Automotive chassis ground minus the chassis

[BigBambu 152-462 MHz]

TL;DR Summary

Grounding to chassis required, but not possible for inverter installation.

I’ve installed an inverter, 12v to 120vac. The inverter has a post meant for grounding to the vehicle chassis. No chassis or other ground is available in this application for the inverter to recognize. I’ve homebrewed what amounts to half of a large capacitor. There’s just one “plate”, with the surface area in this arrangement at roughly 150 square feet. My questions being, 1) will it theoretically work, and B) am I going to get killed by it?

 

[berkeman]

Welcome to PF.

You seem to be talking about a vehicle that is not made out of metal? What exactly is the application? Why are you mounting a 120V inverter on this whatever-it-is?

 

[jack action]

There’s just one “plate”

And your "plate" will produce 12 V with respect to what? Whatever it is, that is your ground.

 

[BigBambu 152-462 MHz]

Welcome to PF.

You seem to be talking about a vehicle that is not made out of metal? What exactly is the application? Why are you mounting a 120V inverter on this whatever-it-is?

Hi, thanks for the interest. It’s part of a portable power supply, for beach camping & such. It’s on/in a big, fat-tire wagon.

 

[DaveE]

Summary:: Grounding to chassis required, but not possible for inverter installation.

I’ve homebrewed what amounts to half of a large capacitor. There’s just one “plate”, with the surface area in this arrangement at roughly 150 square feet. My questions being, 1) will it theoretically work, and B) am I going to get killed by it?

What, exactly, is half of a capacitor? There's more in that formula than plate area. Anyway I can pretty confidently predict that whatever you built it's insignificant at the 50/60Hz frequencies you care about. You seem to be ignoring that discrete capacitors have two wires. Your homebrew version does too, although they may be really well disguised (like the dirt nearby). The question of where to connect your ground post is closely related to this.

Anyway, if you can, connect that ground to whatever the person that might be electrocuted is standing on. A long copper spike pounded into the ground is the favorite choice for houses and such, but probably doesn't make sense in your case. Instead, look at the specifications of the inverter. It must have something along the lines of double insulation or reinforced insulation, like the electric drill that only has two contacts on the power cord. I don't know if they'll tell you that, it may be (ought to be, IMO) a requirement for the safety standards (CE, UL, CSA, TUV, etc.) in which case they might not mention it. What does the operators/installation manual say about grounding?

I would connect it to any metal bits that someone who is trying to kill themselves may be touching and leave it at that. No, I wouldn't guess that you'll kill anybody. But then how would I know since I don't have any data about your parts and system design? OTOH, I wouldn't be the least bit surprised if you told me you got a small shock from small leakage currents.

 

[Tom.G]

As I recall, in the USA, the National Electrical Code allows for a fully floating 120V supply that is fully isolated from the commercial power lines. The main reasons for the 120V commercial power to be grounded are user protection from lightning strikes, and capacitive coupling in, or failures of, the distributon transformer on the power pole. Those transformers have 16,000V volts on the primary side. (Not something you want sneaking into your house.)

In light of the above, in a battery-supplied, stand-alone system the highest voltage is the output voltage, so there is no safety reason for a physical Ground; no matter what you get your hands across it can not be higher than the output voltage.

Unfortunately someone 'borrowed' my copy of the code so I can not quote the specific section. Perhaps someone else here can look up the pertinent section.

Cheers,
Tom

 

[BigBambu 152-462 MHz]

What, exactly, is half of a capacitor? There's more in that formula than plate area. Anyway I can pretty confidently predict that whatever you built it's insignificant at the 50/60Hz frequencies you care about. You seem to be ignoring that discrete capacitors have two wires. Your homebrew version does too, although they may be really well disguised (like the dirt nearby). The question of where to connect your ground post is closely related to this.

Anyway, if you can, connect that ground to whatever the person that might be electrocuted is standing on. A long copper spike pounded into the ground is the favorite choice for houses and such, but probably doesn't make sense in your case. Instead, look at the specifications of the inverter. It must have something along the lines of double insulation or reinforced insulation, like the electric drill that only has two contacts on the power cord. I don't know if they'll tell you that, it may be (ought to be, IMO) a requirement for the safety standards (CE, UL, CSA, TUV, etc.) in which case they might not mention it. What does the operators/installation manual say about grounding?

I would connect it to any metal bits that someone who is trying to kill themselves may be touching and leave it at that. No, I wouldn't guess that you'll kill anybody. But then how would I know since I don't have any data about your parts and system design? OTOH, I wouldn't be the least bit surprised if you told me you got a small shock from small leakage currents.

The comparison to a capacitor is but figurative, to establish my concept. The surface area is intended to mimic the surface area of a non-existent chassis, to convince the unit that it’s not mounted in a fat-tire wagon, on a beach. I’d like it to be happy, in its humble ignorance.

 

[berkeman]

to convince the unit that it’s not mounted in a fat-tire wagon, on a beach.

It sounds like this is no different from a gas-powered generator in such a wagon, no? I would just read the safety manual from a typical generator to see how they handle the output ground connections to their multiple 3-prong AC Mains power outlets...

https://generatorpowersource.com/wheel-kits-for-portable-generators/

 

[BigBambu 152-462 MHz]

It sounds like this is no different from a gas-powered generator in such a wagon, no? I would just read the safety manual from a typical generator to see how they handle the output ground connections to their multiple 3-prong AC Mains power outlets...

View attachment 287487
https://generatorpowersource.com/wheel-kits-for-portable-generators/

Thanks for your input. It’s a bit different (I think), in that the battery negative is the ground, in the absence of a sustantial chassis ground. I’m leery of creating a negative loop in the place of a true “chassis” and its accompanying surface area.

 

[Averagesupernova]

Theoretical situation: Take a vehicle, install your hardware as per instructions. Now start removing parts of the vehicle until all you have left is the conductive parts that connect your desired hardware. Now shrink that down to as small as possible. A metal framework that the holds your hardware in place and serves as a very highly modified vehicle chassis. I don't see a problem here.
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This half capacitor business is nonsense.

 

[DaveE]

The comparison to a capacitor is but figurative, to establish my concept. The surface area is intended to mimic the surface area of a non-existent chassis, to convince the unit that it’s not mounted in a fat-tire wagon, on a beach. I’d like it to be happy, in its humble ignorance.

Unnecessary. It would be happy with no ground connection.

 

[BigBambu 152-462 MHz]

Unnecessary. It would be happy with no ground connection.

Thanks, Dave. I appreciate it

 

[BigBambu 152-462 MHz]

Thanks, Dave. I appreciate it

Theoretical situation: Take a vehicle, install your hardware as per instructions. Now start removing parts of the vehicle until all you have left is the conductive parts that connect your desired hardware. Now shrink that down to as small as possible. A metal framework that the holds your hardware in place and serves as a very highly modified vehicle chassis. I don't see a problem here.
-
This half capacitor business is nonsense.

Thanks, it sounds like I’ve stepped into the Rube Goldberg side

 

[BigBambu 152-462 MHz]

And your "plate" will produce 12 V with respect to what? Whatever it is, that is your ground.

Thanks, Jack… sorry for the lagtime. The plate is attached to what little frame there is, with a single, soldered 10ga ring terminal, so my thinking was/is that it’s now just mimicking a much larger frame, in the event the inverter would function better than just connecting the device ground to the batter negative, in a barely roundabout way. My understanding is that there are some fairly dangerous voltages bouncing around within the inverter. What part a physical vehicle chassis, acting as an already virtual ground plays in the switches doing their 120vac thing is unknown to me, but the manual says I need one. I think my solution is better than relying on a fat-tire wagon to mimic the ideal ground of not even a vehicle, but a proper rod in the Earth.

 

[jack action]

Personally, I have trouble following what you are saying. You do not use terminology that I'm used to.

So you should share a wiring diagram to make sure we understand each other.

Normally, you should have a power source, usually a battery with +ve and -ve pole, but I guess it could be a capacitor, but I'm not an expert at this. As long as it produces 12 V between the +ve and -ve poles. The -ve side is usually connected to a vehicle frame, making the frame the "ground". You then connect the inverted 12 V +ve lead to the +ve pole of the power source and the 12 V -ve lead to the "ground", i.e. the vehicle frame.

If it is not a vehicle, the "ground" can be anything. It doesn't even have to exist at all, i.e. the inverter can be connected directly to the +ve and -ve poles of the battery. It is possible that the inverter casing needs to be connected to the ground as well (in such a case, usually it would require the casing to be bolted to the frame, i.e. the "ground"), check the instructions.