Enabling EPS DIY?

[Kenny000666]

Hi guys

Just reading through this thread - viewtopic.php?p=19952#p19952.

I would like to enable EPS on my Solis RAI ac inverter. My use case is really to keep some key electronics running totalling most probably less than 300w
4x raspberry pi,
Ethernet switch,
Modem
Wifi router

Not too fussed about lighting etc, got phones for that, but once my pis go down, so does my home automation and these clusters start getting all unstable when they come back up. I worry I might even get some HDD failures.

I see the above thread suggests getting a separate CU, most probably on a 16amp RCD, and create circuits off that for plugging things in.

I’m thinking; can I diy connect a cable to a 16amp fused socket and for connecting to an extension lead.

Stupidly I missed out on forward planning, the electrician just wired up the inverter to my Garage CU and has just provided certs. If i thought about this earlier, could have asked him to connect up a socket to the backup (he didn’t want to get involved with the inverter anyway so probably would have refused the extra work)

If I’m going to need to get an electrician on hourly rates to do something simple like this, it might just be cheaper for me to invest in a UPS, anyone any thoughts?

[Oldgreybeard]

Main issue is earthing, but if the power supply you're using for your home automation stuff is double insulated then that problem goes away. For the specific case where the only items to be powered from the EPS are double insulated (i.e. have a two core flex with no earth required) then it's OK to just connect them to the EPS supply. If the power supply needs an earth, then things get more complex, as you need to consider how to ensure there is a protective earth for the EPS system (the incoming supply provided earth may well not be present in a power cut).

Whether they stay up in a power cut depends on the inverter. Some take a short time to switch the EPS on, so there is a chance that the dip in power may cause the Pi's to crash. Depends how long the power supplies take to respond to the dip at switchover, as well as how quickly the inverter takes to switch.

I'm not 100% sure about the speed with which the Solis switches its EPS on, but know that my Sofar takes just a fraction of a second, but that's enough to cause a noticeable dip at switchover. The Solis may be better, perhaps like the Powerwall that seems to switch instantly.

[Kenny000666]

Earthing wise I should be safe, the Solis is earthed at supply as well as having an Earth from the inverter chassis to an Earth block that goes to the metal gas pipe. All the transformer adapters with the devices also don’t have earths, as the 3 pin plugs have plastic pin for the earth.

RE the inverter cutting out; I will need to test it out. I assumed the EPS to be always on supply when connected to battery, I.e getting power from battery at all times so in a power cut it will be seamless. I didn’t think about the inverter itself may shutoff for a couple of seconds. Which would impact that EPS connection.

So maybe an external UPS is a must. Not that I have regular power cuts

[Oldgreybeard]

Earthing wise I should be safe, the Solis is earthed at supply as well as having an Earth from the inverter chassis to an Earth block that goes to the metal gas pipe. All the transformer adapters with the devices also don’t have earths, as the 3 pin plugs have plastic pin for the earth.

RE the inverter cutting out; I will need to test it out. I assumed the EPS to be always on supply when connected to battery, I.e getting power from battery at all times so in a power cut it will be seamless. I didn’t think about the inverter itself may shutoff for a couple of seconds. Which would impact that EPS connection.

So maybe an external UPS is a must. Not that I have regular power cuts

That earthing will not work if the supply fails, though, that's the point. A power cut is as likely to be caused by the neutral being cut (which provides the earth to most homes) as it is from a phase wire being cut. The bonds to the gas pipe etc are not to do with earthing, as such, they are equipotential bonds to keep all the metal in the house at the same voltage in the event of a fault.

If the power supplies are all double insulated, then as before you're OK, as they will be no less safe running on the EPS as they would be on the mains supply.

I don't know about the Solis EPS, but the Sofar EPS is not always on, it switches on when the grid goes down, so there's an external contactor to switch the always-on supply. I believe that the Solis may have this contactor internally. All comes down to how quickly it switches. Best bet might be to do a practical test.

[MrPablo]

If the Solis RAI is like my RHI, then you can set the inverter to one of the following configs:

- Backup output off
As it suggests, no output at any point.

- Backup output on, EPS mode off
This will mean the second output is live all the time, regardless of whether or not there's a grid connection. As long as there's battery anyway...

- Backup output on, EPS mode on
This is the mode designed for use with a changeover switch (preferably automatic). When the grid is available, there's no power at the backup output.
If the grid goes down, then the backup outlet will be energised following the delay time set on the inverter. Once the grid is restored, backup outlet
goes dead again.

In your case, I think the second option will do as you need, especially as there's no earth required for the appliances you'll be connecting.

[Kenny000666]

Thanks yes I should test out that option. Will take a look at the inverter and see if I have option 2.

On the note of Earthing, that is an interesting subject which I’ll follow on the other thread. But long story short, looks like to get a proper “legal” EPS setup would require a some work to put in an Earthing Rod, so there’s no Earth dependency on the mains and also whether or not our Inverters have a an appropriate mechanism in place to comply with IET (I assume it does as it’s fully type tested), but never the less doing this properly needs an electrician.

Nevertheless l’ll test out the theory of if it’s suitable to keep a Pi powered when power goes off. Will have to test next week when I get the battery

[Kenny000666]

Looks like my inverter only has the backup on/off option.

I don’t see any EPS.

[MrPablo]

Interesting, I guess the RAI just has different settings available re. the backup.
Definitely a case of experimenting and see what happens with no grid power.

[Jinx]

Afaik losing earth is far from common, but is still worth considering, most houses will be TNCS also known as PME (protective multiple earth) so whilst the earth and neutral are combined the neutral is tied down all along the supply. I think it is now recommended to add a earth rod on TNCS anyway to cover an earth break, TT systems will already be earth rod.
As for the bonding, in the event of earth break then things like outside taps could become live.

[Oldgreybeard]

Afaik losing earth is far from common, but is still worth considering, most houses will be TNCS also known as PME (protective multiple earth) so whilst the earth and neutral are combined the neutral is tied down all along the supply. I think it is now recommended to add a earth rod on TNCS anyway to cover an earth break, TT systems will already be earth rod.
As for the bonding, in the event of earth break then things like outside taps could become live.

I remember reading elsewhere that there are only a few hundred faults a year where the earth/neutral becomes live as a result of a fault on the local grid. One of those unlikely sort of faults, but not much consolation if it happens to you and you get a belt from something you think should be safe to touch. Worst bit is that it's an invisible fault, things can seem to be fine, but outdoor earthed metalwork could easily kill you if there is a fault at the same time as you happen to touch it, even if that isn't that likely to happen.

Adding an extra earth electrode does nothing useful on its own, because the impedance will likely be far too high to take any appreciable earth fault current. If there's RCD protection then that mitigates the risk to some degree, but obviously an RCD won't trip if the earth coming into the home is at a high voltage, as the RCD won't see any problem.

The only 100% safe way to handle faults like this is to make sure that the incoming earth is not used at all when the back up supply is being used. Lot to be said for having a completely independent earthing system, that works no matter what happens with the grid, but it does then place a responsibility on the home owner to make sure that earthing system is always working and safe.