Missing ground from inverter in system (off-grid cabin)

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outsourcedguru asked Jun 22, '20

Missing ground from inverter in system (off-grid cabin)

Victron MPPT 150/100-TR
Victron BMV-712 Battery Monitor with shunt
Dewalt DXAEPI1000 1000-watt inverter
Four 6V 225AH AGM batteries wired as series/parallel to present 12V
Four 400W Jinko solar panels in series/parallel
Battery cut-off switch
150A circuit breaker
Two 20A inline fuses to the PV

I've created an earth ground outside and I've attached it to the marked ground connection on the MPPT, as expected.

When I connect a TP-Link HS300 power strip to the inverter, it does not indicate that there is a system ground. Likewise, if I plug into the inverter an APC brand UPS, it shows a red LED for "system ground fault".

The manual on the Dewalt is shy on earth grounding; I'm guessing that the product is normally used in a car. Normally, the negative on the car is the car's ground... but just not an earth ground.

Any thoughts?

Phoenix Inverter

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Answer 1 · 2020-06-22T06:28:26Z

wkirby answered · Jun 22, 2020 at 06:28 AM

Where is your Neutral - Earth bond? Does the Dewalt make this inside or are you expected to make it yourself? The Dewalt manual should cover this. I couldn't see a link to the manual on the Dewalt website.

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outsourcedguru commented · Jun 22, 2020 at 07:57 PM

I don't see anything in the Dewalt manual. I've read through it multiple times. They appear to be overly worried that the user will install it into a car with a positive ground. They don't appear to have a chassis screw marked ground that I can see.

"Where is my Neutral - Earth bond?" I have driven an 8' copper grounding stake and I've connected this minimally to the ground-marked chassis bolt on the MPPT. As indicated, this doesn't seem to be making it into the inverter.

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Answer 2 · 2020-06-22T22:04:42Z

Kevin Windrem answered · Jun 22, 2020 at 10:04 PM

You are seeing faults on the plug strip and UPS because the safety ground and neutral are not "bonded". Somewhere in the electrical system there should be ONE connection between safety ground and neutral. In typical wiring that connection is at the point where power enters the building.

Inverters and generators often do NOT make this bonding connection when they should.

This has noting to do with an earth ground connection. Plus a earth ground connection to the MPPT will have no affect on the inverter AC safety grounding.

There should be a low resistance connection between the u-ground and the larger spade (neutral) pins on the AC receptacles. If not, you can buy generator bonding plugs that make this connection externally.

You should also ground the frame of the earth ground. AND that earth ground should also be connected to whatever you have as a "chassis". The earth ground connection provides a fairly high resistance path to drain off static charge and other leakage currents.

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outsourcedguru commented · Jul 04, 2020 at 04:38 PM

What I'm seeing is that the DeWalt inverter is presenting the ground pin in the receptacle as the zero point. Either remaining connection then presents something like minus and plus 60VAC. So it's presenting 120VAC across the two main connections but this isn't the reality you want. I'm thinking that the DeWalt is just badly suited to this exercise.

I attempted to add the following that's bolded:

120VAC output receptacle on the DeWalt
Short three-prong extension cord
The typical two-prong to three-prong adapter plug and I added a wire to the earth ground
Three-outlet power strip
Three-prong test device (again, showing a missing ground via the LEDs)

I worry that adding some sort of direct wire from the DeWalt ground outlet to the NEG receptacle will fry something.

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Kevin Windrem outsourcedguru commented · Jul 04, 2020 at 04:56 PM

As a test to see if you'll damage anything, connect a low wattage incandescent (not LED or CFL) light (e.g., 7watt Christmas light bulb) between safety ground and neutral pins on the outlet.

If the output is floating relative to safety ground, the voltage between safety ground and neutral will drop to within a volt or so of zero and you can safely make your own safety-neutral bond externally.

If the light glows at all or you measure more than a few volts, then shorting the two would cause problems or even damage. Best to replace the inverter to something that presents a proper neutral/safety bond.

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