Earth grounding your rig??
- Thread starter LXRACR
- Start date
TheRoadie
Explorer
As an engineer, I can't see why. Planes and boats have lots of electronics, and they work just fine grounded to the frame or fuselage.
Lightning is a different issue. If you're inside the body, and it's not a fabric roof, then the energy of a strike will run along the metal skin. Just don't be touching the inside of the roof and you'll be very safe. It's called the Faraday Cage effect.
Some folklore says you're safe inside a car because the insulation of the rubber tires "insulates" it from the ground. But the voltage behind a lightning bolt allows it to jump thousands of feet from the cloud to the car - it certainly isn't going to be slowed down by a few inches of carbon-filled rubber, likely also wet to boot.
If you're camping OUTSIDE in an electrical storm, grounding the vehicle won't help. Just jump in and stay away from the windows.
Lightning is a different issue. If you're inside the body, and it's not a fabric roof, then the energy of a strike will run along the metal skin. Just don't be touching the inside of the roof and you'll be very safe. It's called the Faraday Cage effect.
Some folklore says you're safe inside a car because the insulation of the rubber tires "insulates" it from the ground. But the voltage behind a lightning bolt allows it to jump thousands of feet from the cloud to the car - it certainly isn't going to be slowed down by a few inches of carbon-filled rubber, likely also wet to boot.
If you're camping OUTSIDE in an electrical storm, grounding the vehicle won't help. Just jump in and stay away from the windows.
I would agree, even without being an electrical engineer. Auto tires, sitting on dry pavement or asphalt, will conduct static electricity to ground when refueling a vehicle. If they didn't, we would risk static discharge fires every time we refueled the vehicle. If low pressure static electricity will pass to ground over dry tires, lightening will pass over or through a tire to get where it is going, and offering a path with slightly less resistance won't make a difference.
As for having an earth ground otherwise, I can't see that it makes any difference for the electronics on board. These systems operate independent of earth as long as they have a ground return to their power source. Grounding a vehicle to earth would not defeat a ground fault in an electronic system because the power source is not also earth grounded, so I see no value there.
As for having an earth ground otherwise, I can't see that it makes any difference for the electronics on board. These systems operate independent of earth as long as they have a ground return to their power source. Grounding a vehicle to earth would not defeat a ground fault in an electronic system because the power source is not also earth grounded, so I see no value there.
DaveInDenver
Middle Income Semi-Redneck
That's probably right if the inverter is hard mounted to your truck. If it's not a portable installation, then the guidelines might be different.LXRACR said:
NEC 551.20(C):
Bonding Voltage Converter Enclosures
The non-current-carrying metal enclosure of the voltage converter shall be bonded to the frame of the vehicle with a minimum 8 AWG copper conductor. The voltage convertor shall be provided with a separate chassis bonding conductor that shall not be used as a current-carrying conductor.
I should note that that's the NEC rule for recreational vehicles, which is not quite exactly the same. They have distribution panels, outlets and appliances running off the inverter or generator and that changes how you wire things up. In an RV they are concerned with grounding, ground and potential. The ground itself is only relative and the circuits themselves don't care where they are compared to it. As long as the current has a return path to the source the circuit is complete. Both sides of the circuit can be isolated from earth and it will work perfectly fine and be perfectly safe. The potential against earth doesn't matter other than for lightning or other sources (like overhead wires, if your truck touches those you want the current to have a path to ground). So grounding the inverter to the frame is primarily to give all other devices connected to it a common ground for the source. If other things don't have frame mounted grounds, then it won't matter if the inverter is grounded to the frame or not in practice. But being too safe is never a bad move.
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762X39
Explorer
Here is a picture of the inverter on my work truck with the #8 wire bonded to a suitable chassis ground (the inverter itself is wired back to the battery with 2/0 welding cable and fused near (I couldn't fuse it at the battery) with a 150 amp ANL fuse.As far as earthing (grounding) the actual truck, we used to do it to our Radio Trucks when I served in the Canadian Forces but that was because of our comms equipment and the antennas we sometimes erected beside the trucks (although we often just used the big whip antennas and an antenna tuner.
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Robthebrit
Explorer
Grounding a vehicle does remove one hidden safety risk associated with inverters but potentially can cause a much bigger one. This hidden problem is far more common in modern inverters and arises from the fact that theres a direct electrical path between the mains output circuit and the low voltage input circuit (including the battery). This path is usually via the auto turn-on sensing and is especially true in cheaper inverters, you can buy isolated inverters and in these inverters there is no electrical path between the two sides.
When the inverter is being used to power a single tool or appliance, this internal current path normally doesn't pose any safety risk because the complete battery-inverter-appliance system floats above earth. The battery will always be 12V greater than ground, even if ground is at 100v. The system is floating so the absolute voltages don't matter.
However if the tool or appliance is faulty and develops a short or some other severe leakage between its mains wiring and its external metal case or frame (which would normally be earthed, when itﾒs plugged into a mains power outlet), thereﾒs a risk that the battery connections can become dangerously live.
How can this happen? If the leakage path to the appliance frame happens to be from the side of its mains wiring connected to the active side of the inverters output, and the frame of the appliance becomes connected
to earth, this will immediately raise the neutral side of the inverters output to the full output voltage above earth and with it, the low voltage side of the inverters circuitry and the battery terminals and potentially all the electronics in the car. If an unsuspecting person who also happens to be earthed should touch one of those safe-looking 12V battery terminals, they can receive a potentially fatal shock - they basically just touched the output of the inverter! In a negative earth car the situation is much worse because touching the car body can be dangerous (you are the ground wire!!)
While grounding the vehicle will prevent this problem (the system will no longer be floating and the electrical fault will not be able to 'pull up' the voltage of the real ground) but there is always a risk they you'll forget or not get a good contact with the ground (most buildings use a giant metal stake hammered into the ground which is impractical for a vehicle). In order for the ground wire from the car to work you not only need a good connection but you also need to make sure all the inverter sockets are earthed. Grounding the inverter socket but not grounding the vehicle is what makes the earth fault potentially fatal. With an inverter it is much safer to not wire the earth pin and clearly mark the output as INVERTER POWER: FLOATING. In this scenario you have to be outside and touching the case of the bad appliance, the car body will remain perfectly safe.
There is a lot more to wiring an inverter than first meets the eye, espeically when you throw mains powered chargers and generators into the mix. Always think twice before touching the terminals of a battery that you know is being used to power an inverter.
Rob
When the inverter is being used to power a single tool or appliance, this internal current path normally doesn't pose any safety risk because the complete battery-inverter-appliance system floats above earth. The battery will always be 12V greater than ground, even if ground is at 100v. The system is floating so the absolute voltages don't matter.
However if the tool or appliance is faulty and develops a short or some other severe leakage between its mains wiring and its external metal case or frame (which would normally be earthed, when itﾒs plugged into a mains power outlet), thereﾒs a risk that the battery connections can become dangerously live.
How can this happen? If the leakage path to the appliance frame happens to be from the side of its mains wiring connected to the active side of the inverters output, and the frame of the appliance becomes connected
to earth, this will immediately raise the neutral side of the inverters output to the full output voltage above earth and with it, the low voltage side of the inverters circuitry and the battery terminals and potentially all the electronics in the car. If an unsuspecting person who also happens to be earthed should touch one of those safe-looking 12V battery terminals, they can receive a potentially fatal shock - they basically just touched the output of the inverter! In a negative earth car the situation is much worse because touching the car body can be dangerous (you are the ground wire!!)
While grounding the vehicle will prevent this problem (the system will no longer be floating and the electrical fault will not be able to 'pull up' the voltage of the real ground) but there is always a risk they you'll forget or not get a good contact with the ground (most buildings use a giant metal stake hammered into the ground which is impractical for a vehicle). In order for the ground wire from the car to work you not only need a good connection but you also need to make sure all the inverter sockets are earthed. Grounding the inverter socket but not grounding the vehicle is what makes the earth fault potentially fatal. With an inverter it is much safer to not wire the earth pin and clearly mark the output as INVERTER POWER: FLOATING. In this scenario you have to be outside and touching the case of the bad appliance, the car body will remain perfectly safe.
There is a lot more to wiring an inverter than first meets the eye, espeically when you throw mains powered chargers and generators into the mix. Always think twice before touching the terminals of a battery that you know is being used to power an inverter.
Rob
DaveInDenver
Middle Income Semi-Redneck
Rob's scenario depends on whether the inverter has a bonded neutral or not. I would not expect a small inverter to have a bonded neutral. So if there's a path from neutral to DC(-) would be a pretty major design issue IMO.
Robthebrit
Explorer
Most small inverters have some form of small signal continuity between the DC and AC sides. It normally through the load sense (on/off) or the sense circuitry for stability, short protection. Small signal continuity is all you need to float the ground and its very easy to do - very few inverters have nothing connected to the AC side of the output transformer. Old inverters used to use a relay for the on/off sense that was powered by an AC side circuit and would switch the DC side - this is completely decoupled. 99% of solid state relays don't have the same decoupling characteristics of mechanical relays and 99% of the time it doesn't matter.
If you get a nice name brand inverter that has all the required certification then you are good. If you get a $19 made in china special then all bets are off. I just tested my old 200W Vector and it has continuity between the AC side and ground. Looking at the circuit board I can't see any attempt to isolate one side from the other.
EDIT: Not looking to start any problems, I am just pointing out that there are a whole array of problems associated with inverters in vehicles, a lot of it is out of your control and rarely talked about.
Rob
If you get a nice name brand inverter that has all the required certification then you are good. If you get a $19 made in china special then all bets are off. I just tested my old 200W Vector and it has continuity between the AC side and ground. Looking at the circuit board I can't see any attempt to isolate one side from the other.
EDIT: Not looking to start any problems, I am just pointing out that there are a whole array of problems associated with inverters in vehicles, a lot of it is out of your control and rarely talked about.
Rob
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DaveInDenver
Middle Income Semi-Redneck
I'd expect a feedback path for the PWM, to compensate from under voltage condition on the input (i.e. low battery voltage). The only place I'd expect DC to be exposed to something dangerous would be at the boost converter. You have to make LV DC into LV AC to send to a transformer someplace. How the controller or topology for the inverter is built could mean a path to the 12VDC for AC and potentially HV AC. That depends on how cheap or well made the inverter is. There are ways to make less than perfect AC power that don't require as many expensive components.
But I would not have ever expected a path through the chassis (i.e. ground lug, not neutral) to DC, or AC for that matter, on a portable type inverter. Certainly not on a small one like what we'd use in a car. The ground lug in the case of an non-bonded neutral should just be an extension of the enclosure, which certainly could be expected to be bolted to the car body. Pure sine inverters could be one type that have bonded neutrals because they could legitimately be expected to be used for back-up power, for example.
For it to be dangerous the case would have to have a fault condition short that should otherwise have opened the GFI. Certainly anything with a bonded neutral will have GFI outlets. If the outlets are not GFI, then having a bonded neutral could be a bad deal if the distribution isn't safe, which is why I would not have expected a portable inverter to have them. Odd. I would expect anything under about 5kW or so to be a fully isolated AC from ground by default.
But I would not have ever expected a path through the chassis (i.e. ground lug, not neutral) to DC, or AC for that matter, on a portable type inverter. Certainly not on a small one like what we'd use in a car. The ground lug in the case of an non-bonded neutral should just be an extension of the enclosure, which certainly could be expected to be bolted to the car body. Pure sine inverters could be one type that have bonded neutrals because they could legitimately be expected to be used for back-up power, for example.
For it to be dangerous the case would have to have a fault condition short that should otherwise have opened the GFI. Certainly anything with a bonded neutral will have GFI outlets. If the outlets are not GFI, then having a bonded neutral could be a bad deal if the distribution isn't safe, which is why I would not have expected a portable inverter to have them. Odd. I would expect anything under about 5kW or so to be a fully isolated AC from ground by default.
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