# chasis ground



## Ziggythewiz (May 16, 2010)

Correct. High voltage cables should be separate from the 12v system, with no common ground. 

It's fine to use the existing wiring with the chassis as ground for 12v, as is done on most cars on the road. If the existing cabling needed replacing, however, I would run wiring pairs to eliminate the chassis ground; I've created more sparking by accidentally shorting the 12V system than anything I've done in my HV system.

I'm no automotive expert, but IMO the chassis ground is a safety hazard invented to save $20/car.


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## strantor (Sep 4, 2011)

Ziggythewiz said:


> I'm no automotive expert, but IMO the chassis ground is a safety hazard invented to save $20/car.


I've been of the same mind also. I have always though it would be so much easier to troubleshoot automotive electrical systems if not for the chasis ground.


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## Sunking (Aug 10, 2009)

You do not bond or reference the high voltage to the chassis for a few good reasons.

1. Safety as mentioned.
2. Unplanned discharge path and drain on the batteries. Battery terminal post get dirty and contaminated which can conduct current via the frame causing the batteries to discharge. 
3. Likely to cause severe RFI and electrical noise.

Heck not even golf carts bond the 36 and 48 volt batteries for the above reasons. Technically there is no reason to even bond the 12 volt systems, you just need to arrange for over current protection by using Over Current Protection Devices on both polarities.


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## mizlplix (May 1, 2011)

The first vehicle I noticed using a non chassis ground was a Honda 350 scrambler motorcycle. They used a wire for everything. 

Of coarse after I rebuilt the bike, I grounded the wiring and battery to the frame. I afterwards learned why they used a non frame ground ....to cut down on bearing electrolysis . A novel idea at that time.


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## nikita (Aug 24, 2013)

*Chasis ground*

low voltage system should be grounded to chasis....is there any specific region in the chasis and technique to ground low voltage system


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## nikita (Aug 24, 2013)

*galvanic separation*

how can we galvanically separate LV and HV system?


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## dougingraham (Jul 26, 2011)

*Re: galvanic separation*



nikita said:


> how can we galvanically separate LV and HV system?


Don't tie them together? Seriously, the good components are isolated if they touch both the original 12 volt and traction pack systems. They use transformers which are magnetic or optical isolation techniques.


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## dragonsgate (May 19, 2012)

One of my concerns has been getting continuity reading from motor housing to the frame. Has any one else noticed this? Also the DC/DC converters are grounded. Does this cause any problems?


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## dougingraham (Jul 26, 2011)

dragonsgate said:


> One of my concerns has been getting continuity reading from motor housing to the frame. Has any one else noticed this? Also the DC/DC converters are grounded. Does this cause any problems?


The motor housing should be isolated from the windings. It is possible and not unheard of for an accumulation of carbon dust in a brushed motor to bridge to the motor housing. The frame of the car would be very difficult to isolate from the motor housing.

DC/DC converters input and output stage should be isolated. Because they are isolated you can tie the negative output to the chassis of the car without issue. This isolation is done internally with a transformer. A lot of the devices used as DC-DC converters are AC input regulated power supplies often with a PFC front end. These devices will be isolated or you end up with the AC input riding on the output DC.

You can test for isolation by measuring the voltage between the chassis of the car and the battery terminals. If you see much voltage between the chassis and the terminals of the battery you should probably look into it further. There are capacitance effects in some devices that can give you a false reading. If you place a small load across the DVM terminals you will see the voltage bleed off. Something like a 10k ohm resistor should be enough. With a 300 volt traction battery the most current that would flow would be 30ma (9 watts). A small resistor will get hot if the chassis is not isolated. The voltage across the resistor will drop if it is just capacitive coupling.


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## dragonsgate (May 19, 2012)

I figured the DC/DC had a way to isolate but there is still a potential for grounding. As for the motor housing, trace the housing through the transmission to the drive line through axels and suspension. Usually shock absorbers have rubber bushing but there can still be metal to metal. The most likely route is the emergency brake cables hooked to the brakes and to the lever that is attached to the frame. Shifting linkage is another ground from trans to body.


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## nikita (Aug 24, 2013)

*Re: galvanic separation*

thankyou for your reply. but..can u please tell me how transformer can be used to galvanic isolate the LV and HV system. as transformer operates on AC current while our car has DC current only.


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## dougingraham (Jul 26, 2011)

*Re: galvanic separation*



nikita said:


> thankyou for your reply. but..can u please tell me how transformer can be used to galvanic isolate the LV and HV system. as transformer operates on AC current while our car has DC current only.


The input half of the switching supply rectifies the input AC and if power factor corrected has around a 350 volt DC buss. It then generates high frequency AC which feeds an isolation transformer. At high frequencies the transformer can be quite a lot smaller and lighter than one that operates at 50/60 hz. I have seen modern switching architectures that operate at megahertz frequencies.


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