# Understanding the IOTA/Power Max DC/DC grounding Lug



## azdeltawye (Dec 30, 2008)

Yukon_Shane said:


> Can any of the electrical engineers on the forum help me understand the purpose of the grounding/bonding lug that's on the IOTA and Power Max DC/DC converter?...


The UL listing for the device requires that the converter chassis be electrically bonded to the vehicle chassis.

Not connecting this lug to the vehicle chassis exposes the user to risk of shock. 

There shouldn't be any issues if the traction battery pack is properly isolated from the vehicle chassis.


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## Yukon_Shane (Jul 15, 2010)

azdeltawye said:


> The UL listing for the device requires that the converter chassis be electrically bonded to the vehicle chassis.
> 
> Not connecting this lug to the vehicle chassis exposes the user to risk of shock.
> 
> There shouldn't be any issues if the traction battery pack is properly isolated from the vehicle chassis.


Thanks Darren. That's what I would understand the purpose of this lug to be so I appreciate you confirming my understanding.

With that in mind do you have any thoughts on why our friend Pete is seeing a current flow through that lug when connected to the vehicle chassis? Is this a sign of an internal short or leakage from the main battery pack?

thanks again,

Shane


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## Tesseract (Sep 27, 2008)

Yukon_Shane said:


> Can any of the electrical engineers on the forum help me understand the purpose of the grounding/bonding lug that's on the IOTA and Power Max DC/DC converter?


It's purpose is to bond "chassis ground" to *earth ground* when it is powered from the AC mains. This is required by all modern electrical safety codes, btw. Obviously, there can't be an "earth ground" in an EV, and neither are there AC mains, so this presents an odd problem because the EMI filter that is guaranteed to be inside the PSU expects to be able to shunt away any common mode noise into earth ground.

More specifically, it is the "Y capacitors" which perform this function, by providing a pathway for AC current to flow from each side of the AC line (ie - the traction pack in an EV) to chassis ground (ie - the lug).

Safety regulations limit the maximum amount of reactive current that can flow through these capacitors, but this current is based on the highest AC mains frequency in the world, or *60Hz*. Modern PWM motor controllers operate at 8kHz to 16kHz and the ripple which is a byproduct of their operation can cause significant current to flow through the Y caps (and the X caps, which bridge the "AC line", converting any differential mode noise to common mode). This current can cause them to overheat and fail, which can eventually lead to Bad Stuff Happening. Y-caps are supposed to be safety agency approved to NEVER fail short, but that is, once again, assuming they are being used on the AC mains which can deliver a significant fault-clearing current through the earth ground as a return. 

Yada, yada, yada... So should you connect the lug to vehicle ground? Technically there is no absolute answer, but I'd lean towards *not* connecting it, but it's just a lean towards no, not a declaration. The vehicle chassis is not a good ground by any stretch of the imagination, so it's not possible to dump either "fault current" to it, like a true earth ground, nor noise currents from the Y-caps. There is literally no earth to dump them to.


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## DIYguy (Sep 18, 2008)

Tesseract said:


> It's purpose is to bond "chassis ground" to *earth ground* when it is powered from the AC mains. This is required by all modern electrical safety codes, btw. Obviously, there can't be an "earth ground" in an EV, and neither are there AC mains, so this presents an odd problem because the EMI filter that is guaranteed to be inside the PSU expects to be able to shunt away any common mode noise into earth ground.
> 
> More specifically, it is the "Y capacitors" which perform this function, by providing a pathway for AC current to flow from each side of the AC line (ie - the traction pack in an EV) to chassis ground (ie - the lug).
> 
> ...


This should be a sticky or wikki or something with an i on the end of it.


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## Yukon_Shane (Jul 15, 2010)

Tesseract said:


> It's purpose is to bond "chassis ground" to *earth ground* when it is powered from the AC mains. This is required by all modern electrical safety codes, btw. Obviously, there can't be an "earth ground" in an EV, and neither are there AC mains, so this presents an odd problem because the EMI filter that is guaranteed to be inside the PSU expects to be able to shunt away any common mode noise into earth ground.
> 
> More specifically, it is the "Y capacitors" which perform this function, by providing a pathway for AC current to flow from each side of the AC line (ie - the traction pack in an EV) to chassis ground (ie - the lug).
> 
> ...


 
very valuable information and more evidence to convince me that these AC to DC chargers are not adequately design to operate as EV dc/dc converters.

Thanks Tesseract


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## PeterH (Mar 20, 2009)

Sorry guys..... I wasn't participating in this because I was out installing my new GE Watt Station. At the moment my DC/DC isn't seeing any pack voltage because I haven't installed the batteries yet. I'll be doing that over the weekend.

I agree with Shane in that these re-purposed AC power supplies are really just a hack when used as we are doing in our builds. I am patiently waiting for a evnetics purpose built DC/DC converter. 

We will all know if this 'no ground' approach works or not soon. I hope the be able to apply 45 cells worth of battery power within a week.

Thanks,
Pete


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