Can I put the cat amongst the pigeon.

Scenario 1. Single phase in a dwelling working normally
Scenario 2. Single phase in a dwelling working in ISLAND mode

The supply side has now changed.  Is the DNO/DSO/MO worker down the street doing an install or repair still safe?  Could the worker get a shock from a reverse feed?

Sergio Fernandez said:

Scenario 2. Single phase in a dwelling working in ISLAND mode

The supply side has now changed.  Is the DNO/DSO/MO worker down the street doing an install or repair still safe?  Could the worker get a shock from a reverse feed?

Please could you describe how the "reverse feed" might happen under these circumstances (get you from your inverter down the street to the worker, then back to your inverter again to complete the circuit? The live conductors (line and neutral) are disconnected, per BS 7671, but not the protective conductor.

Yes, I can see small amounts of current being shared under very unusual circumstances (electricity usually takes the easiest path), but this is no different to current shared through extraneous-conductive-parts from installations of other DNO distribution circuits that are still energized in the area. Consider that, if we disconnect the distributor's earthing terminal from an islanded installation, extraneous-conductive-parts are still connected, which are:

(a) still connected to the ground, and therefore still connected to the PME earth electrodes, albeit via earth resistances; and
(b) if shared metallic services, still connected to the distributor's earthing arrangements through other installations via main protective bonding.

In summary, in switching the link to the distributor's earthing terminal when moving to island mode, you'd be introducing a potential point of failure in the installations' predominant earthing arrangement (connected mode, via distributor's earthing terminal), for little or no realized benefit.

I do think in time people will refer to LIVE conductors in a single pahse install as

Line Neutal Earth.

No, not Earth. If you change the meaning of LIVE again to include Earth you'd have an awful lot of rewriting to do across an awful lot of text (or have to insulate an huge amount of metalwork).

upgrade the BS7671 regulation to a 3 pole isolator

Isolators aren't perfect. They should be reasonably reliable in opening (i.e. indication of open shouldn't happen until all poles have opened fully) but like any mechanical device they can and do go faulty. Often it's one pole or other that doesn't fully close (or a loose connection at the terminals) - if that happens to be N then things start to go wrong fairly rapidly - on single phase things will be fed by full mains voltage but appear to be dead to an ordinary person which carries some risk - while on 3-phase an open N with line conductors still connected can result in 400V being put across things intended for only 230V - with a high likelihood of damaged equipment and even fire (with all its attendant dangers). That's one reason why 3-phase installation traditionally had 3-pole rather than 4-pole devices.

Start switching PE and you greatly increase the risk of an installation being completely unearthed while powered up.

There are many situations where the regs (indeed physics) give us less than ideal situations - from small overloads of long duration to faults between live conductors of different circuits - where there is the theoretical possibility of all kinds of trouble. In practice the chances are normally acceptably low, or 'the least worst' option - but such things are kept under review as it were.

   - Andy.

Very interesting. We have just ordered a solar panel system with battery backup.

Looking at the inverter/battery spec. it looks as though excess power could easily be inserted into the grid.

When they come to install it all I will simply ask if I have to switch a transfer device when the battery is fully changed and I'm not using all the solar energy generated on a nice sunny day like we have been known to get on odd occasions.

Whats the betting a simple shrug of shoulders will be the answer. (no, I'm not flogging it back at half the price they sell to me at oil/coal generated prices the government insist on using as a charge basis).

Norm 

Normcall aka Norm said:

Looking at the inverter/battery spec. it looks as though excess power could easily be inserted into the grid.

Yes, that is 'parallel operation' as defined in Regulation 22 of ESQCR. Unless you have an export limiting system to G100, excess generation will be delivered to the grid when the battery is full (and when the battery is charging, if the PV can deliver more power than being used to charge batteries and power any loads you have switched on).

Island mode is where the grid is disconnected but the installation continues to operate

Normcall aka Norm said:

When they come to install it all I will simply ask if I have to switch a transfer device when the battery is fully changed and I'm not using all the solar energy generated on a nice sunny day like we have been known to get on odd occasions.

You could just set the system up as an export limiting system with export limit of zero ?

My experience ranges from, "we have to fit an isolator" even when one is already present to, "happy to fit one if we have it in the van". However, in general terms the law is not retrospective. In 1989, removal of the fuse from the service head would have been the norm and does of course provide safe isolation. If one has to pay the DNO or supplier to undertake the task, that does not contravene the EAWR 1989.

If you are tempted to do it yourself, you do need to be properly equipped to cope with the possibility that removal of the fuse disturbs the service head. It may be a rare occurrence, but what would you do if it happened?

I might add that my local DNO, SSE, permits electricians to pull the service fuse in case of emergency.

We have just ordered a solar panel system with battery backup.

Is the battery there to guard against grid failure, or simply to 'time shift' PV output sunny periods to when you're likely to need the power?

Looking at the inverter/battery spec. it looks as though excess power could easily be inserted into the grid.

Almost certainly the case. In simple cases (i.e. what happens with traditional PV setups without a battery) is that the inverter is connected just as if it were an ordinary load (if one that draws -ve power) - no N-PE links, no inverter-end protective devices, no need to regulate output to match your domestic loads. If the grid fails the inverter shuts down completely. So no risk of electrocution from your PV system for DNO workers, but no power available in your home even if it's sunny.

With battery systems (whether part of a PV system or separate) there is a possibility of "islanding" - i.e. if the grid fails then your installation is disconnected from the grid and supplied via an inverter from your battery. The inverter needs to match its output to your load and you need to provide your own means of earthing and N-PE link. Disconnection from the grid and switching in of a N-PE link is normally automated.

Islanding mode requires a significant amount of extra kit and complexity - and extra hassle with the DNO if the total on-site generation exceeds 16A/phase (which is probably likely if you want everything to work normally without the grid) - so just because there's a battery, it doesn't necessarily follow that you'll have islanding mode available. By far the cheaper option would  be for the battery system to operate as if it was a conventional PV system - just shifting its output to co-inside with likely on-site demand.

   - Andy.

AJJewsbury said:

Islanding mode requires a significant amount of extra kit and complexity - and extra hassle with the DNO

That's why I recommended setting an export limit (even if not required by a connection agreement) - there are apparently G100-ready inverters out there, and would just require an import/export monitor (current transformer connected by aux cable, or by CANbus/Modbus) at the origin.

That's why I recommended setting an export limit

But aren't the limits (e.g. ESQCR & G98/99) set by the generating capacity rather than export?

  - Andy.

The system is designed to provide a max of 6Kw and just 3Kw in backup mode with optional automatic transfer switch. I think it should come configered for what we want is simply battery supply overnight (grid will take over when battery flat).

We have 3 phase supply (no gas until recently) and lage transforner feeding the village in the field down the road so  should do us for now.