I too think that the Solis earthing diagram is confusing.
As I see it the situation is this...
We commonly have circuits which are protected by an Earth-Leakage Trip, such as an RCD or RCBO. They 'measure' the current passing through live and neutral. If there's a difference (of 30mA or more) between the current on live and neutral, then it implies there is a fault - some current is passing to earth. The imbalance in the current must cause the Trip contacts to open.
Appliances have a Protective Earth (PE) connection to ensure that a fault allows current to find its way to earth. If that were not so then the metalwork of the appliance would 'float' to a high voltage under fault conditions. That means there's a risk of electric shock if a person then touches the appliance.
If I install an inverter in a house which has its own earth rod (as mine does), then a power-cut doesn't remove my earth.
So it's OK to use the earth rod for both grid-tied appliances, and those which continue to be powered from the 'off-grid' output.
However, if the earth is provided by the DNO, then I can't trust that it's intact (the cable might be cut) nor do I now have a Neutral.
For my Off-grid (Maintained) output from the inverter to continue operating during an outage, it must have its own earth rod, and the Neutral must be tied to it.
That relay may or may not already be included inside the inverter. The installation instructions must therefore be followed.
Once the mains power is restored, the relay becomes re-energised and the connection between Earth and Neutral is removed for the appliances which have been operating off-grid.
The Neutral-Earth bond at the Service Fuse (TN-C-S) or the substation (TN-S) again provide the required earth reference.
If that relay weren't present, then the RCD supplying the inverter from the Main Distribution Board would trip out. It would sense the connection between Neutral and Earth and act as if there were a fault.
The actual instructions may still vary between inverter manufacturers. Some inverters will have the Neutral terminals connected together permanently, and others won't during a power-out.
This post was modified 10 hours ago by Transparent
However, if the earth is provided by the DNO, then I can't trust that it's intact (the cable might be cut)
I cant see how that is the case if the earth is provided by means of a bond to the metal outer sheath of the incomer; no cut (other than one in your driveway of which you would surely be aware) can remove it. This is 'usual' for TN-S according to the IET diagram and text here
I can see how this is the case if the earth is provided by your DNO 'remotely'. This is 'usual' for TN-C-S according to the diagram in the same location
Which makes me wonder about how TN-C-S is safe, because isn't it the case that 'earth' potential can vary from location to location so its normal (required?) to earth near an installation
Or did I misunderstand?
This post was modified 9 hours ago 2 times by JamesPa
This is what the Solis Engineers sent me about earthing, it does not mandate a rod in all situations, but it may apply:
When no voltage is present at the inverter’s grid port, the neutral on the backup port is bonded to the protective earth (PE) terminal on the same port. In typical installations, this is the point where an earth rod is connected. The Solis design offers flexibility by allowing either a dedicated earth rod or the grid’s PE connection to be used at this port. Important: Always consult and comply with local electrical regulations and standards when configuring neutral-to-earth bonding and earthing methods.
This was what led me to believe the requirement for the rod is related to the inverter backup/UPS/off-grid, but this does not stop it covering all earthing requirements in the site.
My recommendation is still to have the distributor review the overall plan and diagram. I approached Solis for this, but I do not think that should be necessary, and the disadvantage is they are not always that familiar with all local regulations. I could have oversized a few circuits as a result.
Now in terms of earthing, in most sites, if the DNO provides an earth connection, is it not reliable?
I wonder what happens with purpose built flats, is earthing via bonding to metallic pipes or is there a rod for the block?
I too think that the Solis earthing diagram is confusing.
Hopefully I helped explain it, my recommendation is to get the distributor to do some work for their margin. They may suggest an installer, but that could mean thousands extra for an MCS certified one. And they would probably not be able to recommend one that works in your area..
So with the diagram, I managed to get electricians to do the work. But it should be said that far too many NICEIC professionals are too busy fitting new sockets and lights to want to get into higher powered circuits.. Even fitting an EV charger is not common knowledge.
Also, most electricians that had "heard" about inverters immediately pointed out that the idea of treating an inverter as a socket that you can supply via a 30ma RCBO was a non-starter. And that was even before there was any discussion of the type of inverter.
It is not an absolute requirement, the electrician is the best one to confirm what is needed.
For my Off-grid (Maintained) output from the inverter to continue operating during an outage,
it must have its own earth rod, and the Neutral must be tied to it.
That relay may or may not already be included inside the inverter. The installation instructions must therefore be followed.
Once the mains power is restored, the relay becomes re-energised and the connection between Earth and Neutral is removed for the appliances which have been operating off-grid.
The Neutral-Earth bond at the Service Fuse (TN-C-S) or the substation (TN-S) again provide the required earth reference.
If that relay weren't present, then the RCD supplying the inverter from the Main Distribution Board would trip out. It would sense the connection between Neutral and Earth and act as if there were a fault.
The actual instructions may still vary between inverter manufacturers. Some inverters will have the Neutral terminals connected together permanently, and others won't during a power-out.
The Solis S6 is a "key in hand" kind of islanding/UPS capable inverter. So the operation can be seamless. But it should be said this may the same with others, like the givEnergy inverter I subsequently looked into. The Solis delivers a bit more for similar cost, has that extra smart port that can work as lower priority UPS or to take an AC coupled PV inverter but this could be trumped by other factors.
Perhaps when the initial off-grid inverters became available, they required manual or ATS switches?
