Grounding Inverters & Chargers

[GordMay]

Grounding and Circuit Protection for Inverters and Battery Chargers
http://www.bluesea.com/Article_detai..._ID=292&id=392

Battery chargers, inverters, and inverter/chargers form an electrical bridge between a boat's AC and DC systems. When installing these devices:
• Follow the installation rules that apply to each system.
• Take additional steps to assure that there is proper grounding between AC and DC.

Builders, installers, and well-informed boat owners follow ABYC E-11 Standards document when installing devices in boats. However, there are additional documents for the installation of charging devices:
• ABYC A-20 provides guidelines for installing battery charging devices.
• ABYC A-25 provides guidelines for installing power inverters.
To properly install an inverter or charger, follow requirements listed in E-11 and A-20 or A-25.

Check Boat Wiring Before Installation According to ABYC E-11
1. Before installing a charging device, confirm that there is a good connection between AC and DC grounds. If there is not a good connection, the charger/inverter grounding, when it is installed, may become the main connection between AC and DC grounds and must be sized for the boat's entire electrical system in order to avoid the risk of shock or fire. The AC grounding system and the DC grounding system should be firmly connected at a main grounding bus or at the engine block.

These connections help prevent faults in a boat's wiring from becoming lethal leakage currents that flow into the surrounding water or energize exposed metal on a boat. Some boat builders omit this grounding connection as a means of reducing galvanic corrosion resulting from coupling to adjacent vessels through the power system. A better way of preventing galvanic corrosion is to install this ground connection and install a galvanic isolator or isolation transformer in the AC power system.

When Installing Charger, Inverter, or Inverter/Charger Follow A-20 and A-25
A-20 and A-25 contain specific installation directions that are not described in E-11.

2. Install the AC wiring to the charger or inverter including an AC grounding conductor of a size equal to the current carrying conductors unless the circuit exceeds 30A, in which case the grounding conductor may be one size smaller (E11.16.1.3.8.2). This is the typical grounding conductor that you would see with any AC appliance and returns with the other AC conductors to the power distribution panel.

3. Install a DC grounding conductor sized not less than one size smaller than the DC positive conductor and have a capacity such that the DC positive fuse has an amperage rating not greater than 135% of the current rating of this grounding wire. As a practical matter, this wire will be much larger than the AC grounding conductor. This requirement is the latest addition to the standards when it was discovered that faults in the DC side of an inverter or charger could provide sustained high currents that could start a fire from overheating the AC grounding conductor.

Diagram missing, but available at original link (abpve) grounding_circuit_protect

Discussion
The rules for grounding electrical systems have evolved over time. Boat builders, installers, and electricians continue to recognize hazards and increase safety measures. Battery chargers were originally treated like any other small appliance, first without having any safety ground as was common through the 1950's, and then by adding a safety ground to reduce shock hazards during faults.

It was found that faults in the DC wiring or the DC side of chargers could generate fires because high current could flow back from the batteries, so a fuse was added between inverters or chargers and the battery system. As the capacity of chargers increased, and with the introduction of inverters, these DC fuses became quite large. It was then determined that a fire hazard exists when a DC fault in a charger or inverter can pass DC current into the AC safety ground wire. The AC safety ground was not sized for the high DC currents, so a high capacity DC grounding wire is now required by standards A-20 and A-25.

Now three critical grounding wires for these systems have been identified. This may seem excessive, but this combination of grounding conductors has been shown to give protection against a wide variety of faults.

Copyright Blue Sea Systems

[colemj]

I have a question about my inverter installation that has been bothering me, and I think it is related to Gord's post here. I have a Xantrex Prosine 3.0 3kW inverter/charger, which is mounted within 3 feet of my batteries. There is a chassis safety ground connection independent from the AC safety ground. The installation book says to connect this to the DC ground system and provides suggested wiring sizes based on length (standard table). They also suggest that the engine is a good place to ground this to.

The problem is that my engine is 20 feet away and they suggest 4/0 wire for this run. The connector provided will only accept a maximum 6 gauge wire. My engine block is also directly connected to my battery bank negative post with a 4/0 wire (return path for starter). So I naturally thought of connecting the inverter directly to the battery negative terminal 3 feet away instead of to the engine, which is directly connected to that terminal anyway.

But I called Xantrex first (and was on hold for an hour like every other time I have tried to contact Xantrex, but that's another topic). The engineer I talked with didn't seem to think the connection was necessary at all! But he did think that it needed to be connected to the engine block if I decided to connect it. He also told me a 8 or 10 gauge wire would be sufficient, even given the 20 ft run.

So, the manual tells me one thing and a company tech guy tells me another. This is what I did - I connected the chassis ground to the engine block using a 6 gauge wire.

But it has been nagging me ever since.

Any thoughts from the forum?

Mark

[colemj]

Doh! I missed the link in Gord's post above the first time I read it. The way I read that article, I can simply connect the chassis ground directly to the battery ground 3 feet away.

Any disagreement?

Mark

[ssullivan]

I would agree, Mark. I am set up the same way. Ground is ground, so long as everything is attached to it properly. BTW: Same thing on my bat chargers. They suggest a certain AWG cable, but fail to have connectors large enough to accomodate the size cable. The smaller ones have worked fine though.

[Sandero]

You can get lug type adapters for large gauge wire for smaller terminal posts. Audio equipment mfgs offer them in gold for speakers I suppose. They are not terribly expensive... and when you do a long run you have to up the wire diameter and the terminal posts on some equipment is not geared for the large lugs. UGH.

Jef
Shiva
Contest 36s

[DeepFrz]

I would just like to point out that the Blue Sea diagram doesn't really make it clear that the neutral and grounding wire from the boat are not connected together on the boat (that connection should be done in the dock wiring). That is the meaning of the 2 dashed lines.

Deep

[Rick]

The ABYC standard specifying that an inverter/charger be installed using a cable between the chassis ultimately connecting battery negative via a cable size capable of carrying all of the current possible from the positive battery inverter/charger cable creates a conundrum. To make such a connection without creating a ground loop through the ships bonding system from the battery negative requires that a huge cable be used from the chassis to the ac neutral bus as well as the "green wire ground" between the ac neutral buss and the ships 12V negative distribution buss. Such an installation is usually impractical and burdensome and, therefore, if anyone actually endeavors to adhere to the standard they will take the short-cut of installing the large cable between the chassis directly to the engine block or battery negative thereby creating the aforementioned ground loop.

All grounds are not at the same potential. If you don't believe me use a DVM capable of measuring microvolts and place the probes between ANY two so-called "grounds" and you WILL measure a non-zero voltage. BECAUSE inverter/chargers internally have filter capacitors as well as parasitic capacitance between the 12V supply and the chassis, along with leakage coupling between the transformer and the chassis, there will be an ac current flow from the chassis via any connection back to the input ac neutral buss. By providing a huge cable between the chassis and the battery negative you force that ac current through a ground-loop to raise any ships bonding system above that of the sea water, especially if a galvanic isolator is installed.

The idea is that if one has adhered to all of the other ABYC standards AND if a fault occurs between the +12V cable to the inverter/charger chassis THEN a fuse located at the battery positive terminal will blow (yeah, it might be a 400Amp fuse or larger). That also means that you cannot install a 30 Amp fuse in either the chassis "ground" wire or any part of the neutral wiring in order to prevent burning up a smaller #6 AWG wire should such a fault occur.

The reality is that if the inverter/charger adheres to U/L and NEC and CE standards for construction no such fault will occur. The reality is that we are dealing with the law of diminishing returns in terms of probability of failure versus risk. The real risk is if some idiot makes an installation whereby a huge current carrying cable might short externally to the chassis. Well, that could apply anywhere in the ships system, could it not? Again, if one adheres to the ABYC standards for cabling such a fault will not likely occur with any reasonable risk probablility.

Previous inputs to this thread recommending that the chassis wire be connected directly to the engine block or battery negative are ill-advised when considering ground-loops. Proper installations have a battery negative distribution system separate from a lightning and/or galvanic bonding distribution system separate from a neutral bonding system all of which are brought together at one and only one point so that ground loops do not occur either by design or by fault.

One must at least understand these principles before willy-nilly making cable connections merely because it is easy to assume that "ground is ground". It is not.

[colemj]

Hi Rick,

Thanks for your input. To clarify, I didn't suggest that the chassis be connected to the engine block - the Xantrex manual and tech support person did. I just claimed confusion . Also, the Blue Seas link Gord provided seems to show the chassis ground connected to the battery negative, which is connected to the AC ground.

So do you suggest I disconnect the chassis ground completely? I understand the issue with ground loops. On my boat the AC and DC grounds are brought together at a single point on the distribution panel, with the exception of the inverter chassis ground now in question. I don't have a galvanic ground system (transmission electrically isolated from engine and no metal skin fittings). So even if I brought the inverter chassis ground to the same single connection point, I am still introducing the possibility of ground loops due to capacitance coupling, as you describe. In addition, I think my inverter connects the AC ground to neutral on the boat when operating. Doesn't this produce another connection point in the system?

I replaced a perfectly good inverter that didn't even have a chassis ground or recommendation to install one (I wanted a sine wave inverter, but that's a different debate). The new one has the same rating and charger, and came from the same company (Trace vs. Prosine, but still Xantrex), so why the difference.

Still confused...

Mark

[Rick]

Hi Mark,
First realize that when in the "invert mode" the inverter/charger is a "source" and, as such, must provide a connection between the white neutral wire and the green safety wire. Internally the green safety wire is already connected to the chassis as per U/L specifications. This neutral connection is provided by a "neutral bonding relay" internal to the inverter control board. So, in the "inverter mode" (as opposed to "off" or in a "charger mode") the chassis, green safety wire and white neutral wire are all made to be common.

Disregarding for the moment the ABYC recommendation to make huge 2/0 wire chassis connection back to the battery negative consider that the chassis voltage ought to be kept at the same potential as the other metal objects in the boat and that should be at the outside water potential. In the case of a land-based installation it would literally be "earth" potential with a proper grounding rod. Hopefully all of the metal objects are "bonded" together with a #6 AWG green bonding wire each in turn connected to a central bonding point. That point is connected by a single wire to the neutral distribution point as is your house battery negative distribution system (again, by a single wire). The idea is that no ac loads or faults cause a current in the bonding system and that no house dc loads or sources or faults cause a curent in the bonding system or in the ac neutral system either.

So, the answer to your question is that any/all chassis get connected to the bonding system and not the battery negative distribution directly. If you map this all out on paper you can readily then understand how big the problem can be in terms of replacing various connecting wires with 2/0 wire to meet ABYC standards and also not cause ground loops. I feel that the ABYC standard to make the chassis green bonding wire a 2/0 (in the case of a 2kW or so rated inverter/charger) conductor is ridiculous in the face of existing standards for the construction and installation of the inverter/charger.

[DeepFrz]

Oh my gosh, I made a misteak...<gr>

I just read Nigel Calder's article in Professional Boatbuider magazine again and the green wire and neutral are tied together on the boat when an isolation transformer is installed.

Sorry.
Deep

[ssullivan]

Excellent input, Rick. Thanks for correcting my mistake as well. I hadn't considered the ground loop currents you describe above. My boat already had the 3 "grounds" (ie: bonding ground, "green wire 120VAC" ground, and 12VDC Neg ground). Each are brought together at one point. By lucky chance, the bonding system was the closest "ground is ground" point to my inverter, so I grounded to that. I got lucky. I conected to it by sheer luck.

Great input... and thanks for stepping in and setting us all straight here.

Sorry for the incorrect advice, Mark.

[Extemporaneous]

Because this is Great Stuff!! !!

Best Regards,
Extemp.