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Power inverter earth ground.

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dragonf1y
1st Mate

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USA
34 Posts

Initially Posted - 05/14/2012 : 21:28:29

I have a 1981 Catalina 25. I bought a Cobra CPI 880 inverter. It has the power outlets built-in so it's just directly wired to the battery with no fancy inverter/shore power switching, etc. I bought the permanent install cable kit Cobra sells for it. I've set everything up as advised, including an inline fuse on the hot side, tinned/crimped/heat-shrunk ring connectors, etc. However, I'm unsure where to connect the green earth ground that goes from the inverter's case to the 'chassis ground'. I'm installing the inverter on the external vertical surface of the starboard settee toward the aft end (below the seat lip overhang). My batteries are located under the starboard settee aft, so this provides extremely short cable runs and the inverter is out of the way, but very accessible. My breaker panel and battery selector is in front of the port side galley sink with the electrical connections in the port side cockpit lazarette.

After cruising multiple forums online looking for info, most instruction is regarding units that are wired into the boat using the existing AC outlets. Advice is always to make sure the AC and DC grounds are tied together on the same bus bar. Mine is stand alone. Would I still want to run a wire all the way over to the ground bus bar in my port side lazarette, which is a hassle (I assume that's the closest/only one)? Or can/should I be doing something else with it either safer or more convenient?

----------------------

Backstory if anyone cares:

For the past few years of ownership we've used the accessory socket to charge our phones when away from shore power. This worked fine with the exception of the poor placement of the socket right by the galley sink.

Last year we added a tender/dingy with a Torqeedo 401 outboard for it. I have two battery packs for it and these usually last a week long trip on mooring buoys and at anchor. However, last summer this wasn't the case. We needed ice for our ice chest and not wanting to lose our spot on a prime buoy location we had to go about 4nm roundtrip to get ice as the place listed as having ice nearby was no longer in business. While we were able to make the trip, it was the second to last day of our trip and our batteries were low. We had to make the trip on the low power saving setting and it took ages. My wife wasn't pleased (nor was I). I decided then an onboard inverter would make phone/device charging much more convenient away from the sink and we'd be able to top off our dinghy's batteries if necessary.

So, the inverter's primary use will be charging phones (away from the sink), iPad charger, handheld VHF charger, and outboard battery charger. Noticing a theme here? I use my iPhone as a handheld GPS so it needs to be topped off frequently. I understand the inverter will have a larger draw than the straight accessory plug, but convenience outweighs that for me. I do have a 9.9hp outboard Yamaha kicker with an alternator so it charges the batteries when running, we have a small portable solar panel to charge phones when sun is available, and I installed a battery charger when we're connected to shore power. So while we only have one house battery it's generally topped off to start with and other than charging electronics and providing cabin/nav/anchor lighting it really doesn't have any other draw on it.

1981 C25 TR/FK Dragonfly

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cat25
Navigator

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USA
140 Posts

Response Posted - 05/15/2012 : 02:59:30

My 1990 cat 25 did not have a ground connection between the shorepower switch and the 12volt panel neg bar. The grounds are supposed to be connected together. That might bring the ground connection closer for your inverter. I used number 10 wire. Hopr this helps.

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Davy J
Master Marine Consultant

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USA
1511 Posts

Response Posted - 05/15/2012 : 05:03:56

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Advice is always to make sure the AC and DC grounds are tied together on the same bus bar.<hr height="1" noshade id="quote"></blockquote id="quote">
I'm no electrician, but this doesn't sound right to me. Hopefully someone with boat electronics experience will chime in.

I installed an inverter, but did not connect any wire to the chassis ground because I could not conceive how I would achieve an earth ground inside the boat. Like I said, hopefully someone with experience will speak.

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Sloop Smitten
Master Marine Consultant

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USA
1181 Posts

Response Posted - 05/15/2012 : 08:21:43

I think DavyJ is right. The reason to use the inverter is because you do not have access to an AC source to provide an earth ground. You do not want any part of your DC power grid to be connected to your AC power grid, even the earth ground. Begging for trouble.

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JimGo
Admiral

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USA
962 Posts

Response Posted - 05/15/2012 : 08:49:22

Here's a good thread on the topic:
http://forums.parallax.com/showthread.php?97575-Should-DC-and-AC-circuit-have-common-ground

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Voyager
Master Marine Consultant

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USA
5369 Posts

Response Posted - 05/15/2012 : 18:58:12

According to the US Power Squadron's course Marine Electronics that I teach, this is the most poorly understood problem on boats there is. If you have shore power, DC power, an inboard engine and other metallic components in your boat, you can ground the AC incoming power and all AC receptacles and appliances to a single heavy duty ground point and you should also use that same connection for all DC circuits too. This prevents you from getting shocked if you touch any metal on the AC appliance and another piece of electrical equipment, either AC or DC. this also prevents your zincs from disappearing.
That said, I don't use shore power and have installed a 300W inverter and NEVER ground it to anything. I usually use a UL or CSA-approved wall wart device and check the terminal voltage against the metal enclosure of the inverter. The green AC ground lead is electrically connected to the negative DC lead, so I assume it leads back to the boat's ground. The battery (-) terminal.
To be "belt and suspenders" sure, you can always measure the voltage between the case of something plugged in and the battery's negative lead. This should read zero volts DC and zero volts AC.
If not, then rewire the boat, replace the inverter or install GFI or GFCI outlets on your boat.
Some inverters also supply USB connections. I've seen some pretty sketchy circuits where some residual AC can be found on the USB connector.
These are trash and dangerous to equipment and people.
None of this discussion includes anything about ground isolators - that's a whole 'nother story. Only use them if installed by a marine electrician and only if you are losing your zincs in under 1 year. You've got more problems than can be relayed here if that is the case.
Also, AC and DC grounding equipment does nothing to prevent lightning damage.
That's my 2c if that's worth anything ...

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Davy J
Master Marine Consultant

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USA
1511 Posts

Response Posted - 05/16/2012 : 05:13:47

<blockquote id="quote">quote:<hr height="1" noshade id="quote">That said, I don't use shore power and have installed a 300W inverter and NEVER ground it to anything. I usually use a UL or CSA-approved wall wart device and check the terminal voltage against the metal enclosure of the inverter. The green AC ground lead is electrically connected to the negative DC lead, so I assume it leads back to the boat's ground. The battery (-) terminal.
<hr height="1" noshade id="quote"></blockquote id="quote">
Again my disclaimer that I only have a basic understanding of electrical design, but this statement seems contradictory. If the green ground on the chassis of the inverter is connected to your DC circuit, and if there is a short at the inverter, you would have AC current going to your battery bank. Is this correct? Also, is the DC ground on the boat an earth ground or simply a ground back the batteries? Thirdly, if there is a short at the inverter, you cannot become the earth ground because physically you are not grounded? And last, can an earth ground be achieved by connecting the inverter to a keel bolt or swing keel hardware?

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Stinkpotter
Master Marine Consultant

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Djibouti
9080 Posts

Response Posted - 05/16/2012 : 09:25:09

Bruce: I'm obviously no electrical engineer--I'm an example of those who don't understand the concept of connecting the negative ("so-called "ground") side of your DC system to the ground used in the AC system. If something in the AC system shorts to ground and you have an inverter or charger connected to your battery in the DC system, is something going to melt down? (Of course, everything should be fused, but I've seen a fire started by a fused 12V DC circuit.)

To me, "ground" in a 12V DC system (car or boat) is just the negative side of the circuit. In a car, it's the chassis, which is not grounded--it's just connected to the negative pole of the battery. In a boat, it's the black wires that lead to a bus that's connected to "-".

Like I said, it's not that I don't believe you, it's that I don't understand it--and I hate to connect wires in a way that I don't understand, no matter what the instructor or instructions say--especially when there's 120V involved. (About the only thing I'll do in my house is connect a switch or light fixture in an existing junction box. If I find a nest of wires in the box, I call the electrician.)

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BCG-Woodbury
Mainsheet Editor

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USA
396 Posts

Response Posted - 05/16/2012 : 09:51:46

I just looked at your manual and it has no references to connecting the "chassis" ground. According to what I found in a Xantrex Freedom Marine Inverter (a high-end inverter) they specifically say NOT to connect the DC(-) to the AC "safety" Ground. I would have to say that if they printed it, it is most likely accurate based on the price their products cost ($1500+).

5. Do not connect the inverter battery
negative cable to the vessel safety
ground. Run the negative (-) cable directly to
the battery bank. If the positive (+) and negative
(-) cables run parallel to each other, twist the
cables together. This will minimize the
inductive adverse effects of cable length. Be
sure the cable size meets with NEC
requirements for your installation.

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JimGo
Admiral

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USA
962 Posts

Response Posted - 05/16/2012 : 09:58:15

Have you checked whether the inverter already has a connection between the grounds?

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dragonf1y
1st Mate

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USA
34 Posts

Response Posted - 05/16/2012 : 10:30:52

Last night I finished installing the inverter without the chassis ground. Tested it charging my outboard battery pack and my cordless drill battery pack as it was low. Worked like a charm. Now I just need to fix the electrical taped ring connectors that were already on the boat on the leads from the battery selector switch. They are corroded and looking sorry.

As for installation instructions I got them via the kit that Cobra sells for the inverter.
Cobra CPI-A4000BC 4-AWG Heavy-Duty AC Power Inverter Cable Kit

In my reading, seems the chassis ground is more for static electricity than anything else in this installation. With the fan running and a partial metal case apparently there is the potential for static electricity. I assume the protection in that install is more to protect the hardware than the user.

Regardless I think I'm fine. I'm sure the install I did is far more robust than most weekend boater installs of similar hardware.

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Davy J
Master Marine Consultant

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USA
1511 Posts

Response Posted - 05/16/2012 : 10:48:04

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Last night I finished installing the inverter without the chassis ground.<hr height="1" noshade id="quote"></blockquote id="quote">
Congratulations, I'm sure you'll find it as useful as I have. Wished I would have put one on the boat earlier.

I think the confusion about the inverter is that most of the installations are on a land based structure. The chassis ground or "safety" ground as Brian G found out, to my understanding, is there in case the inverter's case or "chassis" becomes energized.

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BCG-Woodbury
Mainsheet Editor

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USA
396 Posts

Response Posted - 05/16/2012 : 17:49:47

If the ground is only to fight static build-up then you could wire it through a 0.25A fuse which would blow if any current exists but will operate all day long as the static drain.

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Voyager
Master Marine Consultant

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USA
5369 Posts

Response Posted - 05/21/2012 : 19:22:01

OK, I had a chance to check all three of my inverters:
(1) a 250W unit on Passage,
(2) a 220W unit for the car and
(3) a small cigarette lighter unit for my wife's car.

I stand corrected - there is no connection between the metal frame, the negative battery lead and the ground lug on the 3-pronged outlet. In fact, the AC portion of the circuit basically "floats". The 110VAC power is only delivered between the hot and neutral lines of the electrical outlet of the inverter. The neutral side is connected to my outlet ground, but that has no bearing on frame ground or the negative DC power line to the battery.

Therefore, if the frame or grounding lug on the outlet is connected to the negative battery lead of your boat, you risk placing a 110VAC shock hazard on it in case of a fault.

Therefore, don't make the connection, let the AC circuit float. Before you connect the inverter to your boat's 110VAC circuits, please make sure that the neutral and ground leads of the circuit are NOT connected to the battery negative lead.

I apologize for any confusion. Live... ... and learn!

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