klaus-l avatar image
klaus-l asked

Groundin Battery Minus?

I install an offgrid system withe an Easy solar II GX 48/3000/35-32.

I have a question to the grounding. I have to connect the ground (eath) toi the chassi of the Easy Solar. But do I have to connect even the batty Minus to the ground (earth)?

Phoenix Inverter
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3 Answers
ddickov avatar image
ddickov answered ·

Hi there I have built a 48v system with a Quattro installed the battery minus should always go to earth terminal also, I would imagine the easy solar has a grounding point with a large nut and washer, I have connected my minus bus bar from the battery bank to the chassis of the Quattro and all working fine, i hope this helps a bit thanks david

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Hi, can you please give a little bit of info why the minus „should“ always go to earth. That would be great.


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It depends on which school of thought you think is safer. Floating/Non Grounded DC system can be safer but also less safe in other ways. I personally prefer floating DC system in stationary applications and Bonded in Boats, RVs, Vans. This will also vary by local regulation. Some codes require DC- be grounded. Others require it to be floating but with Isolation monitoring.

Victron recommends Bonded systems for all applications but Ground and DC - must only be bonded at the battery negative terminal. Not in the inverter or anywhere else.

@Markus I really wish Victron would give an explanation for this in the wiring unlimited guide. Especially when to do a floating DC bus and when not to do a floating DC bus.
Page 64

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Yes, that depends on local regulations.

In real life practice, I have seen lots of installations, especially mobile ones, where grounding the minus was just done the wrong way, producing ground loops and other problems.

Reality over here looks like that: The installer is responsible for the proper installation and safety.

I have never met a single person, that would be able to explain, why grounding a LV battery minus is a must from a safety perspective. But maybe someone here can…

My personal opinion: If you don’t know why, it’s better to not ground the battery minus.
(If this is about boats, galvanic corrosion is indeed a thing)
If you do it, do it the right way.
@shaneyake Isolation monitoring on a LV battery? For what reason? Maybe you are referring to floating AC nets (IT net)?

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Yeah, I agree with you. Also seen installs that are grounded incorrectly or not grounded or have some AC voltage on the ground that gives you a nice zap.

I have also not been able to find a perspective that I fully agree with or was explained in a way that made complete sense. The answer I get are always for AC and specifically Ground referenced AC that have just been copied pasted over to DC.

My personal opinion would be if you don't know why follow manufacturer's guidelines.

Galvanic corrosion is definitely a thing but can be mitigated in a few diffrent ways tho.

Where I work 50V and above is considered HV, AC or DC. So if I install a Victron system with 16S LFP battery the voltage at 100% SOC is 56-57v this is then considered HV so it will need isolation monitoring if the system is floating. This will just be continuously monitoring the resistance between DC+ to Ground and DC- to Ground if a resistance is lower than the set value an alarm is triggered and in some systems batteries are isolated. The reason is because if you have a double fault the system could have current flowing through the structure or though cabling that isn't designed for the load. In my opinion double faults are very unlikely and single faults can be easy detected with a multimeter at sub 100V systems.

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Interesting discussion. May I ask you where >50V DC is considered HV? Over here in Austria <60V DC is considered safe extra low Voltage.

Let’s agree that theory and practical application from time to time go different directions.

My personal „favorite“ is a MultiPlus without a RCD installed and Ground Relay „on“ and perfectly earthed to the chassis but that’s a different story.

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I am in the US which does class things as

Extra Low voltage < 120VDC
Low voltage < 1500VDC
High voltage >1500VDC

but OSHA considers all voltages of 50 volts or above to be hazardous.
So things are classed as LV, anything below 50V, this is mainly 24V control systems and HV anything above 50V. If more than 50V volt there is a whole list of things you have to do for safety, doesn't matter if it is 51V or 500V.

I think 60VDC makes more sense.

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I have noticed in the US, some things are rather different :o)

There are much more rulesets out there than over here.

the AT local regulations say: Anything <60VDC is considered safe to touch.
Or at least not able to pass through a dangerous ammount of current through a human body.

It’s ok to have e.g. electric miniature railways exposing <60VDC on unisolated rails. Batteries are also mentioned there.

The regulations say: grounding „is allowed“ to be done on <60VDC circuits.

Lastly the installer is responsible for a safe installation. He can follow the ruleset or can do different it’s up to him.

If something bad is going to happen, the installer will most probably have a QandA session in front of a court. If you not have followed the rules, the burden of proof is up to you.

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From what I understand if installed as a stand alone system then an earth spike should be installed and be of the correct ohm reading to be considered a suitable ground, the victron instructions are very clear as well as Victron wiring unlimited manual which explains everything.

I installed two earth spikes connected together and driven into the ground until the ohm reading went under 5ohms, an inverter is effectively a mini power station and thus requiring an earth spike for grounding, the ac supply coming into a home has earth and neutral bond at the termination of the property or at the transformer for the street.

I followed all the manuals and read and read them again and my system works fine and the way it should, if a fault develops then the current will flow to earth and trip the breaker or rcd, if there is a fault from the dc side the current has then got a path to earth also or else the human body is the earth connection, to avoid all risk earth everything. I’m sure there is very technical guys who could explain the maths behind it etc.

If an inverter powered unit also becomes faulty say a toaster then where would the current flow to without a ground spike or other earth connection, it would need to flow to yourself as there is no path to earth unless the inverter is powered from the grid in that case the inverter uses the grid earth, when in dc only to ac mode the inverter then doesn’t have an earth thus the need for an earth spike and ground relay to be on to give the battery a grounding to earth ensures the whole system is safe from electrocution.

Hope this helps a bit thanks david

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Quite interesting topic this. Still, nobody have chimed in to explain why battery minus "always should" go to earth.

I have an installation in a shipping container which is part of a building. On the second floor I have a big 24V battery bank and MPPT's and inside I have a Multiplus-II, grid connection and all automation and distribution centrals. I have connected the grid PE to all AC circuits, even the generator which is in another part of the building, but I have not connected battery minus to PE. Also, I have not grounded the container itself, do you guys have any idea if that is wise to do?

I did some measurements today and found that between the container and PE I have 1.75 MΩ and between the container and DC- I have 90 kΩ. These measurements doesn't tell me much, but maybe someone knows whether I increase safety by connecting any of them to the container or possibly both, which would create a common ground for both the AC and DC circuit. But I am hesitant to do that, just think when working on the battery bank or the DC circuit, the walls and the floor up top is part of DC-. Feels safer to have DC floating...

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Hi Mr. Happy,

"Also, I have not grounded the container itself"

If the purpose of that container is to supply AC electricity on it's own to the outside world, producing a Neutral, I would ground the container in any case. I am sure, there are local rules how the AC grounding of that installation has to be done.

I would install an earth spike connected to that container and measure with an installation tester the Earth resistance to be within local regulations and if the RCDs inside that installation are operating as they should. And document all that!

Since I have never seen this installation and don’t know your local rules, I can only give a general advice. If you are still not sure what to do, involve a local electrician.


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@Markus, Thanks for the help. I now have connected PE from the Multiplus-II to the container. I have RCD's on every sub-circuit and I want them to work the way they are supposed to. Strangely enough, I measured resistance again, and the resistance of 90 kΩ I had between the container and DC- is now gone. My fluke 117 reacts a little bit when probing but shows OL which means open circuit.

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To do earth and isolation measurements you will need an installation tester. A multimeter is not the right tool.

Something like this:

To operate such a tool, you most probably need to be trained.

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Ok. Just trying with what I've got. My fellow the electrician will inspect my installation when it is fully finished. He has all the equipment.
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That sounds good to me.

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I agree that a installation tester is required for testing but a multimeter can be used to ballpark for sub 50/60V DC systems as if there was a short or some connection it should detect it.

I would not use a multimeter for testing AC or higher voltage DC systems as you won't get any indication of a connection but at higher voltages there may be.

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markde avatar image
markde answered ·

Why ground the DC Side?

-> Everything works fine without grounding the battery. BUT: There is a risk of an undetected single failure and that is quite dangerous. Imagine the following: A fault occurs inside the inverter (e.g. poor insulation or damaged insulation between two wires and this leads the AC mains voltage to the DC side). This fault stays undetected until someone touches the battery terminal (it is usually safe because there are only 48 volts but not now) and due to the single fault inside the inverter, AC grid voltage is present on the battery terminal and can flow through your body to earth (a dangerous electric shock).

If you earth the battery: The single fault in the inverter leads to a current flow through the earth connection on the battery and the circuit breaker on the AC side trips. This makes the system more secure, as a single fault does not stays undetected.

Please comply with your lokal regulations.

This is my personal opinion and it must not be right!

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Hi markde,

I agree with what you wrote and very much like to discuss this topic.

In your example, AC mains (grid power) is present on the battery minus terminal, because of a internal insulation fault inside the inverter.

If the inverters safety mechanism fails to detect that and does not open the backfeed relay and if there is no RCD involved on AC mains input and if the battery minus is exposed somewhere in the system, so can be touched, then you will have a problem. -still some "ifs"

As you (and I) said, the local regulations are important. If you act against the rules and something bad happens, the burden of proof is up to you.

There is no general rule that says "the battery minus should always go to earth" in some local regulations there might, in some others there might not.

The installer is responsible for a safe installation. He has to consider the dangers, have to comply with the local or installation specific rules and is lastly responsible for a safe installation.

I have never read, that Victron states that the battery minus always have to be wired to ground for safety and/or functional reasons. Rather: follow the local regulations and it is allowed to ground the minus.

"This is my personal opinion and it must not be right!" I would not say "must". I would say "may" ;o)

My personal opinion "may" also not be everyone elses, but this discussion is (in my opinion ;o) a good thing to have.

Best Regards

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I agree with what you have said however in the Wiring Unlimited Guide from Victron, on page 64.

"Ground close to the battery. The battery poles are supposed to be safe to touch. The battery ground should therefore be the most reliable and visible ground connection."

They don't say must but they do say Ground. They also include this picture.

In my opinion this does suggest that Victron thinks you should ground the DC-, however I personally don't ground my DC bus on 48v systems. I also don't assume the battery terminals are safe to touch. As 57V can give you a little zap.

Also in this picture if the hot wire did get connect to DC- in the inverter the RCD/GFCI would not trip.

The inverter would probably overload.

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as far as I interpret that: if you do ground the battery minus (according to local rules), do it the right way like described here.

"in this picture" is no mains input shown. The error would rather be "battery Voltage is converted via the inverter to AC and backfed to ground and the battery" I doubt that the Multi wont quit inverting when this happens, but yes - "I" cannot garant that here for sure.

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I think you have convinced me haha.

I think I interpret it the same way now.

@Guy Stewart (Victron Community Manager) Maybe Victron can clarify it's policy on DC- Grounding and maybe offer it's own reasoning.

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The AC breaker will only trip if it can detect a difference in current between the 2 conductors.

However this changes when DC is introduced to the AC circuit.

Most AC type RCD will not trip when over 50ma of DC is introduced into the AC circuit. Most only operate when above 6ma of AC differential. They absolutely will not trip when >250ma of DC is introduced on the AC circuit.

If you have a probability of DC becoming connected to AC side then you must use an RCD type B, that will operate when DC is present on the AC circuit.

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A Multiplus has a Transformer. Luckily, no DC is able to pass a Transformer.


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johnsmith avatar image
johnsmith answered ·

People need to understand the difference between earthing and bonding of supply and devices.

Current DOES NOT return to earth, current returns to the source of the generated current (whether that current returns through the earth as a conductor back to the source is another matter), but it does not just return to earth and magically get absorbed into nothingness.

Adding multiple ground rods will NOT IMPROVE grounding. There should only ever be 1 ground rod to avoid potential difference between any grounded circuits. A ground rod WILL NOT save your life, the only way to stop the fault is TO TURN THE CIRCUIT OFF, whether that is a fuse or an MCB or MCCB or even a switch, although a switch is obviously a bad idea.

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