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Mischief Managed 09-24-2019 09:17 AM

Inverter case ground connection
I installed a 3000 watt true sine wave inverter in my boat over the Summer. I have not connected the chassis ground lug to anything yet and I have convinced myself to leave it that way. My reasoning is that the unit is grounded by the shower power connection (when it's connected) and if there is a short to the case, I don't want AC or DC power going to my bonding system as it could kill someone and will not achieve anything useful regardless since it's not a true ground and won't trip a breaker.

The installation instructions are vague for marine applications:

"Connect the grounding cable between the chassis ground
point and the DC grounding point for your system.
In an RV or vehicle installation, the DC grounding point
will usually be the vehicle chassis or a dedicated chassis
ground bus.
For marine installations, refer to the applicable local code
for marine DC grounding detail."

I was, however, thinking that maybe I should check for continuity between the grounding lug and the AC ground connection and if there is none, ground the case to the AC in ground wire. That said, I will be very surprised if there is no continuity there already.

Any thoughts would be appreciated.

DavidM 09-24-2019 09:42 AM

I have also wondered about this installation requirement or recommendation.

As noted in your text above, it applies to the DC grounding system. The inverter has its own AC load, neutral and ground connections independent of the case DC ground.

What good is a separate case ground. DC pos and neg cables are connected between the inverter and the battery. I realize that the case is not connected to either the AC ground or the DC negative. Why another separate DC ground for the case?


psneeld 09-24-2019 09:56 AM

I am assuming it is because of a potential DC short...the inbound wire is huge....what could happen?

Ka_sea_ta 09-24-2019 09:58 AM

Actually ABYC a-25 speaks to this, in practice a case ground is sized to one size less then the size of the positive feed from the battery. The language is that the OCD on the positive side is 135% larger then the ampacity of the DC bonding wire....

diver dave 09-24-2019 09:58 AM

The overriding shock safety issue, with a metal case, is to insure there is no hazardous voltage present on the case, vs local earth. If hazardous voltage exists within the box (and there is), then either a double insulated rating is used, or, a locally grounded case. The instruction manual should give guidance.
Unless otherwise indicated, I would bond the case to a local ground point.

psneeld 09-24-2019 10:04 AM

The ground in this case I believe is to protect from a DC short (fire I am guessing due to amperages involved) thus the huge gound cable required....the AC side has its own ground which I would think would be for shock hazards.

Maerin 09-24-2019 10:55 AM

It's a DC ground. ABYC requires it to be grounded to the DC ground point- buss, battery, not the engine, because then the ground path between engine and DC ground buss or battery is only the size of the ground conductor on the engine. As others posted, it can be one wire size smaller than the (+) conductor on the DC input on the inverter.

ABYC also requires this grounding conductor from case to DC grounding point on your battery charger.

It can pop on a survey, so on a new install, you're well-advised to include the case ground. On already installed gear, it depends on how thorough the surveyor is. Mine remarked that my charger & inverter had the case grounds, he typically finds them missing. So that's one surveyor who looks for them.

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