# electrical isolation?



## PeterH (Mar 20, 2009)

Hi there,

I'm in the process of doing all of the various wiring now that all of the mechanical work is done. Well, it is done till I find that I forgot something...

I am *not an electirical engineer* so perhaps I'm being overly cautious. However, I just have to ask to make sure...

I was looking at the wiring diagram for the J1772 circuit. The diagram indicates that the ground wire from the 240 VAC supply should be attached to the vehicle chasis.

That suggests that the 12 DC system is sharing a ground with the 240 VAC circuit.

This makes me a little uncomfortable. Would this put anything at risk? 

I think everything on the high voltage DC side of things will remain completely isolated from the 12 VDC ciruit and the 240 VAC circuit. 

I am using a shunt in the main pack circuit to monitor current flow. But the power to the instrument that will be connected to the shunt will be isolated from the vehicle 12 supply by a DC to DC isolating power supply.

Another instrument sensor on the high voltage DC circuit usings a hall effect sensor so there isn't any physical connection between that sensor and the high voltage DC. So I don't see any risk with that portion of the system.

One area where I am still a bit concerned about the high voltage DC finding a path to the vehicle ground is the main DC to DC converter I'll be using to keep my 12 accessory battery charged and to run the normal 12 VDC vehicle circuits. I've been told that the input side (connected to the main pack) is isolated from the output (connected to the vehicle ground and the accessory battery positive).

Hope someone can give this a bit of informed thought and let me know if I'm ok, or if I'm about to let out $10,000 worth of smoke. 

Thanks,
Pete


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## TigerNut (Dec 18, 2009)

The 240VAC ground should be the bare conductor or green wire (on the power cord / house wiring side), and its use in the wiring scheme is for safety ground only; it should never carry any current. If there is a current carrying 'neutral' in your 240VAC circuit it would typically be a white wire, while the two 240VAC phases would be red and black. That's Canadian electrical code, anyway...

It's absolutely essential to have your vehicle grounded to the house when you're charging... otherwise, if you have any open circuit, your vehicle could become live with a 120V potential to ground, and you'd get the surprise of your life when you touched any part of it. If you have a block heater in your car, it also connects the power cord ground to the chassis, for the same reason.

Notwithstanding any of that - you have to make sure that none of your HV equipment is tied in any way to the chassis, or you will have major problems when you connect everything and plug in the charging connection.


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## peggus (Feb 18, 2008)

The GND wire has to be connected to the chassis so that the EVSE can detect any ground fault and open up the contactor preventing damage/injury. 

If you don't connect the GND wire the chassis could be live (due to wiring failure) and you won't know until you touch the vehicle and get zapped....


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## dougingraham (Jul 26, 2011)

PeterH said:


> I was looking at the wiring diagram for the J1772 circuit. The diagram indicates that the ground wire from the 240 VAC supply should be attached to the vehicle chasis.
> 
> That suggests that the 12 DC system is sharing a ground with the 240 VAC circuit.
> 
> This makes me a little uncomfortable. Would this put anything at risk?


The reason to connect the J1772 ground to the chassis of the car is so that the GFI (Ground Fault Interrupter) built into the EVSE can detect a problem and cut off the AC. The GFI looks for current flow on the ground wire and if it sees 10 to 20 ma this is a fault condition. If say you poke a finger where you shouldnt it will cut the AC before you can be hurt.

Your DC-DC converter better be isolated or you will probably let the smoke out. And if it isn't isolated you don't want to be using it in your car. If it isn't isolated then the chassis of the car will likely end up at the same potential as the battery pack negative. You don't want this because it makes your traction battery quite a lot more dangerous. 

A voltmeter is your friend. You can verify that your DC-DC is isolated with one. Turn on the DC-DC and measure the voltage between the chassis of the car and the traction battery positive terminal using a scale that is higher than your pack voltage. You might see a voltage briefly but it should fairly quickly drop to nearly zero. Move to the most negative point and you might see negative pack voltage briefly but after a few seconds it should again show close to zero. The drift towards zero might be fast enough your meter never shows it. And this is good. If you have a significant voltage either way you have something that isn't isolated and you will want to figure out what it is and correct it. You will probably want to turn on all your systems that connect to the traction battery like your heater and make sure it doesn't do something odd.

Your charger might or might not be isolated. And it really shouldn't matter if the traction pack is isolated from the chassis of the car. You don't really want to be poking around the battery much even if it isn't being charged at the time. It is dangerous! I consider big batteries more dangerous than the AC coming out of the wall. After all if you short out the AC in the wall there is a circuit breaker that will open. If you drop a tool on your battery pack there is pretty much nothing that is going to stop the tool from being vaporized. Just cover everything so it is touch safe.

Best Wishes!


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## PeterH (Mar 20, 2009)

Thanks everyone for the great advice. I've clearly got a few things to check before any power is applied but at least I am confident about what to do with that ground from the AC source!

Pete


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