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I recently came across a fairly old car inverter. It plugs into a 12v DC lighter socket, and outputs 115 Vac at 60 Hz, and has a normal American outlet on the side of it (with a ground pin, which seems questionable) and is rated for 100W continuous or 150W peak. This tool could be very useful to me, mainly to run a laptop in the car (when someone else is driving, of course). The output is also non-sinusoidal, but, as far as I understand, that does not matter for switch mode power supplies, and all of the things that I want to power in the car run on switch mode power supplies.

More concerningly, however, it says "NEUTRAL FLOATING." My understanding of electrical engineering tells me that this means that the neutral wire isn't attached to anything, except maybe a capacitor. This may still work with lower current loads (perhaps up to 100W based on the labeling, although that seems high) due to the capacitance on the neutral wire, but it would also cause the device that I am using to start floating above 0V on its neutral or ground side, a dangerous situation. It seems, however, that such a dangerous device would not be legal to sell in the United States, where I am pretty sure that this was bought (it is very old, so I don't know exactly).

As such, it seems like I am misunderstanding what "NEUTRAL FLOATING" means. What does it actually mean? Is it safe to use my inverter, or do I need to buy another one?

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  • \$\begingroup\$ Did you take the cover off and see where the neutral line goes? \$\endgroup\$ – Ron Beyer Jan 13 at 16:32
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    \$\begingroup\$ My guess would be that the neutral is not connected to the frame of the vehicle. \$\endgroup\$ – relayman357 Jan 13 at 16:33
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    \$\begingroup\$ Also, as for the people saying "I think" or "My guess," I appreciate that you're trying to help but I can guess too. I am looking for a more definitive answer, since safety is at stake. \$\endgroup\$ – john01dav Jan 13 at 16:36
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    \$\begingroup\$ Since safety is at stake, as you don’t seem to have the original instructions then don’t use it and buy a new one. \$\endgroup\$ – Solar Mike Jan 13 at 16:41
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    \$\begingroup\$ These could be for 2 pronged plugs only and double insulated devices. only. RF X2 Cap to earth ground for noise reduction. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 13 at 17:38
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Many smaller inverters (and my Honda EU2000 generator) have "floating neutrals".

I don't think "floating" is quite the right term here, as the Neutral will alternately be connected to +170 V or Ground. What happens is that the inverter has a single high voltage supply (+170 V or so), and connects the Line to that supply on one half-cycle, and Neutral to that supply on the other half-cycle of the AC waveform.

A load connected between Line and Neutral will see an alternating voltage. As neither Line or Neutral should be exposed to the user, this technique should not present a hazard, and simplifies the inverter as it does not need to generate a negative high voltage supply.

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  • \$\begingroup\$ "As neither Line or Neutral should be exposed to the user, this technique should not present a hazard" Unless whatever you plug in has a metal chassis and a fault occurs. \$\endgroup\$ – Mast Jan 14 at 8:12
  • \$\begingroup\$ @Mast: Then it ought to have a ground pin. Devices without one should be safe even if a fault occurs. \$\endgroup\$ – MSalters Jan 14 at 8:20
  • \$\begingroup\$ @MSalters Yes, those should be double-isolated or equivalent. But since there's no RCD on most inverters, is a ground pin going to be good enough? \$\endgroup\$ – Mast Jan 14 at 8:41
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It means that the output 115VAC is not connected to the input power in any way (not common with the minus, which is connected to the car chassis, nor to the +12V input). Similar to adding an isolation transformer to your mains that comes out of the wall (well, without the pesky design, safety testing and certification ... ).

That's a fairly subtle difference with an inverter or a generator.. it's generally likely safer because getting your body between the "hot" and car chassis ought not to give you a shock (though I would not suggest testing this given the typical slapdash construction of cheap inverters), however it might have some (non-destructive) ill effect on the operation of devices that use capacitive touch sensors for operation such as phones and computers with touch screens. Possibly they have incorporated a capacitor to AC-ground the output, or maybe not. If one is added it should be a Y-rated safety capacitor.

The receptacle ground pin is likely connected to the (-) lead (car chassis). It is not connected to the neutral.

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What it means is that the neutral output of the inverter is not connected to ground. On a car, ground would be the chassis.

The ground pin might be connected to the car chassis, or it might be totally disconnected.

If you are just plugging one appliance in, it isn't necessarily unsafe. If the appliance is double insulated, with a 2-pin plug, it's largely irrelevant anyway. Even if the appliance has a 3-pin plug, then the lack of a connection between the 115V neutral and the vehicle ground doesn't mean that you will be electrocuted if you touch the appliance and the car bodywork at the same time - because there's no connection between them.

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