How to add batteries & more PV

[openspaceman]

Thanks for the clarification; that is how I thought it must work with a changeover switch which is what I intend to do, then using the EPS or a generator to keep the FITS system and the additional panels alive.

Can the earthing be implemented at the changeover switch with ground spikes if the earth is normally provided by the grid?

[Umski]

At this point the inverter can draw from the directly connected PV and from the batteries BUT as there is no grid the "legacy" inverter will disconnect and stay disconnected until the grid is restored, so you will not get any power from the PV connected to the "legacy" system.

Thanks, this is the clincher for me as in EPS mode, I would be reliant on the batteries and my legacy grid-only PV would be redundant during a power cut

I have mine set up so that when we have a power cut I have to manually enable the EPS output and plug it in to my existing "generator" input and then I have a manual 100A changeover switch which switches the whole house over. This was an existing bit of wiring as I have a large diesel generator which used to cover the frequent power outages we get - when I say frequent it's really around 5 or 6 long (> 6 hours) outages a year - enough to be a bit more than annoying, the Solis EPS output is a lot more convenient - and quieter!!

I have a similar arrangement I think as the previous owners had a changeover switch for the whole house with a commando plug outside - is that what you mean by plugging into your "generator" i.e. take a 'socket' from the EPS output and plug it into 'the other side' of the changeover switch Presumably you don't want your legacy PV to think your EPS is a 'grid' so you need to flick this off manually too?

Oh yes and good point about earthing too

[Oldgreybeard]

I just followed the instructions that came with the inverter, which was to install a changeover contactor so that the emergency supply consumer unit (that just runs the lights, water pump, sewage pump and a strip of sockets) is powered from the grid normally, then when there's a power cut it automatically changes over to run from the islanded inverter. Earthing was not an issue here as the house was already TT anyway. Means we don't need to do anything in a power cut, the lights stay on, we can run the TV, use the toilet, etc, as usual, just need to use the camping stove to cook and make cups of tea.

[marshman]

Thanks for the clarification; that is how I thought it must work with a changeover switch which is what I intend to do, then using the EPS or a generator to keep the FITS system and the additional panels alive.

Can the earthing be implemented at the changeover switch with ground spikes if the earth is normally provided by the grid?

NO !! the EPS or generator WILL not keep the FiTs system alive unless you have a set up like Nowty - in fact the instructions with the Solis specifically state you cannot connect inverters together on the EPS output. A standard grid tie inverter needs to see the correct impedances, which it checks during its start up phase - it will not get the correct readings from the EPS output or a generator. Think about the situation where your batteries are fully charged and the house is drawing a minimal load and the sun is shining, the Solis will throttle back it's own PV inputs but what happens to the FiTs system? no where the power to go and nothing controlling the inverter output, that's if your particular make of inverter can actually do it. SMA Sunny Boys can be controlled by a "Sunny Island" but that is a whole different set up.

Earthing, I won't comment as each situation is different. You may or may not be on PME, you may have a separate earth stake, you may have both. Plenty of info but no definitive answers on the t'interweb. Just to confuse things further the regulations, guidance and advice on earthing keeps changing. My view is you just have to make sure you have a decent earth under ALL conditions - but be aware of what a "decent" earth is.

[Stinsy]

Typing this has just prompted me to check the earth impedances of my earth stakes as the recent (actually long term) lack of rain and low groundwater levels could affect it. Normally it is a fairly constant 11 Ohms when ever I have checked. Just checked it and now it is 14.1 Ohms, still pretty good in my opinion, however for comparion, the PME earth on the house is 0.47 Ohms.

The problem is that your PEFC isn't big enough to reliably trip the overcurrent protective device. Some people fit an additional "time delay RCD" to ensure they still have circuit protection and additional protection. In any case it would be prudent to test the function of your RCDs because you're relying on them to keep you alive!

[Oldgreybeard]

The problem is that your PEFC isn't big enough to reliably trip the overcurrent protective device. Some people fit an additional "time delay RCD" to ensure they still have circuit protection and additional protection. In any case it would be prudent to test the function of your RCDs because you're relying on them to keep you alive!

It is worth remembering that the absolute maximum possible prospective earth fault current for an ancient TT installation is only 100mA, and for any modern installation with RCDs on all the circuits that fault current will only be 30mA, so even an earth loop impedance of a few hundred ohms will be fine. The regulations allow a maximum earth loop impedance of 1667Ω for a TT installation, although the guidance (and it is only guidance) is that this resistance should be less than 200Ω at initial installation testing or whenever the installation is subsequently tested as a part of a periodic inspection.

The only reason that the earth loop impedance needs to be massively lower for any non-TT installation is because there may not be an RCD at the incoming supply, so there the prospective fault current on the supply to the consumer unit (or any circuit that doesn't have an RCD protecting it) can be a great deal higher. IIRC, a PME supply must have an earth loop impedance of under 0.35Ω, for example, for this reason.

Last time I measured our installation, back when I did the testing and produced an electrical installation chit (as an arse covering exercise) for the battery inverter system, I found that it was down at 24Ω, which is very good indeed. The three other places we've lived that had TT installations were a fair bit higher than that, around 70Ω to 100Ω IIRC. If anyone has a Renault Zoe, though (I do) then it's worth noting that if they have a TT installation (best bet for car chargers in my view) then the earth loop impedance needs to be under 150Ω, as the Renault checks this and throws a wobbly if it is higher than this.

[marshman]

The problem is that your PEFC isn't big enough to reliably trip the overcurrent protective device. Some people fit an additional "time delay RCD" to ensure they still have circuit protection and additional protection. In any case it would be prudent to test the function of your RCDs because you're relying on them to keep you alive!

No practical TT system will ever carry enough current to operate an over current device - though mine will carry at least 16A at 230V so should operate the 5A MCB on the lighting circuits! My loop impedance via the earth stake is a pretty damn good at 14.1 Ohms, even in these drought conditions (normally around 11 Ohms). This is only on my outbuildings, at the moment the house is still TN-C-S, though I am considering changing to TT to simplify the earthing when running off the inverter EPS. (All the PV, inverters and batteries are in outbuildings). All the consumer units have 30mA RCD's that I have tested properly (not just pressing the "test" button), and all operate as they should within the specified no. of cycles.

?? why a time delay RCD, or is that where the feed to the outbuildings is from the house consumer unit instead of a separate connection via a Henley block?

[Oldgreybeard]

No practical TT system will ever carry enough current to operate an over current device - though mine will carry at least 16A at 230V so should operate the 5A MCB on the lighting circuits! My loop impedance via the earth stake is a pretty damn good at 14.1 Ohms, even in these drought conditions (normally around 11 Ohms). This is only on my outbuildings, at the moment the house is still TN-C-S, though I am considering changing to TT to simplify the earthing when running off the inverter EPS. (All the PV, inverters and batteries are in outbuildings). All the consumer units have 30mA RCD's that I have tested properly (not just pressing the "test" button), and all operate as they should within the specified no. of cycles.

?? why a time delay RCD, or is that where the feed to the outbuildings is from the house consumer unit instead of a separate connection via a Henley block?

Absolutely correct. The only current that the earth electrode ever has to carry is the trip current for the main RCD, usually 100mA for a TT system, 30mA for circuit protective RCDs.

The requirement is that the circuit protective conductors (aka "earth wires") must not rise above 50V (which is considered to be safe to touch) above earth before the RCD on that circuit trips. This is where the maximum allowable earth loop impedance for a TT system of 1667Ω comes from, good old Ohm's Law, 50V / 30mA = 1667Ω. For the main 100mA RCD to trip needs an earth loop impedance of not greater than 500Ω, but as most homes now have 30mA RCD protection (or should have, really) then the RCDs protecting the individual circuit are what really matter.

Not usually a problem to get below 500Ω anyway, and the recommendation is that the earth loop impedance of a TT installation shouldn't really be greater than 200Ω, which is a pretty high value in the real world.

I left our house as TT as it had to be TT when it was being built (not allowed to have PME during construction). Glad I did, as it made life easier when it came to connecting up the inverter and EPS.

[openspaceman]

NO !! the EPS or generator WILL not keep the FiTs system alive unless you have a set up like Nowty -

I see, it seemed to me that the sunsynk inverter could be programmed to run islanded.

Nowty has explained he uses a sunnyisland but a drawing would help.

Even if the FITS system cannot be kept alive without the grid I shall continue to look into how an additional array may be able to do so, a local company that deal in Victron kit have said they can do it and should be getting back to me, also the chap who did my original installation says he can start work on a second system in October, though he lapsed his MCS certification but still is registered NCEIC.

Now I have not experienced cuts of more than several minutes for over forty years but with the way the energy market is changing I wonder if load shedding may become more likely.

As to dealing with excess generation when the grid isn't present this cannot be different from programming the inverter not to export.

[nowty]

Info for SMA whole house backup is here, there are diagrams, but unless you have specific hardware it wont be possible to keep your Solar inverters running.

http://www.windandsun.co.uk/media/93853 ... es-v21.pdf