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.

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 ?

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.

AJJewsbury said:

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

Yes ... and we must also consider the fact that, at present, there are no islanding requirements in G98 which means that many DNOs require G99 for islanding.

However, you said that islanding requires a significant amount of extra kit ... and that wouldn't be needed, if you didn't need "power backup" but wanted to set a zero export limit ... the wider issue, though, is that even if you do have islanding, it won't be long I don't think, before there will be more stringent conditions on islanding (i.e. can't "just do it" but power has to go off for a length of time first before it happens).

it won't be long I don't think, before there will be more stringent conditions on islanding (i.e. can't "just do it" but power has to go off for a length of time first before it happens).

Curious- I can't immediately see any benefit for the grid for doing that (the load will have been dropped already - I would have thought that having installations present near-zero load on grid return would have been beneficial if anything*). I could see that a random delay before switching back to grid after power return might help. A delaying in islanding on loss of power would seem to undermine any UPS type requirements for the installation too.

  - Andy,

* I recall back in the 1970s power cuts my dad following official advice and going around switching everything off after the power went off  - save one light that was left on to signal the end of the power cut.

AJJewsbury said:

Curious- I can't immediately see any benefit for the grid for doing that

If the installation is in direct feeding mode, and it just happens, yes that can definitely potentially cause instability.

However, let's say that we get to mid-day on a hot day, and suddenly all the parallel-connected generation is turned off because the battery is full ...

BUT

I think the issue will be resolved, overall (at least for those who "own" their installation ... energy poverty is a whole other discussion) by novel approaches to tariffs, which have already started with companies like Octopus.

However, let's say that we get to mid-day on a hot day, and suddenly all the parallel-connected generation is turned off because the battery is full

Bear with me - I'm sure I'm missing something obvious - if the battery is full then in effect one internal load (the battery charger) is disconnected - PV continues to generate and supply the other on-site loads and any excess is exported (i.e. the system degrades to a conventional PV inverter-only system). Load on the grid is reduced by the rating of the battery charger (probably not as bad as that as most multi-stage chargers reduce the power to the battery gradually as it nears capacity) - but even in the simple case of switching directly from full power to off situation it's hardly any worse than say a heat pump or EVSE switching off (or an immersion or hot tub) and the grid seems capable to coping with that. There's likely to be a significant variance between installations (different systems, different battery capacities, differences in panel positioning and shading) not to mention sun position across the country, so I don't see a huge risk of all such system across the country acting simultaneously (unless triggered by some external event - say a tariff change - but that sort of thing is under the control of the supply industry anyway).

   - Andy.