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Earthing in an off-grid mobile system (campervan)


moshulu

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The system I am putting together is for a (mostly) off-grid campervan and consists of:

- Axpert MKS 3KVA 48V Charger/Inverter

- ~1kW of 60 cell PV in 2x2 strings

- Pylontech US2000B Plus 2.4kWh 48V LiFePO4 battery.

When mains 240VAC is connected to the Axpert AC input the system will be earthed via the supply earthing system.  However when the van is in off-grid mode the tyres ensure there is no natural connection to earth. 

I've noticed that in the Axpert manual it only says: 

" GROUNDING INSTRUCTIONS -This inverter/charger should be connected to a permanent grounded wiring system.  Be sure to comply with local requirements and regulation to install this inverter."

I've tried to think this through logically and research all the possible implications of the different ways of dealing with this issue however I'm struggling to come to a definitive conclusion.  I would be most grateful if anyone can enlighten me on the way to proceed?

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My research on this suggests one of two methods. The one is an IT-earthing system. This means that appliances (and the earth pin in sockets) are tied to the vehicle chassis, but neither side of the supply is bonded. When you're using an external supply (shore power in boat speak), you use the earth connection from that supply line as well.

The other way, which seems to be what Victron inverters use, is to tie the enclosure of the inverter to the chassis, and then the inverter has an internal bonding relay that bonds neutral to the case (aka chassis) whenever it's in invert mode. In other words, tie the neutral of the inverter to the chassis.

I am however not an electrician.

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Thanks Plonkster!  Wow, that was reassuring to read - those are exactly the two options I had come to in my mind after thinking this through for several days and many hours of research!

I was trying to think through the implications of the Victron approach and that's the one that I wanted to clear up in my mind.  Obviously a dual pole RCBO needs to be installed DOWNSTREAM of the inverter for that Earth to Neutral bonding to be safe and achieve its purpose.  Without that RCBO downstream of the E to N bonding at the inverter this would just be downright dangerous.

Curious if anyone can think of anything I'm missing?  What I'd really like is to figure out how to have an IT earth system WITH a means to automatically shutdown/isolate the inverter if a single earth fault develops, since in an IT system there will be no return path for the E leak to flow, so no imbalance in the L and N conductors.  I know there is a way to do this, as ships (isolated N system) need to achieve something similar (a warning rather than an automatic shutdown - keeping your steering motors at sea is more important than isolating a single earth fault instantly).  I wonder if the capacitance between the isolated PE wire (with a fault connection to L) and the adjacent N wire in a short cable run of a couple of metres would provide enough capacitive coupling to create an imbalance between L and N enough to trigger an RCD?

Is there any way the Axpert inverter might malfunction without an grounded PE input - I'm thinking along the lines of a ground plane in RF transmitters?  @Coulomb - is this within your area of expertise?

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11 hours ago, moshulu said:

(a warning rather than an automatic shutdown

You seem to be quite informed on this. When looking into this yesterday I noted the same, that one reason for an IT system is precisely that you can find the fault without shutting things down. There are monitoring equipment available for this, though apparently it is not cheap. The other listed benefits are a lower risk of fire and so on as well.

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  • 2 weeks later...

I've just noticed that in the more recent versions of the Axpert manuals (2016+) it lists this same Victron style inverter output N-E bonding feature, albeit without the built-in bonding relay.  It's configurable via the menus as an option that does not exist in the circa 2015 and earlier era manuals.  The external relay is designed to be triggered by the dry contact port on the inverter to bond the output N to E when the inverter is in battery or power-saving mode.

I wonder if that's just a firmware upgrade or if the hardware is any different?

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2 hours ago, moshulu said:

I wonder if that's just a firmware upgrade or if the hardware is any different?

I believe it is just firmware, for control of the dry contact relay.

But models that came with firmware 73.00 have a neutral to earth connection via an internal relay, in fact one of the relays went from single contact to changeover type. The connection happens in battery mode and when off, but not in line/bypass mode. That's obviously a hardware modification. 

Curiously, not all recent models have the internal neutral to earth relay feature. 

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Does each Axpert model have its own firmware update files or are they all in one blob and you select the correct model during the installation process (ie will a firmware update file you have for the PIP-4048MS also have what I need for the MKS 3KVA-48V for example)?

Does this mean that updates only happen when you (the customer) contact the vendor and harass them for one?

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9 hours ago, moshulu said:

Does each Axpert model have its own firmware update files... 

Yes. No big blob. 

> Does this mean that updates only happen when you (the customer) contact the vendor and harass them for one?

They might fix the problem some time after they are aware of it, and may produce a minor update (e.g. 72.61) if there are enough changes to warrant one. But you will only get the update if you were the one complaining about it. 

You might be very lucky and get a fix someone else complained about, or the manufacturer initiated, if it ends up being available on a third party web site. 

Any fixes are only as a result of complaints from a customer who has bought significant quantities, and only for faults or things that might affect safety. The neutral to earth relay change was an example. Feature requests are most unlikely to be considered. 

It's far from what I'd like to see, but that's the way it is at the moment. 

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I only just realised that the Steca Solarix PLI 5000-48 is an Axpert MKS 5KVA-48 with a different cover on it!  I was looking at the specs again when suddenly the numbers started looking very familiar! They have an improved (better written/better explanations) version of the manual and do have that built-in N-E bonding relay you mentioned...

I gather someone must have hoarded various firmware update files for the different models somewhere? ;-)

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1 hour ago, moshulu said:

I gather someone must have hoarded various firmware update files for the different models somewhere? ;-)

No central site has them all. This forum in the downloads section hosts some Infini and a few Axpert firmwares, the AEVA PIP-4048 and PIP-5048 topic hosts most of the 4+ kW Axpert firmwares I know of including patched firmwares, and this Czech site hosts a few of the smaller and/or older model firmwares. There are a few others, e.g. this Voltacon site, but it says it has 5 kVA duo/tri MPPT firmware when it's actually for 3 kVA and 1.5 kVA models (12/24/48 V), the files are password protected, and they don't give the password! (If you really need it, it's mentioned on some PV forums). If anyone is aware of other sites, I'd love to hear about them.

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8 hours ago, Coulomb said:

this Voltacon site, but it says it has 5 kVA duo/tri MPPT firmware when it's actually for 3 kVA and 1.5 kVA models (12/24/48 V), the files are password protected, and they don't give the password! (If you really need it, it's mentioned on some PV forums).

Google isn't giving up the location of those PV forums that easily... any further clues? :-) 

I still haven't received a replacement MKS 3KVA-48 from the supplier who sent me the used/tampered with unit in error(?)... I'm in two minds whether to just cancel the order and seek out a MKS 3KVA Plus elsewhere instead.  The manuals on the supplier's site I purchased from are from 2015 or earlier, the mfg date on the unit I was sent was 2015 and this does not fill me with confidence that l'll be able to get firmware updates out of them...

8 hours ago, Coulomb said:

If anyone is aware of other sites, I'd love to hear about them.

Ditto!

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  • 2 weeks later...
On 2018/10/24 at 1:18 AM, HermanP said:

I have mistakenly upgraded it with the 5 kva 4 kw firmware.

That's interesting (though I'm sorry you have this problem). I've noticed that there is a different command to start flash programming for the 64 V models (those capable of running safely with 64 V at the battery terminals). These presumably have a different transformer ratio and all sorts of things. They have a control board (the daughter board with the processor on it) with a different part number. That different flash program command makes it very difficult to accidentally flash the wrong firmware. So I'm surprised that they don't do this for other firmware combinations that could cause problems.

What firmware revision did you have, and what do you have now?

The HS indicates that it's a model with a PWM SCC (Solar Charge Controller). For whatever reason, I don't seem to have any of those in my collection, though I should make a list when I have more time to be sure.

It seems to me that this is a mistake that your supplier can and should address for you. If you have updated to a patched firmware, I'd quietly update it again to the factory equivalent, then complain to your supplier that your machine which you've paid good money for is now not performing to advertised specifications, and they should supply a firmware update file for you. I'd say it's the only way to resolve this issue. Firmware update files are very scarce these days, even for the most popular (5 kVA) models.

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