The right inverter for battery storage and some other Qs

[sharpener]

A good diagram which may or may not suit your purposes here https://community.victronenergy.com/sto ... yout-2.pdf

RCD100mA + Curve C 25A breaker. Would it sit inside in a mini cu between main CU and inverter?

Re diagram - the earth rod is confusing. Do I need one?

Yes, many people install a 4-way garage unit as a convenient way of housing these bits. If you are upgrading anyway you might also think about

(i) a Surge Protection Device
(ii) a bypass switch e.g. https://a1electrics.com/circuit-protect ... rator.html to connect the house to the incoming mains if the inverter is out of commission

If your inverter has got a UPS capability (like the Victrons) so it will power your installation in a power cut and it is type-approved for use for this purpose in the UK then it will have anti-islanding relays which will isolate the incoming mains and then connect the N to the E.
You then need your own earth electrode with an impedance of less than 200 ohms as you cannot rely on the incoming TN-C mains earth (e.g. they may have disconnected the cable somewhere to repair it). If no UPS capability you don't need it.

[Caesium]

If your inverter has got a UPS capability (like the Victrons) so it will power your installation in a power cut and it is type-approved for use for this purpose in the UK then it will have anti-islanding relays which will isolate the incoming mains and then connect the N to the E.
You then need your own earth electrode with an impedance of less than 200 ohms as you cannot rely on the incoming TN-C mains earth (e.g. they may have disconnected the cable somewhere to repair it). If no UPS capability you don't need it.

Does this also apply (the own earth electrode) for non-whole-house backup? By this I mean my inverter has a separate UPS circuit that stays powered (but not backpowering the entire CU) in a powercut. I have this wired up just to a couple of sockets for emergencies at the mo.

Do I need any special earth considerations for this socket as until now I was relying on the earth going right back to the CU through the inverter.

[Oldgreybeard]

If your inverter has got a UPS capability (like the Victrons) so it will power your installation in a power cut and it is type-approved for use for this purpose in the UK then it will have anti-islanding relays which will isolate the incoming mains and then connect the N to the E.
You then need your own earth electrode with an impedance of less than 200 ohms as you cannot rely on the incoming TN-C mains earth (e.g. they may have disconnected the cable somewhere to repair it). If no UPS capability you don't need it.

Does this also apply (the own earth electrode) for non-whole-house backup? By this I mean my inverter has a separate UPS circuit that stays powered (but not backpowering the entire CU) in a powercut. I have this wired up just to a couple of sockets for emergencies at the mo.

Do I need any special earth considerations for this socket as until now I was relying on the earth going right back to the CU through the inverter.

The simple answer is "yes". If there is a power cut, then there is a very good chance that the protective earth will disappear along with the power, leaving the house with no form of protective earth at all. This assumes that your house is like most, and is PME, where the earth is really the incoming neutral.

Obviously there are some risks associated with having no earth, even for a single backup socket. The fix is to provide a local earth, separate from the neutral of the incoming supply, so that this can be used to protect from electric shock when running some appliances from the battery system when there is a power cut. If all the appliances that you run are double insulated, so just have a two core flex, there's nothing to worry about, as they have no need of an earth. If any have a three core flex, which usually means they have exposed metal bits, then an earth needs to be provided, as there's a small risk that the metal bits could become live relative to the earth at the house.

The fix I've adopted was was easy. We live in a rural area, and the PME earth was already pretty dodgy, because of the length of the cable, so we had to ignore it and install an earth electrode plus a Type S RCD on the incoming supply, anyway. When I got around to fitting the batteries and emergency supply this was a blessing, as it meant that I didn't need to make any changes to the earthing at all. Our earth electrode provides a good earth whether we are drawing power from the grid or drawing power from the battery inverter when there is a power cut. This also means that I can connect up my small and ancient Honda generator as an emergency supply safely, without the need to worry about earthing.

[Caesium]

The simple answer is "yes". If there is a power cut, then there is a very good chance that the protective earth will disappear along with the power, leaving the house with no form of protective earth at all. This assumes that your house is like most, and is PME, where the earth is really the incoming neutral.

Ok, sounds like this is the point where I should probably get quotes to get this done properly

Sounds like this will involve an earth spike, do I have to get my house earth isolated from incoming earth then? Or will it still be the same, just with an additional earth point at my house?

[sharpener]

The simple answer is "yes". If there is a power cut, then there is a very good chance that the protective earth will disappear along with the power, leaving the house with no form of protective earth at all. This assumes that your house is like most, and is PME, where the earth is really the incoming neutral.

Ok, sounds like this is the point where I should probably get quotes to get this done properly

Sounds like this will involve an earth spike, do I have to get my house earth isolated from incoming earth then? Or will it still be the same, just with an additional earth point at my house?

No, you can keep the TN-C earth and connect the ground spike to the Main Earthing Terminal as well. But better IMO is to convert to a plain TT arrangement, then you can't import your neighbours' interference or fault currents. See here https://electrical.theiet.org/wiring-ma ... e-systems/

OGB, is there a way of putting this stuff in a sticky note so we don't have to keep typing out the same advice?

[Oldgreybeard]

The simple answer is "yes". If there is a power cut, then there is a very good chance that the protective earth will disappear along with the power, leaving the house with no form of protective earth at all. This assumes that your house is like most, and is PME, where the earth is really the incoming neutral.

Ok, sounds like this is the point where I should probably get quotes to get this done properly

Sounds like this will involve an earth spike, do I have to get my house earth isolated from incoming earth then? Or will it still be the same, just with an additional earth point at my house?

No, you can keep the TN-C earth and connect the ground spike to the Main Earthing Terminal as well. But better IMO is to convert to a plain TT arrangement, then you can't import your neighbours' interference or fault currents. See here https://electrical.theiet.org/wiring-ma ... e-systems/

OGB, is there a way of putting this stuff in a sticky note so we don't have to keep typing out the same advice?

To be clear, adding an earth electrode in parallel with a PME earthing system WILL NOT necessarily provide protection in the event of a power cut that cuts the neutral in the street (and so removes the very, very low impedance earth loop fault current path).

Unlike PME, with its very, very low earth loop impedance, an earth electrode does not provide a high fault current path. It cannot, as it will be very unusual for the earth loop impedance from a simple earth electrode to be lower than about 20 ohms, and that is far too high to carry any sort of significant fault current without the entire earth system rising to a dangerously high voltage relative to the local earth ground.

If adding an earth electrode in parallel to a PME system, so adding another intermediate earth to the chain of them connecting the supply neutral to ground all along its length (hence the "multiple" bit in PME), then to make the installation safe in the event of a power cut taking out the supply neutral, it's essential that every circuit has RCD protection. There are still an awful lot of homes with no RCDs, or only partial RCD protection. Best bet is to just add a100mA Type S RCD on the incoming supply, and add an earth electrode. That way the installation just reverts to being a properly protected TT one in a power cut, with no high earth loop fault currents because of the Type S RCD on the incomer, or more likely a 30mA Type A RCD tripping if there happens to be one protecting that circuit.

[sharpener]

Unlike PME, with its very, very low earth loop impedance, an earth electrode does not provide a high fault current path. It cannot, as it will be very unusual for the earth loop impedance from a simple earth electrode to be lower than about 20 ohms, and that is far too high to carry any sort of significant fault current without the entire earth system rising to a dangerously high voltage relative to the local earth ground.

And another complication of the high electrode impedance is that if you have an SPD fitted directly on the intake, and it develops an internal fault in the L-E section, the fault current may not be enough to take out the company fuse. (In my case the impedance is 60 ohms so only 4A will flow.) So you are advised to swap the E and N connections (i.e. wire it L-N and E-N instead of L-E and N-E) so then the current will be enough to blow the fuse. Sorry I can't immediately find the reference for this advice (and it is not in the installation instructions) but it makes sense to me.

[ducabi]

Is there another option to make it save in UPS mode, eg PEN protection? I have TN-C-S system early 90s build in estate development.

[sharpener]

Is there another option to make it save in UPS mode, eg PEN protection? I have TN-C-S system early 90s build in estate development.

Not to my knowledge other than fitting an isolating transformer and having an IT system (completely isolated and not earthed at any point), but this is not used in domestic settings for the reasons outlined here https://en.wikipedia.org/wiki/Earthing_system.

You cannot rely on any type of incoming supply earth in a power cut, the cable might have been cut in two by a digger or been disconnected at the substation prior to re-routing. The inverter cannot tell what the situation is and will start generating automatically so its output needs a separate earth. Even if the loads plugged into your UPS sockets do not require an earth there will be the exposed metalwork of the inverter itself.

OGB describes what is required in post #63 above, the rationale is explained in the IET link.

Sorry

[A cautionary tale: when I was seconded to the DTI there was a terrible case where a man had wired a toaster plug with the L and E reversed. Even though it did not (could not) work he left it plugged in and switched on. His baby son climbed on to the work top, touched the toaster and was killed instantly. Some might say this was 20th century Darwinian evolution in action.]

[ducabi]

Yes, many people install a 4-way garage unit as a convenient way of housing these bits. If you are upgrading anyway you might also think about

(i) a Surge Protection Device

Would the 4-way unit accommodate everything,ie:
-RCD100mA + Curve C 25A breaker.
-Surge protection Device?
Do you have an example of a suitable set including above or what SPD shall i buy?
Do I still need AC isolator if I have above?

(ii) a bypass switch to connect the house to the incoming mains if the inverter is out of commission

Does this mean that using a standard connection if something goes wrong with inverter I won't have power at home from the grid?