  and drives in just like a big nail. Just avoid the drains. and the gas pipe ...

. 

Oddly I have been looking at a battery and solar system something like this - a colleague at work keeps telling me how good it is...  And most of the companies I have spoken to have no problem putting the battery indoors, in the loft or indeed in an waterproof cupboard on the roof of my attached garage (!), so your suppliers sound like they may be more nervous than many. Cable underground in ducting is a perfectly sensible thing, and done all the time so again I am surprised.

That said all but one of the folk I have been talking to did  wither a bit when I start asking deeper questions about fault currents and Island modes, and I rather fear/ suspect many work by repeating the same recipe rather than a deep grasp of the design fundamentals.

(my own domestic electrics are a slightly non-standard shall we say... )

Mike.