Earthing home PV/battery system for use in power cuts

[SolarSystem]

Hello folks, I've searched forums for similar but couldn't find answer! I've had a quote for domestic solar PV system (4.6KW) with batteries (2x3.5kw) for 3 bed south west facing bungalow on the coast. Usage would be mornings and evenings mostly, one adult all week long and then two at the weekends. We have a combi boiler so gas water/heating. but I'd like to be able to run an electric radiator in a bedroom for an hour or two sometimes. Will switch to Octopus for export.

We get a lot of storms and some power cuts, so I asked about having the system work during a power cut, i.e. off grid, and the installer said it'd have to be earthed at the property so that anyone working on the grid wouldn't get zapped if I have a live system when they're down. Can anyone explain to me how the earth works and what's required? I understand you need a rod in the ground, I'd appreciate knowing what that means in reality!

Any advice much appreciated.

Quote is £8800 installed (but not including any earthing to be off grid). The guy is an electrical engineer and seems very competent. It's pretty tough getting anyone around here as they're chocca!

 

[binky]

Not sure earthing has anything to do with it. Most batteries have a UPS function so that a cct with limited loading can be wired to it, to power essential items only, like the freezer. This would not backfeed the grid nor need any changes to the earthing arrangements.

 

[SolarSystem]

I've since also found this:

"It should be noted that a battery will not give backup ‘out of the box’ so it needs to be planned for ahead of the install. Furthermore, most people are not aware that a standard UK home solar array is required by law to turn off in a power cut, for the safety of grid workers, so it is usually of no help in a power cut (the exception being our advanced Level 5 option below)." from https://tanjent-energy.com/blog/power-cut-backup-options-with-battery-storage/

So this concurs with what the fella told me. ANd then this: "Update 2022: In line with the latest IET Code of Practice on Electrical Energy Storage Systems we now aim to fit a separate earthing system whenever we install a battery system with off-grid capability, so that extra cost will be included in all future quotes."

So I'm just wondering what that involves.

 

[steptoe]

The earthing probably refers to the fact that most properties rely on the DNO to supply an earth, if you are off grid you cannot do this for 2 reasons, (maybe more)
1 your supply cable may be disconnected, and therefore your earth,
2 if a fault were to occur you would send a fault current down the DNO cable endangering anyone working on said cable

Edit, just a thought, is it still the case that not all inverters are suitable for TT systems,?

 

[binky]

Edit, just a thought, is it still the case that not all inverters are suitable for TT systems,?

Not seen that written on any inverter instructions for many years. I'm also not sure how well protected the UPS function is on invererters, many have built in RCD type protection for AC and DC, the UPS circuit always has an earth connection, so the need to TT the house is probably dependent on the make of inverter - I shall have a read of a manual and see what it says.

 

[binky]

this is form a Solax hybrid inverter - seems set up to not want an earth. Solis hybrid inverters, which is what I normally fit, have RCD protection on the UPS, as do other slightly dearer makes that I have a quick look at.

 

[binky]

this is from GivEnergy, a popular hybrid inverter, and seems to need an earth, so it would seem to be determined by whoever makes the kit. A check of the inverter data sheet isn't showing built in RCD protection on the AC side either, I've always regarded GivEnergy as cheap and nasty, perhaps I'm not wrong in that opinion

 

[SolarSystem]

this is form a Solax hybrid inverter - seems set up to not want an earth. Solis hybrid inverters, which is what I normally fit, have RCD protection on the UPS, as do other slightly dearer makes that I have a quick look at.

Thanks folks - and for this reply - it is a Solis Energy Storage 3.6kW Hybrid 5G Inverter that the installer has quoted for - so this does not need earthing? Apologies I have zero electrical knowledge so following the thread is a challenge!

We won’t be off grid generally - just if power goes out but I think someone earlier was referring to those that are? Or maybe they meant just that, when grid/line goes down.

 

[binky]

The only from of earthing that could be installed is a TT system, ie bang an earth rod in in case of failure of the grid connection. But I'm puzzling effectively having 2 earth systems within the same property, which is what that would leave you with. One for @steptoe.

Most of Europe uses earth free systems, with RCDs, so I thinking the whole earthing thing is possibly overkill for an irregular occurrence.

 

[SolarSystem]

The only from of earthing that could be installed is a TT system, ie bang an earth rod in in case of failure of the grid connection. But I'm puzzling effectively having 2 earth systems within the same property, which is what that would leave you with. One for @steptoe.

Most of Europe uses earth free systems, with RCDs, so I thinking the whole earthing thing is possibly overkill for an irregular occurrence.

Thank you, yes he mentioned a rod in the ground. Let's see what @steptoe thinks then.........

 

[binky]

Thank you, yes he mentioned a rod in the ground. Let's see what @steptoe thinks then.........

I suspect he will convert the whole house to TT then. Steps is our resident earthing guru, so I'll leave any further comment to him.

 

[steptoe]

I suspect he will convert the whole house to TT then. Steps is our resident earthing guru, so I'll leave any further comment to him.

I've been thinking about this,
I think you would have to convert the whole place to TT,
As I don't think an inverter would disconnect the earth on a powercut, then by default it doesn't disconnect the neutral either on a TNCS .
So a fault current could still present itself to anyone working on the DNO cable.
Wonder how this works with a 100mA TD RCD,?

 

[binky]

As I don't think an inverter would disconnect the earth on a powercut, then by default it doesn't disconnect the neutral either on a TNCS .

thinking about that, on the odd occasion I've installed an isolator for the mains supply to allow use of a gennie in the event of a power cut, I don't remember isolating the mains earth, just live and neutral on the meter tails. So I'm wondering how worried DNO staff would be about the earth. Shame Miss Sweden isn't still here to advise us.

 

[steptoe]

thinking about that, on the odd occasion I've installed an isolator for the mains supply to allow use of a gennie in the event of a power cut, I don't remember isolating the mains earth, just live and neutral on the meter tails. So I'm wondering how worried DNO staff would be about the earth. Shame Miss Sweden isn't still here to advise us.

In theory, your Genny should be TT'd, but, it would still be an easier path to earth to go down the neutral, I've only ever done change over switches in fairly remote properties that were TT anyway, I'd not really considered the number of properties where your scenario probably exists.
There are still DIY idiots out there that use widow-makers.

 

[binky]

There are still DIY idiots out there that use widow-makers.

My next door neighbours son in law, a bus driver was telling me how he installed 2 single sockets under the floorboards and constructed a 'ring' for some sockets in a bedroom with plug tops - all on the grounds that he was legally allowed to do that under part P. That conversation didn't go much furhter, although I did manage not to call him a useless ****!

 

[SolarSystem]

Thanks folks although I can’t follow the thread exactly, should we consider not trying to have a power cut proof set up?

 

[Gian]

thinking about that, on the odd occasion I've installed an isolator for the mains supply to allow use of a gennie in the event of a power cut, I don't remember isolating the mains earth, just live and neutral on the meter tails. So I'm wondering how worried DNO staff would be about the earth. Shame Miss Sweden isn't still here to advise us.

As I have been advised in small generators up to 10kw that the Genny it's self must be linked at the Genny end . Neutral CPC link and a permeant sign not to use other than with Genny. And the Genny then connects as TT. This will allow the RCD protection to work using the change over switch as you have said it's the live and neutral that's used.

 

[steptoe]

As I have been advised in small generators up to 10kw that the Genny it's self must be linked at the Genny end . Neutral CPC link and a permeant sign not to use other than with Genny. And the Genny then connects as TT. This will allow the RCD protection to work using the change over switch as you have said it's the live and neutral that's used.

The problem is if the DNO supply is TNCS,
Unless you disconnect the supply earth, then by default, if a fault was to occur the supply DNO neutral will be subject to the fault current.

 

[Gian]

The problem is if the DNO supply is TNCS,
Unless you disconnect the supply earth, then by default, if a fault was to occur the supply DNO neutral will be subject to the fault current.

Yes and it's really not 100% clear , you would think that it would be written but everyone has differant takes on this subject.

 

[SolarSystem]

I was sent this:

https://electrical.theiet.org/wirin...ractice-on-electrical-energy-storage-systems/

Contractor mentioned Island Mode.