3 prong AC for main cabin

Hi there, I just got my 2023 Prime and still in the throws of getting it set up. Fun times! It replaced my 2003 Gen1 Prius, so a huge step up in technology.

The built-in AC adapter is big for me. We go on road trips and my wife is usually using her laptop so we've been using a stand-alone inverter. The weird thing is the AC outlet in the main cabin is only 2-prong. Laptops clearly need 3-prong and running an extension to the trunk doesn't seem great. I have a ground "cheater", but curious what solutions others have used for ground connection. I'm thinking of running a ground wire to one of the bolts holding down the driver or passenger seats. Does anyone have a better idea?

will

 

Tom_06: Ugh, good point on not messing with that bolt. That seemed the easiest access to something grounded. Over the decades all my laptop power cords have been 3-prong, so just assumed they were all that way. Maybe it is because I get "desktop replacement" class laptops with heftier power supplies. If neutral is indeed hooked up to chassis ground, then life is good. I've never had an in-car inverter before, so not sure how it is wired up. I guess I could (VERY CAREFULLY) try to measure resistance between neutral and chassis ground and see if they are connected. I'm pretty sure that is how my stand-alone inverter is wired. I do want it grounded somehow.

will

 

Aforkosh: Thanks. As mentioned to Tom, interesting that I've only ever seen three-prong cords for my laptops. I'll do some poking around to see if neutral is indeed connected to chassis ground. I feared it may be kind of like the "double insulated" setup you see on some equipment that just isolates everything.

 

Sam:

Fair question. As mentioned earlier about some folk getting a tingling feeling on a double-insulated device. Even with a plastic case, there is probably some metal somewhere and certainly metal if you plug something in like USB. I worked for a satellite TV company until retiring and we made the move from 3-prong to 2 prongs for some set-top boxes and without ground having the chassis float to half-way between live and neutral was "just how it works" and you did what you could to make sure it was safe. That just seemed not great to me, so must admit decent grounding is more a preference than a necessity.

The car itself clearly isn't grounded, but better to be at the same potential as the chassis while you are inside it at least!

will

 

Chapman: Yea, I'm probably being a bit overly paranoid!

In other news:
- I measured resistance between chassis ground and live and neutral coming out of the socket in the center console. No measurable connectivity for either. Kinda makes sense, it is whatever the source equivalent of "double insulated" is. I also measured between chassis ground and the USB plug grounds and those are near-zero ohms.
- I measured resistance between the mains ground and ground on the laptop-side plug and get *exactly* 1Mohm. That rings a bell, zero ohms is bad in case there is a failure of some kind with hot, a dead short can make lots of smoke. Again, makes sense after realizing that.
- I measured resistance on my old external 12V-120V inverter and that was zero ohms from the 12V ground to the AC ground plug.

Armed with that, I figured I just needed a "light ground" for the ground pin, so resorting to just an alligator clip that I'll attach to the chair rail when traveling. With that, TADAA! My solution below. The wire is trimmed to be just long enough to as not to snaggable.


will

 

Sam:

It DOES make me feel better! Given a three-prong cord and a two-prong outlet I have to do something and grounding a chassis the designers intended to be grounded seems a good thing even if in the absolute sense the failure conditions where it matters are even more obscure than with real mains. So not COMPLETELY useless, but agree the value is pretty minimal. It is as good a solution as an extension cord to the outlet in the baggage area without needing an extension cord.

But mostly it makes me feel better!

will

 

Gokhan: Alas it isn't that simple. Even Toyota has a three-prong plug in the back! Just two prong in the center console. Ground is there only for when things (and in general multiple things) go wrong. The odds of truly needing it are nearly but not exactly zero so I agree mostly with Sam Spade. To me, if a device was designed to assume it is grounded I want to support it. The ground "cheaters" aren't intended to disconnect ground, they are officially to connect ground to the plate screw they say should be grounded. Yea, not how people use it, but none of us speed either, right? It clearly isn't earth ground, but in a car, chassis ground is your ground reference. The difference as best I can tell is for the built-in inverter, both live and neutral look to be floating, so not presenting the same hazard you'd get with house mains.

To be clear, we're going down rabbit holes here, the practical side of our differences are minor if non-existent. For giggles tomorrow I'm going to confirm the ground pin in the back plug is indeed connected to chassis ground as I suspect. I also have a mains tester that also can test for GFCI. If the car trips on the GFCI test (which I suspect it will), that is even better support the ground pin being superfluous. Since Neutral and Ground are not connected, I think it will get grumpy about that though.

will

 

ChapmanF: Interesting and that seems along the lines of what I would have expected as at least an option, grounding the inverter chassis to the car chassis.

I got two expected and one very unexpected result from the Prius though!
- In the back, the plugs ground pin is indeed hooked up to chassis ground (no surprise)
- With the mains tester I get an "open ground" error indication. This is expected too given that the car is floating neutral relative to chassis ground. Just the center light in the pic below for this test.
- The weird thing was when I hit the GFCI test button. Nothing tripped (a definite bummer), but the Not/Neutral reverse light lit??!!

I need to think about this a bit more. The way I understand the GFCI test button is it makes a relatively high resistance connection from hot to ground. If there is a GFCI breaker, it detects a current mismatch between hot and neutral and trips. It didn't, so wondering if that light is just a natural consequence of the GFCI test failing. By pressing the test button, hot is "slightly" connected to ground. Since both hot and neutral are floating, if hot gets referenced to ground via the test button, then neutral will then look hot relative to ground and that lights the red light??? It certainly does so with enough current to light the neon bulb.

The car not tripping based on the GFCI test is definitely a disappointment. Can anyone else check this just to see if it's maybe a one-off error with my car? That seems odd. How this plays into the original discussion on the importance of ground I'm not sure yet.

Definitely learning new things!

will