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12VDC Battery Power/Solar Power Backup/AC Invertor Portable Display: Grounding Problem

12VDC Battery Power/Solar Power Backup/AC Invertor Portable Display: Grounding Problem

12VDC Battery Power/Solar Power Backup/AC Invertor Portable Display: Grounding Problem

I am active in our county's Emergency Response Program, and am working on a battery-backup/Emergency AC power demonstration model for training different groups around the region.

It will be a "working model" on a two-wheeled cart with a 12 V lead-acid battery below and a 16 inch x 42 inch plywood panel above the battery to mount a 120 VAC inverter, a battery charger-trickle maintainer, DC fuses, a DC volt meter and ammeter, a couple of DC adapter stations (cigarette lighter-style adapters) and USB charger connections, etc. Add space and connections for a nominal solar panel tie-in, if I get fancy later, or other essentials as you recommend here.

In the far future, if the training works out, I'd do something more elaborate on a separate cart and larger plywood to show backup power options for a typical house circuit: That would require the AC tie-in isolation switch, a 120/240 generator connection and switch, a typical house breaker panel, emergency light for the panel.

But let's stick to the simple display right now.

My inverter is rated for 1500 watt, 120 VAC/60 Hz continuous - obviously it will be connected to the 12 VDC battery. If I were permanently connecting to a set of lead acid batteries at home, I'd tie the inverter chassis ground point to the house ground system with a #6 or #8 AWG ground wire, as the wiring instructions call out. No problem. If I were in a vehicle or boat, I'd use the vehicle ground.

But with a portable system, one on a isolated cart by rubber tires and a non-conducting panel, where is the proper ground point? The two inverter AC outlets are GFI-protected, but it does not seem right to ground the inverter chassis to "nothing", to the cart frame (and then to ???), nor to a "handy" 8 foot grounding rod pounded into the classroom floor.

Nor does it seem right to ground the AC part of the system to the battery negative cables, although, in car, RV, truck, or boat, that's is what you're essentially doing. Yes, I could ignore the ground entirely, but that isn't right for a training system.

Am I over-thinking the problem? Or missing the intent of the chassis ground?

RE: 12VDC Battery Power/Solar Power Backup/AC Invertor Portable Display: Grounding Problem

Theoretically all generators have an obvious ground lug that you poke a wire in and screw down on after bringing your ground wire over from wherever.

Correct usage would include you driving a ground rod next to the generator and correctly proceeding. Your application with no power really being supplied off cart doesn't exactly need that connection to earth ground just as motor homes don't usually have. After all, since no power is being delivered off-cart the possibility of being shocked via earth return doesn't exist.

However, as soon as you power something like a house or facility or power tool you have provided an extended network that will include earth-return as a possibility.

In the classroom I'd bring in a ground rod and show what they look like and how to wire to them and do everything but, of course, drive it anywhere. The message should get thru.

You should include in your cart design the ability to mount a large solar panel above. A 260W panel could charge a truckload of laptops and cell phones fuel/noiselessly.

You should also cover GFI protection as portable stuff often includes powering things sitting on dirt and quasi-chaotic power routing that should usually include GFIs for life safety.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 12VDC Battery Power/Solar Power Backup/AC Invertor Portable Display: Grounding Problem

I don't know if you are aware of it, but many inverters in that power range require a floating neutral. Grounding the neutral will either short out 1/2 of the H bridge output or cause the battery to be at a lethal potential. Stress this in your education.

I remember the first inverter I repaired. Noticed that the AC outlet ground pin was just connected to a 470K resistor to battery common. Just enough to drain off a static charge.

RE: 12VDC Battery Power/Solar Power Backup/AC Invertor Portable Display: Grounding Problem

Thank you.

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