Inverter earth/neutral bonding (1 Viewer)

[Guzziando]

I am installing a NDS 1500Wi Inverter, and want to take advantage of the automatic switching between the battery and 230v supply (mains hook-up). I am connecting it to the existing sockets through a 10A RCBO. I am planning to take the feed directly from the electric hook-up connection, and bypassing the existing consumer unit. Given the fact that the M/H is not in direct contact with the ground, but insulated by the tyres, do I need to connect the neutral and earth leads together as shown in the diagram? Here is the manuals for the inverter: www.roadpro.co.uk/userfiles/PDFs/product%20information/NDS/Smart-In%20Inverter%20Manual.pdf also: www.roadpro.co.uk/userfiles/PDFs/product%20information/NDS/Inverter%20with%20earth-neutral%20bonding%20leaflet%20-%20for%20web.pdf

 

[Lenny HB]

The RCBO won't work unless neutral is bonded to earth so yes.

 

[Guzziando]

Thank you Lenny. The RCBO also has a functional earth wire, does this make any difference, and do I connect this to the neutral wire as well?

 

[Gromett]

VERY complicated subject. Be aware if you travel on the continent that they can swap phases. So on pass through you may have live bonded to ground.

 

[autorouter]

Be sure to use double-pole RCBO. RCDs are almost all double-pole, but the majority of RCBOs available in the UK are single pole. Double pole RCBOs are needed for mobile use (motorhome, caravan, campsite hookup post etc.). All the ones I've seen that have extra wires coming out of them are single pole, and are not suitable.

You may have to go to a specialist electrical supplier in the UK. For example CEF:

https://www.cef.co.uk/catalogue/products/5153275-10a-double-pole-2-module-b-curve-10ka-30ma-type-a-rcbo

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[Raul]

Thank you Lenny. The RCBO also has a functional earth wire, does this make any difference, and do I connect this to the neutral wire as well?

No, you don’t, the bond gets done in one place ONLY, near the source as shown in your diagram. After that, each wire do not touch each other again downstream, otherwise you create a loop, and sending energised neutral via PE.
Be aware, this set up only works in stand alone mode, no grid. If you plug in to ehu, this bond needs to be disconnected, as there is one already provided by the ehu distribution up stream.
If you don’t understand this requirements and the implementation of PE N bond, don’t do it,

 

[autorouter]

If you were simply using an inverter with a single appliance, plugged into the inverter output plug, then no earthing is necessary. The two power conductors (the 'live' and 'neutral' wires) would be isolated from any surrounding metal, earthed or not, in a similar way to an isolation transformer. Touching a single power conductor would not result in a high voltage between the power conductor and earth, so no current would flow, and no electric shock.

This 'isolation' principle is used in high risk situations, like for example shaver sockets in bathrooms, which all have small isolation transformers. Isolation transformers are big, heavy and expensive for higher power appliances, so the idea of earthing one of the wires to make it a proper neutral is used in all domestic circuits apart from shaver sockets.

There needs to be a connection between neutral and earth for Ground Leakage Protection (like an RCD or RCBO) to work. That means it won't work for the simple case first described, but in that case there is no necessity for an RCD/RCBO anyway.

Or you might be doing a full integrated mains installation, with mains hookup, generator and battery backup, and mains power distributed to different devices and sockets. Then you would use something like a Victron Multiplus 2 Inverter/charger. This is built to work either with a mains hookup or generator, or entirely independently as an inverter.

This setup has a relay built into it, which automatically connects the 'neutral' wire to the earth when not connected to the grid. It automatically disconnects the neutral wire from the earth when it detects the presence of grid power. That's because the grid connection already has its own neutral/earth connection, and you don't want two of those.

Your proposed system is somewhat between those two extreme cases. It's an inverter with connections to more than one socket/appliance, so you need to consider whether there is any necessity to have any connection between neutral and earth, and if so how to make it automatic. If there was never going to be an EHU connection, then you could wire it permanently as in the diagram, and the RCBO (Ground Leak Protection) would work OK.

However if you intend to sometimes have EHU connected, you could have a changeover switch (transfer switch) between the two. If you make the neutral-earth link on the inverter feed to the transfer switch, it will automatically be connected for inverter power, and disconnected for EHU power. That would work with either a manual transfer switch or a transfer relay to switch automatically.

 

[Guzziando]

Thank you for your answers so far. On further investigation I came across 2 versions of the manual, one from 2022, and one from 2023. They contradict each other, so not sure if there was any changes to the circuitry between those dates? Could someone explain the difference between TN and TE grounding, could this have anything to do with it? I have ordered a double-pole RCBO as recommended by autorouter. Could I fit a relay which would switch between earth and earth bonding, or would this be a bit dodgy?

 

[Guzziando]

Sorry autorouter, just reread your post, thank you for such a clear and detailed explanation. I am interested in your solution for a transfer relay, do you have a wiring diagram and what would be a an appropriate relay? This would seem a good way forward, and resolve the problem of earth bonding.

 

[Gromett]

I came across 2 versions of the manual, one from 2022, and one from 2023. They contradict each other

Which is why I said;

VERY complicated subject.

There really is no general answers to this question. It is EXTREMELY specific to the install, how it is done, what equipment is being used etc.
I used to understand all this stuff but I haven't done it for 15-20 years and am rusty beyond belief. So I don't try to advise.
But I do know that when I did my first motorhome install there was much head scratching between me, an electrical engineer and an autoelectrical technician. In the end we settled on a compromise that I could live with. We could not come up with a perfect solution that fitted all scenarios.

But, you won't be steered wrong by autorouter and Raul

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[Guzziando]

If I use a relay as David has done here, and run a neutral wire through the NC contact to earth, so the relay is energised when I am electric hook-up, and the neutral would bond with the earth when I am on the inverter? Would this work?

 

[Guzziando]

This is the best explanation I have seen about the earth/neutral bonding issue (apart from the excellent replies from Funsters ). The solution suggested by Jolly is to use an AC Contactor.

 

[Guzziando]

Having done some reading around this, and also the contributions made, I have made an attempt at a wiring diagram using a 3 pole relay. I am sure there are errors, and better ways of doing this, but your views would be appreciated. Please be gentle, and as always, constructive UPDATE [Looking at it again, I think I could use a double pole relay, with contact 2 NC (C2) going to earth, and then switching to neutral NO]

 

[Guzziando]

Having done some reading around this, and also the contributions made, I have made an attempt at a wiring diagram using a 3 pole relay. I am sure there are errors, and better ways of doing this, but your views would be appreciated. Please be gentle, and as always, constructive UPDATE [Looking at it again, I think I could use a double pole relay, with contact 2 NC (C2) going to earth, and then switching to neutral NO]

Please ignore the diagram, I have spotted an error. Not sure how to delete or edit posts, as the option does not appear to be available.

 

[autorouter]

I don't think it needs to be as complicated as that. Briefly, if the transfer switch is internal to the inverter, then a relay to switch the neutral-earth link is required. If the transfer switch is external to the inverter, then a permanent earth-neutral link on the inverter output is possible.

External Transfer Switch:

If the transfer switch is external to the inverter, then you can have a permanent connection between the inverter neutral and earth, somewhere between the inverter output and the transfer switch.

The output from the transfer switch (Live and Neutral) can go to the input of the RCBO, then to the sockets. The earth to the sockets can go from the common earth point, which is connected to several things: the incoming hookup earth wire, the inverter case, the chassis and the earth wire for the sockets. And in this case, the earth is also connected to one of the 'floating' power wires of the inverter output, to make it a proper neutral wire. Note that when the hookup is connected, the transfer switch isolates the inverter output, so the earth-neutral link is isolated from the hookup supply.

Internal Transfer Switch:

If the transfer switch is inbuilt into the inverter, then you can have a relay to connect the inverter neutral to earth when the hookup is not connected, and disconnect it when the hookup is connected.

You need a single pole changeover relay, energised by the hookup connection. Use the COM and NC connections for the earth-neutral link. Ignore the NO terminal. The relay for the neutral-earth link should be between the inverter output and the RCBO.

The output of the RCBO can go to the sockets. The earth to the sockets can go from the common earth point.

There is a third possibility, that I don't have enough information about. Maybe there is a way to have a neutral-earth link inside the inverter if it has a transfer switch. The manual talks about TN and TE systems. I know about TT, IT, and 3 types of TN (TN-C, TN-S and TN-C-S), but not TE. Maybe it's a bad translation from Italian. Anyway, apparently you can put in a fuse to make a connection of some kind, and I strongly suspect that what it does is make that neutral-earth connection inside the inverter. So no extra connections required to allow the output to work with an RCBO. But I think further information is needed before you go for that option.

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[Gromett]

If the transfer switch is external to the inverter, then you can have a permanent connection between the inverter neutral and earth, somewhere between the inverter output and the transfer switch.

Nice one

 

[Ray-Vic]

You need a single pole changeover relay, energised by the hookup connection. Use the COM and NC connections for the earth-neutral link. Ignore the NO terminal. The relay for the neutral-earth link should be between the inverter output and the RCBO.

Whilst this is what I have done with my Renogy inverter, and it works absolutely fine, my main concern when considering this option was that on the N/C side it is not fail safe. With the relay likely to be hidden away (it is in my case as it's under a seat) it could be very difficult to tell if the relay coil had failed. I fitted an indicator lamp on the N/O contact on mine and fitted this in a discrete but visible location just so that I can see the relay has operated or, more importantly, hasn't.

 

[Guzziando]

Whilst this is what I have done with my Renogy inverter, and it works absolutely fine, my main concern when considering this option was that on the N/C side it is not fail safe. With the relay likely to be hidden away (it is in my case as it's under a seat) it could be very difficult to tell if the relay coil had failed. I fitted an indicator lamp on the N/O contact on mine and fitted this in a discrete but visible location just so that I can see the relay has operated or, more importantly, hasn't.

Thanks, that is a good idea.

 

[Guzziando]

I don't think it needs to be as complicated as that. Briefly, if the transfer switch is internal to the inverter, then a relay to switch the neutral-earth link is required. If the transfer switch is external to the inverter, then a permanent earth-neutral link on the inverter output is possible.

External Transfer Switch:

If the transfer switch is external to the inverter, then you can have a permanent connection between the inverter neutral and earth, somewhere between the inverter output and the transfer switch.

The output from the transfer switch (Live and Neutral) can go to the input of the RCBO, then to the sockets. The earth to the sockets can go from the common earth point, which is connected to several things: the incoming hookup earth wire, the inverter case, the chassis and the earth wire for the sockets. And in this case, the earth is also connected to one of the 'floating' power wires of the inverter output, to make it a proper neutral wire. Note that when the hookup is connected, the transfer switch isolates the inverter output, so the earth-neutral link is isolated from the hookup supply.

Internal Transfer Switch:

If the transfer switch is inbuilt into the inverter, then you can have a relay to connect the inverter neutral to earth when the hookup is not connected, and disconnect it when the hookup is connected.

You need a single pole changeover relay, energised by the hookup connection. Use the COM and NC connections for the earth-neutral link. Ignore the NO terminal. The relay for the neutral-earth link should be between the inverter output and the RCBO.

The output of the RCBO can go to the sockets. The earth to the sockets can go from the common earth point.

There is a third possibility, that I don't have enough information about. Maybe there is a way to have a neutral-earth link inside the inverter if it has a transfer switch. The manual talks about TN and TE systems. I know about TT, IT, and 3 types of TN (TN-C, TN-S and TN-C-S), but not TE. Maybe it's a bad translation from Italian. Anyway, apparently you can put in a fuse to make a connection of some kind, and I strongly suspect that what it does is make that neutral-earth connection inside the inverter. So no extra connections required to allow the output to work with an RCBO. But I think further information is needed before you go for that option.

Thank you for this. I have finally got round to fitting my inverter. Having wired up a relay, I checked the earth / neutral connection. To my surprise, the inverter is already bonded, so I have not included a second bonding wire. This was without the mains hook-up cable connected, so this was not where it was connecting. In order to connect to the sockets, I added an additional wire to the socket circuit (spur?) which is connected to the load of the RCBO, this has caused the sockets MCB from the hook-up circuit to play up, and cause the RCD to trip. If I leave the MCB switched off, all is well, as the sockets are protected from the RCBO even on hook-up. Is there a simple way around not needing to have the MCB switched off?

 

[autorouter]

Can you clarify what exactly you have connected - live, neutral and earth - and how exactly you connected the sockets. I think if the sockets are running from the inverter then they should only be connected to the inverter, and nothing else. If you connect the hookup supply to the inverter, then the internal switch will automatically switch the sockets to hookup power when the hookup is connected to the campsite hookup post.

Reading the NDS 1500 manual, if that glass fuse is inserted then it does indeed link the earth and neutral. In that case, if the fuse is inserted, the manual says you need an RCD or RCBO on the inverter output, and it should function properly. And that's exactly what you want, I think.

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[Guzziando]

Thank you autorouter. I realised that I still had one lead still connected from the power supply unit (there are 2 to the sockets ), this is now all sorted. The socket MCB is now redundant, and the sockets go through a separate RCBO rated at 10A. The glass fuse makes sense, and this is giving the earth/neutral bonding. I also came across this, which verifies this:

I have learnt a lot from installing the inverter, and while I was dealing with 230v AC, I prefer to work on my MH rather than get someone else to do it. At least if something goes wrong while I am away, I have a better chance of knowing what is wrong, and how to fix it. Thank you again for all your help and support with this.