Earthing for Critical Loads

DIY
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1

Hi,

I have G99 Approval for a MultiPlus II, but it says: “Island operation: No”.

I wanted to be able to power some critcal loads (my server/network cabinets and some, some other radial circuits, possibly lights etc in the future from the inverter on AC OUT 1 - which I assume is what they are classing as island operation?

I’m assuming the issue here is probably the earth - if the grid goes off, then the critical loads on AC OUT 1 will still be powered. If there was a fault and power went to earth, then that would flow back out of the grid connected earth, where someone could be working. Is that correct?

What i’m thinking is, if I get an earth rod installed outside my garage (where the inverter is), the inverter and critical loads would be on it’s own earth and i’d not connect the grid earth. That way, if the grid power goes off, then i’d be effectively off-grid and the DNO wouldn’t be involved and it should in theory be ok??

Is it ok for the inverter to be earthed from an earth rod, while taking grid supply (AC IN) from a supply with a different earth? - and for loads in the house to be running from the grid earth, while there are some from the earth rod?

As long as everything is earthed either way and the two don’t get joined together, I don’t see why there would be an issue?

Anyone do similar?

My grid connection has a separate earth, so not PME (TN-S I believe?).

Thanks,

Ian

2

Islanding refers to powering a portion of the grid when grid has fallen away.

Anti islanding protection is needed if the inverter doesn’t already have the double disconnect.

Your neutral earth bond will be made in the inverter when this happens.

3

Hi @lxonline,

Thank you for the reply! - but are you able to elaborate more as that just made things less clear… :grinning_face:

By “powering a portion of the grid”, do you mean feeding back in to AC IN when the grid is down, rather than continuing to power the loads on AC OUT 1?

Does the MultiPlus II have “anti islanding protection”? - which if that is what you meant above I believe the answer is yes as if the grid disappears it will disconnect it and continue to power just the loads on AC OUT 1 from the battery?

Please could you elaborate on: “Your neutral earth bond will be made in the inverter when this happens.”?

Thanks!

Ian

4

With the correct grid code loaded, the inverter uses LOM detection to island itself on grid loss, all input relays open so the inverter is physically disconnected.
The ground relay then closes joining neutral and earth for the critical loads.
If any of these processes and related tests fail, the inverter will shutdown.

5

You really need to talk to your DNO to find out why they specify no island mode, no one can I give a real answer to that other than them you have a supply that isn’t very common due to its cost of running a separate earth to the property known as Terra Neutral Separate the TNCS was supposed to be more of a fail safe as you have multiple earth points PME and the earth separated from the neutral at the service head disadvantage to this setup is if the power from the grid goes which the earth is part of that means the earth has also gone and you can have issues with broken PEN that can piggyback from your neighbours house through your earth which joins back to neutral and as we bond all of our melt pipe work from the ground makes it super easy for it to finish the connection.

TT is the route that I have gone, it’s a lot more involved ie whole install needs upstream RCD, the earth ohms reading is a lot harder to get into the lower values, you have to maintain it that’s your job now. Doesn’t help but hopefully more information so you can ask the DNO if you could proceed down that route, the rod will still go to the star point of the transformer but resistance is the issue as your using indirect connection through earth to achieve it.

6

Ok, so I think this confirms I was correct in my initial post?

When there is a loss of grid (which presumably gets detected by either a loss of live, neutral or both?), it effectively disconnects from the mains, meaning it’s “off grid”.

(I think) the only issue would be the earth - that’s a solid bar, so there is no disconnection, which means the earth stays connected in the case of grid failure and therefore a fault could in theory make the grid earth live if it was say disconnected for street works.

So if I therefore do not connect the grid earth to the inverter, but connect a local earth rod - the loads behind the inverter will all use the local earth rod, while the house and garage loads connected to the grid will continue to use the grid earth.

Does that seem sane?

Thanks,

Ian

7

No wouldn’t advice that at all whole point of earthing is to try and make sure nothing has a different earthing potential ie two metal things to the same earth, if you have a system that has a lower earth and touch both when there is a fault that fault current is going through you. You would really want to use one earthing system. I know DNO’s on TNCS are saying you can supplementary bond but this is more for them as it’s just another PME point that’s helping there earthing system and as on a TNCS the earth is gone when the grid is down it doesn’t have an effect on that end. PS they do earthing relays but that is a cost and is also a moving part so you will have to maintain and check its operation. I did see it on Efixx if I can find it I’ll post it

8

Good advice from @Daza there.

@ichilton based on your responses here it seems like you don’t have an electrical background (apologies if that isn’t the case). If you don’t have a good grasp of the safety functions of your homes earthing system and other protective devices then this is definitely the point to consult a qualified electrician (ideally one who is familiar with Victron, or at least backup power systems) on the specifics of your install before getting too much deeper into it.

9

Do you have a write up / diagrams of your setup anywhere?

10

Ah yes, of course - that’s obvious when you put it like that!

If you are going to go TT with an earth rod, you would have to do so for the whole installation/house.

11

Hi mate no this is more for a sparkie as there are resistance values that need to be measured to make sure the RCD MCB RCBO’s operate as they should, an upfront RCD is also needed on the whole install which a time delay/selectivity is what you would want. You can use the disc to get your earth ohms lower easier than an earth rod.

For my system I have gone down 4.5meters which I don’t think any spark will take the time to do and used 25mm earth cable, which is an expensive that most wouldn’t go through I’ve managed get it at 5ohms which is really good for TT system but I measure the readings once a year to make sure the value is still good, that’s what I’m talking about maintaining is all you. Talks to a spark as just driving in a rod might not give you the values that you need but one thing to note this value can never be too low, the low the value the better get it as low as possible

12

Yes you would have to TT the whole install

13

i’m a qualified electrician but new to Victron.

You can have more than one earthing system. Each needs to be safe in all senarios. In Island mode the installation needs to operate without using the DNO earth so the most likely earthing system for Island mode is TT. This can be achieved with a local earth rod (earth rod resistance < 200 ohms) and a RCD (or RCBOs). As Daza says, make sure nothing has a different earthing potential ie two metal things to the same earth so connect the DNOs Earth and the Rod at the Main Earth Terminal.

Routinely measuring the earth rod resistance and the correct functioning of RCDs and/or RCBOs is good practice.

14

Hi David!

So are you saying the right thing to do to connect the DNO earth and rod together?

That solves all of the problems - different earth potentials issue, makes sure there is an earth for critical loads if the grid goes away and means if anything happens to the earth rod or cabling then you have a backup earth.

I assume the earth rod would normally be lower resistance so should be where any leakage flows.

However, I was working on the assumption that when the DNO finally come back, they’ll say that for islanding the DNO earth must be disconnected - otherwise you could be sending fault current back down their cabling where someone could be working on the supply?

I was thinking that one way would be to have an earth rod installed right outside my garage, which is a standalone “building” - about 1m from the house. That’s where the inverter/battery is/will be and where the majority of my critical loads are (network / server cabinets) - so I assume I could make that TT and earth from the rod, while leaving the house on the grid earth - but that has the disadvantage of not being able to have any critical loads in the house.

Thanks,

Ian

15

Just phone the DNO and ask if it can be supplementary bonded earth to operate island mode as per BS 7671 Regulation 551.4.3.2.1 as per @David2 suggested but this should have been on the G99 diagram which is probably why they said no to island mode as no earthing arrangements outlined in the diagram? I’m only guessing that could be the reason

16

Just heard back from the DNO - they’ve now granted islanding and said:

Earthing

Whilst the existing earthing arrangements can be retained, it is your responsibility to ensure that adequate earthing arrangements are made for your installation in accordance with the current IEE Wiring Regulations (BS7671).

You must ensure the earthing of your generator complies with the requirements of G99.

Anyone familiar with all of the requirements there?

I believe the important part is this? -

When a low voltage generator (such as a standby generator, solar inverter, or battery energy storage system) operates in island mode as a switched alternative to the public supply, it must not rely on the distributor’s TN earthing system.

So i’ll need a TT (earth rod) to be able to operate any (non double insulated) loads in island mode.

What i’m not 100% clear on is - can I have an earth rod installed and join that to the existing earth from the grid (TN-S) - that way I get “the best of both worlds” and have both a local earth and the reassurance of the grid earth being there should there be any issues/increases in resistance/impedance of the local earth rod - as well as not having an issue with nearby surfaces with different earth potentials.

Reading this guide, I read that as I can have the TT/earth rod connected to the existing main earth supply, which fits with what (if I understand correctly) @David2 said - and if I read it correctly, it says that you can leave the supply earth connected in island mode (it’s the line and neutral connections which must disconnect, which is handled by the Multiplus), you just can not rely on it - i.e you need a TT earth as well.

If I am correct here and the answer is yes [a TT earth electrode an be connected to the existing TN-S earthing system so you have both], is it ok for that TT earth to be connected to an outbuilding?

In my case, I have henley blocks splitting off my house consumer unit with a supply (and 80A MCB) to [16mm2] armoured cable feeding my garage which is a separate “building” at the side of the house. This is where the Multiplus II and solar inverters are and it would be very convenient to have an earth rod installed next to that and fed in to there, which would connect to the garage consumer unit (and earth fed in to there via the armoured cable).

(All end user circuits in both the house and garage are behind RCBOs).

The DNO letter also states:

Earthing Export

The star point of your generator must not be connected to our earth when operating in parallel with our Distribution System.

I believe this refers to not connecting Neutral and Earth when connected to the grid?

In which case I believe that is automatic on the Victron, per Nick’s post above (on loss of mains it opens a relay disconnecting from the grid, before joining earth and neutral - which it will reverse before connecting back to the grid).

Thanks,

Ian

17

As per their letter [quote=“Ian Chilton, post:16, topic:57991, username:ichilton”]
The DNO letter also states:

Earthing Export

The star point of your generator must not be connected to our earth when operating in parallel with our Distribution System.
[/quote]

You can’t connect it to there system ie your star point live neutral earth. Yes the live and the neutral do disconnect from the grid supply, your neutral then bonds to the earth which is connected to the DNO supplied earth so by all accounts your doing what they said not to. You need a relay for your island earth but I can’t seem to find that video from Efixx about it

18

Ah drat, you’re right!

Is there any way to disable the neutral/earth bond on the inverter? - as that shouldn’t be needed if there is a local TT earth there to use?

Otherwise, it seems like the options are:

  • Use a relay to disconnect the grid earth on loss of mains.
  • Stop exporting the grid supply earth from the garage - use a separate TT earth for the garage.
    • Means I can’t have any critical loads in the house, only the garage.
  • Convert the whole house to TT and disconnect the grid supply earth completely.

Unless there any other suggestions?

With either of the TT solutions, I believe i’d also need to replace the MCB on my garage board feed with a time delay RCD.

19

Yeah the relay option never really liked all depends on that moving part, an earth rod far less maintenance but you own it maintenance and all, resistance values are your issue with a TT as it can be hard to get a really low resistance but out of all of this is a far more straightforward way just to TT the whole thing

20

Actually, i’m now wondering if that can be correct.

The MultiPlus II is certified for use in the UK and the DNO specifically requested/checked that.

If what you said there was true then any MultiPlus II connected in the UK and not using TT would be against regulations, but the approval states the current earthing arrangements. There is no requirement to go TT, just a requirement to confirm to BS7671?

EDIT: I’ve stared at this more and I think the key words here are “when operating in parallel”:

The star point of your generator must not be connected to our earth when operating in parallel with our Distribution System.

When operating in parallel with the grid, the earth/neutral bond in the inverter is not connected.

When it detects LOM, the backfeed relays open so live+neutral are no longer connected to the grid, so therefore you are not operating in parallel with the distribution system and therefore not under that clause? (and only then does it bond earth and neutral).

So I feel that clause is irrelevant here as the inverter is setup to handle that correctly, hence it was certified for use under G99?

Ian