# 12v grounding



## greif (Jun 26, 2010)

if the 12v system is ground to frame would this posw any problem with high voltage drive system since motor would be mounted to and thus ground too?


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## dtbaker (Jan 5, 2008)

greif said:


> if the 12v system is ground to frame would this posw any problem with high voltage drive system since motor would be mounted to and thus ground too?



your high voltage traction pack voltage should NOT be grounded to vehicle ANYWHERE.


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## frodus (Apr 12, 2008)

Traction motors aren't case grounded. If it is, you've got problems.

To isolate 12V from HV, most people either use an Aux battery or an Isolated DC-DC converter.


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## Rational (Nov 26, 2011)

Industrial electricians use a Megger to check motor insulation integrity and establishing a trend over time will show how quickly the motor is degrading.

Since a vehicle is ungrounded because of the rubber tires, this brings up interesting safety scenarios for grounded or ungrounded [e.g., wearing sneakers] people touching the car, inside it or outside it. House wiring scenarios are easier.


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## dtbaker (Jan 5, 2008)

Rational said:


> Industrial electricians use a Megger to check motor insulation integrity and establishing a trend over time will show how quickly the motor is degrading.
> 
> Since a vehicle is ungrounded because of the rubber tires, this brings up interesting safety scenarios for grounded or ungrounded [e.g., wearing sneakers] people touching the car, inside it or outside it. House wiring scenarios are easier.



WTF Rational?! 

once again you astound me with your ability to drag in unrelated tidbits of wisdom or equations to sound smart. The OP is concerned about HV ground mixing with LV 12v system..... The simple answer is that the HV and LV systems should NEVER mingle and that the HV should NEVER be grounded to the chassis or anything 12v.


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## piotrsko (Dec 9, 2007)

OK as I understand the OP question: 

NORMALLY the HV pack does not share a ground. YES it could become intermingled because of the motor being grounded mechanically to the chassis. I would think there would be other problems showing if this grounding occurred. Perhaps lousy performance and or smoke. As long as the opposite side remains isolated this grounding would not be much of a concern unless the controller is internally grounded. I do believe it would show up if you were using a ground fault breaker on the charger because it would not stay set or some other effect while connected to house voltages.


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## dtbaker (Jan 5, 2008)

piotrsko said:


> YES it could become intermingled because of the motor being grounded mechanically to the chassis.



gosh, I hope not!

....if it did, and HV positive happened to hit chassis, then you'd have *big sparks*.


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## piotrsko (Dec 9, 2007)

well, yeah, but that was exactly my comment to the OP.


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## greif (Jun 26, 2010)

frodus said:


> Traction motors aren't case grounded. If it is, you've got problems.
> 
> To isolate 12V from HV, most people either use an Aux battery or an Isolated DC-DC converter.


thanks
gary


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## Jan (Oct 5, 2009)

Just to make it confusing: I think the word 'grounding' is often incorectly used. Especially with cars. Correct me If I'm wrong. 

There is plus and minus in case of dc, and there is life and neutral in case of ac. Beside that there is grounding with the earth. If any of the first touches ground, you're able with a kind of breaker to cut of power, because there is electricity leaking to ground. There is a difference in current between what's going in and out.

Cars aren't really connected to the ground, because of their insulated tires. And because cars normaly use the chassis for the minus, it is confused with ground. If a cars plus touches chassis you get a short circuit, not ground leakage.

I've read a while ago about a device for electric cars that can detect ground leakage and cut of the high voltage battery pack. It was bloody expensive, something like 2500 euro. And I doubt any DIY car has it.


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## dougingraham (Jul 26, 2011)

greif said:


> if the 12v system is ground to frame would this posw any problem with high voltage drive system since motor would be mounted to and thus ground too?


Under normal conditions there shouldn't be any problem. The motor case is electrically isolated from the high voltage connections to it.

If the motor were to ingest some debris and fail it is possible that the traction pack could become connected to the frame of the car through a short inside the motor. This could be a dangerous situation depending on how everything is wired up. I can see a situation where your battery disconnect isn't unless it breaks both leads.


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## Rational (Nov 26, 2011)

greif said:


> if the 12v system is ground to frame would this posw any problem with high voltage drive system since motor would be mounted to and thus ground too?


Check it. First measure the voltage from frame to earth ground. If it's less than ~0.3 v almost no one would feel a shock, ever, much less that it would be dangerous. If it's higher, post back.

There is vehicle capacitance to ground so if there is AC anywhere in your car there will be displacement current even with perfectly insulating tires. Just for grins, also check for AC voltage. Info either way is worth something and the test is free so it's a win-win.

C. Dalziel did the work on shock but his book is probably out of print by now.

I feel your pain, dt.


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