Author Topic: car 120v power inverter open ground  (Read 1542 times)

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Offline jsi

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car 120v power inverter open ground
« on: June 10, 2018, 06:26:57 pm »
My truck has a factory installed 120v power inverter that doesn't seem to be working correctly.  I plugged in a simple 3 wire plug tester like this one:

and it showed an open ground. Having done a lot of 120v house wiring I'm familiar with the required safety precautions for high voltage work, so I got my multimeter out and measured the outlet from hot to neutral there's 120 volts which is fine.  BUT, measure from neutral to ground or hot to ground and you get 60 volts which is most definitely not normal.  Google isn't much help on the subject so before I go spending much time troubleshooting I'm wondering if these measurements would be considered "normal" in an automotive application. 
 

Offline Francesco Gozzo

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Re: car 120v power inverter open ground
« Reply #1 on: June 10, 2018, 10:27:16 pm »
Hello, maybe I’m off topic and I know it’s an ac measurement but I watched a video on dc and ground measurement and thought maybe It could help.
https://youtu.be/mJL11UF1arQ


Sent from my iPhone using Tapatalk
 

Online Zero999

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Re: car 120v power inverter open ground
« Reply #2 on: June 10, 2018, 11:15:05 pm »
My truck has a factory installed 120v power inverter that doesn't seem to be working correctly.  I plugged in a simple 3 wire plug tester like this one:

and it showed an open ground. Having done a lot of 120v house wiring I'm familiar with the required safety precautions for high voltage work, so I got my multimeter out and measured the outlet from hot to neutral there's 120 volts which is fine.  BUT, measure from neutral to ground or hot to ground and you get 60 volts which is most definitely not normal.  Google isn't much help on the subject so before I go spending much time troubleshooting I'm wondering if these measurements would be considered "normal" in an automotive application.
That sounds normal. The inverter is not the same as the mains supply.

The test plug assumes the neutral is at the same potential as ground, which is normally the case with the mains, but it doesn't have to be connected like that to be safe. Your inverter most likely has a floating output, which is perfectly safe. Another possibility is it's centre tapped to ground, but unlikely. I suspect it's floating and you measure 60VAC because of capacitive coupling.

Indeed if you measured the 110V supply where I work, you'll find it measures 55V to earth, from either side of the supply. This is because it's derived from a transformer with a 55V-0-55V centre tapped secondary winding. It would be interesting to see what that socket tester would show if I plugged it in, where I work.
 
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Offline Whales

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Re: car 120v power inverter open ground
« Reply #3 on: June 11, 2018, 12:37:08 am »
Indeed that 60V measurement may be a phantom.  Try putting a 100K resistor between Earth and Neutral, then measure this voltage again.  If it drops significantly then it's probably your earth is simply disconnected and you are chasing capacitive phantoms.

Of course you might actually have a severe and dangerous fault instead, so don't make any assumptions until you're sure.  I would not trust an inverter like that until I opened it and saw how it was wired myself.

Offline blueskull

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Re: car 120v power inverter open ground
« Reply #4 on: June 11, 2018, 01:44:21 am »
Car inverters are designed with low cost in mind. They either have no ground, or have ground tied to input negative.
Internally, those inverters are not isolated. Input ground is high voltage DC bus ground, therefore they can save money on digital isolators and low side gate driver level shifters.
 

Online Zero999

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Re: car 120v power inverter open ground
« Reply #5 on: June 11, 2018, 07:53:05 am »
Car inverters are designed with low cost in mind. They either have no ground, or have ground tied to input negative.
Internally, those inverters are not isolated. Input ground is high voltage DC bus ground, therefore they can save money on digital isolators and low side gate driver level shifters.
If there's no DC blocking capacitor, the mains would also be floating at half the DC bus voltage, which doesn't sound good.
 

Offline jsi

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Re: car 120v power inverter open ground
« Reply #6 on: June 12, 2018, 02:45:00 am »
Indeed that 60V measurement may be a phantom.  Try putting a 100K resistor between Earth and Neutral, then measure this voltage again.  If it drops significantly then it's probably your earth is simply disconnected and you are chasing capacitive phantoms.

Of course you might actually have a severe and dangerous fault instead, so don't make any assumptions until you're sure.  I would not trust an inverter like that until I opened it and saw how it was wired myself.
:-+
I did like you suggested and put a 100K resistor between neutral and earth and the measurements were what I was expecting. Zero-ish volts between neutral and earth, 120 volts between hot and neutral, and 120 volts between hot and earth.  House mains wiring seems pretty straight forward so I don't really understand why the truck inverter would be any different.  But I do understand how referencing neutral to earth works as it's essentially what happens in the home breaker box.
 

Offline IanB

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Re: car 120v power inverter open ground
« Reply #7 on: June 12, 2018, 03:13:29 am »
Your truck has an inverter, but it is sitting on rubber tires, and rubber is an insulator. So it would be a miracle if any part of your truck had a low impedance path to the earth beneath your feet.

However, what is the impedance of the path between the ground pin and the vehicle chassis? That might be close to zero ohms. As far as a vehicle is concerned, the frame/chassis is ground.
I'm not an EE--what am I doing here?
 

Offline blueskull

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Re: car 120v power inverter open ground
« Reply #8 on: June 12, 2018, 03:58:50 am »
If there's no DC blocking capacitor, the mains would also be floating at half the DC bus voltage, which doesn't sound good.

So don't touch the output!
 

Online Zero999

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Re: car 120v power inverter open ground
« Reply #9 on: June 12, 2018, 07:44:19 am »
If there's no DC blocking capacitor, the mains would also be floating at half the DC bus voltage, which doesn't sound good.

So don't touch the output!
Well duh.  ;D

The real problem is the additional 85V of voltage stress on the insulation. It should be a problem in most cases, as mains insulation is hugely overrated, but it still isn't right.

Your truck has an inverter, but it is sitting on rubber tires, and rubber is an insulator. So it would be a miracle if any part of your truck had a low impedance path to the earth beneath your feet.

However, what is the impedance of the path between the ground pin and the vehicle chassis? That might be close to zero ohms. As far as a vehicle is concerned, the frame/chassis is ground.
Of course the vehicle chassis is considered to be earth or ground and the same for an aeroplane. If the vehicle is parked and equipment is plugged in to the inverter and used outside, then it's recommended that the vehicle is connected to real earth, ground, via a metal stake.
 


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