# Isolation fault detector



## steelneck (Apr 19, 2013)

Is there some very simple circuit to wire up for isolation fault detection, i mean just as simple as the Lee Hart Batt bridge. I like to have something like that in my car to just light some idiot lamp telling me about the fact.


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## dougingraham (Jul 26, 2011)

This is another one of those things that could easily create the situation it is trying to detect. It is fairly easy to check the isolation with a DVM and a 100k ohm resistor. Place the 100k ohm resistor across the DVM leads. Turn on the car systems so that the controller has closed the contactor and the vehicle would be ready to drive off. Measure the voltage between the chassis ground and the traction battery negative terminal. You might see a quickly decaying voltage which should go to zero volts. Now measure the voltage between the chassis ground and the traction battery positive terminal. Again you might see a quickly decaying voltage. If you don't see zero volts after just a few seconds then you have an isolation fault. The 100k ohm resistor will see nearly 1/4 watt dissipation with a 144 volt traction pack if there is a fault. It will get hot in that case but will remain cold if there is no fault. Don't forget to turn off the car when you are done.

I think hand checking this after the addition or a change of anything that connects to the traction pack is a good idea. An automatic device could be made but a failure of that device could easily cause the situation you are trying to avoid.

Note: Your traction pack will not be isolated during the time you have the DVM + resistor connected between the chassis and the traction pack. The current would be only 1.4 ma max with a 144 volt pack but it is still not isolated during that interval. So don't let anyone touch the chassis of the car and the traction pack while you are making this test.


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## Nabla_Operator (Aug 5, 2011)

a few ideas were mentioned here and here.


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## Semper Vivus (Apr 13, 2011)

Hello,


Nabla_Operator said:


> a few ideas were mentioned here and here.


Similar concepts to the second one are also called the "passive" method sometimes. A passive isolation detection will not detect faults behind an AC converter and will not work in case of an symmetrical isolation fault (i.e. short circuit from the pack center to the chassis).

The method mentioned by Doug may work in some DIY EVs. At least in production EVs there is a predefined high-impedance path between HV+ to chassis and HV- to chassis (and also an predefined capacitive path). So you may meassure some voltage without having an isolation fault by definition...depends on the HV concept of the EV.

Kind regards
Tom


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## steelneck (Apr 19, 2013)

Yes, i know this is a problem, to detect a fault without causing just the the thing we want to avoid. Maybe there is no simple solution to it. I could wire up a test button to do exactly the test that Doug proposes, just push a button and it gives a green or red light. Maybe take it a bit further, put a micro switch to the hood that does the test if i open it and if there is an isolation fault the car honks the horn, or something a bit more discrete than that, then i know i have to be more careful than usual about where i put my fingers. Hmm.. wonder how that could be wired up..?


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## dougingraham (Jul 26, 2011)

If I were doing a motor controller it would be a natural place to do the test. When the controller is waking up it could do this test and generate an error if it detect a fault. That way you would get a fault indication at least once per drive. I don't like this idea much from a liability standpoint because during the power up you are causing a low current fault when testing. The other device that has connections to all the stuff would be the DC-DC converter. But those don't need to have any real smarts so the motor controller would be a better place to put it.


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## dougingraham (Jul 26, 2011)

Semper Vivus said:


> The method mentioned by Doug may work in some DIY EVs. At least in production EVs there is a predefined high-impedance path between HV+ to chassis and HV- to chassis (and also an predefined capacitive path). So you may meassure some voltage without having an isolation fault by definition...depends on the HV concept of the EV.


Tom, can you elaborate on the predefined high-impedance path in production EV's. I have not see anything written about this and doing so on purpose seems counter productive unless the sole purpose would be to discover a fault if one occured.

The concept of an isolation fault detector would be a good thing to put in a BMS. Coming up with a safe way to do it is something to consider.


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## hbthink (Dec 21, 2010)

These are required in commercial high powered 10kW and up fast chargers (ie CHademo, SAE) for safety. I know some DIY high power systems don't include them. I would say use at your own risk as a system that makes high voltage to the vehicle is inherently dangerous and must ensure that the human operator does not become an unknown leakage path!!!!

Commercial boards from Bender exist for $$ or if you look in google patents you'll get a good idea about what commercial folks use. Most circuits are quite simple

Steve


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## Nabla_Operator (Aug 5, 2011)

dougingraham said:


> ... The concept of an isolation fault detector would be a good thing to put in a BMS...


Instead of building my own safety device, I've ordered the Elithion BMS with the HVFE feature (High Voltage Front End), which does the Isolation Resistance Detection. That feels much better.

---


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## Semper Vivus (Apr 13, 2011)

dougingraham said:


> Tom, can you elaborate on the predefined high-impedance path in production EV's. I have not see anything written about this and doing so on purpose seems counter productive unless the sole purpose would be to discover a fault if one occured.
> 
> The concept of an isolation fault detector would be a good thing to put in a BMS. Coming up with a safe way to do it is something to consider.


The capacitive path is caused by the so called "Y-capacitors", that are used for EMC. The resistive path may result from the inverter circuit (gate drive unit etc., I'm no expert in that). Or it may result from an isolation detection as you mentioned. But having i.e. 1 MOhm from high voltage to chassis is not an "isolation fault" by definition. According to some european regulations the minimum isolation resistance shall be higher than 500 - 1000 Ohm / V.

Regards
Tom


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## steelneck (Apr 19, 2013)

Solved it, at least in principle, but i need a little help.

I have this, at least 10 year old and trusty test probe with two LEDs, it works from 4 to 400 volts AC or DC. If i had a couple of these i could use the parts out them to build a simple push-button ground fault test. If a LED lights i have a problem severe enough to require investigation, if it just blinks a second or so the fault is capacitive and so small that the LED dischage it. 










I have not seen a test probe like that for sale for some years. So, how could the circuit in it look like? (i do not want to break apart the one i have)

As i wrote, it works on both AC (both LEDs light) and DC (plus or minus LED lights) and it can measure anything between 4 to 400 volt.


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## steelneck (Apr 19, 2013)

Let me rephrase my question from my last post, and bump this thread at the same time.

How can a basic circuit look like that limit current to a reasonably constant 20mA if the voltage differs from, say 10V up to 300V?

If i knew this i could build an isolation fault tester in the car that i would be happy with.


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## Tesseract (Sep 27, 2008)

steelneck said:


> ...
> How can a basic circuit look like that limit current to a reasonably constant 20mA if the voltage differs from, say 10V up to 300V?..


The short answer is with a "two-terminal" constant current sink/source, which can be made with a bipolar transistor, an LED/diode/zener and a resistor.


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## steelneck (Apr 19, 2013)

Tesseract said:


> The short answer is with a "two-terminal" constant current sink/source, which can be made with a bipolar transistor, an LED/diode/zener and a resistor.


Sorry, i do not have enough knowledge to get anything out of that. Googled for hours without any real success to know what to buy and how to wire it.

LEDs are so common today i think it ought to exist some two pin component to just wire in series with a led to drive it with a wide range of voltage.


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## Tesseract (Sep 27, 2008)

steelneck said:


> Sorry, i do not have enough knowledge to get anything out of that. Googled for hours without any real success to know what to buy and how to wire it.


There are literally dozens of ways to make a two-terminal current source, but the circuit illustrated in Figure 4 of the Wikipedia entry is what I was specifically thinking of, mainly because you can get small bipolar transistors (in, e.g., the TO-92 package) rated for 300V or more. In fact, an excellent choice chosen at random from the Mouser online catalog would be the KSP44TF which is rated for 500V, has a DC current gain of around 50, minimum, and which costs a mere $0.50US. Referring back to Fig. 4 in the Wikipedia article, R1 and DZ1 [sic - should be ZD1] apply a constant voltage to the base of Q1 which will result in a constant voltage at its emitter (minus a diode drop), and since the emitter feeds a fixed resistance, R2, the current flowing into the collector of Q1 will be constant (assuming the voltage exceeds that of DZ1). The indicating LED, then, would be inserted in series with Q1's collector (cathode band towards Q1). 

The current gain of Q1 is what makes this circuit practical, because it lets you deliver a reasonable amount of current to the indicator LED (2mA is enough for a "superbright" red LED) but limit the current through the zener so that R1 does not have to be ridiculously large. For example, with a 5.1V zener for DZ1 and a 100k/2W resistor for R1 you can deliver a constant 2mA to the indicator LED from ~9V all the way up to 400V and stay well within the power rating of R1 and the voltage rating (Vceo) of Q1.

The Wikipedia entry shows a much simpler circuit using a JFET with a single resistor from gate to source (which is explained in more detail on this page) which would appear to be even better, but as is usually the case in electronics, the devil is in the details, and the devil here is that few JFETs are rated for more than 60V or so. In fact, Mouser does not have any in stock rated for more than 40V.


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## steelneck (Apr 19, 2013)

Tesseract said:


> There are literally dozens of ways to make a two-terminal current source, but the circuit illustrated in Figure 4 of the Wikipedia entry is what I was specifically thinking of, mainly because you can get small bipolar transistors (in, e.g., the TO-92 package) rated for 300V or more. In fact, an excellent choice chosen at random from the Mouser online catalog would be the KSP44TF which is rated for 500V, has a DC current gain of around 50, minimum, and which costs a mere $0.50US. Referring back to Fig. 4 in the Wikipedia article, R1 and DZ1 [sic - should be ZD1] apply a constant voltage to the base of Q1 which will result in a constant voltage at its emitter (minus a diode drop), and since the emitter feeds a fixed resistance, R2, the current flowing into the collector of Q1 will be constant (assuming the voltage exceeds that of DZ1). The indicating LED, then, would be inserted in series with Q1's collector (cathode band towards Q1).
> 
> The current gain of Q1 is what makes this circuit practical, because it lets you deliver a reasonable amount of current to the indicator LED (2mA is enough for a "superbright" red LED) but limit the current through the zener so that R1 does not have to be ridiculously large. For example, with a 5.1V zener for DZ1 and a 100k/2W resistor for R1 you can deliver a constant 2mA to the indicator LED from ~9V all the way up to 400V and stay well within the power rating of R1 and the voltage rating (Vceo) of Q1.


Thank you very much, that was very helpful!


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## crackerjackz (Jun 26, 2009)

just to inform others .... the hpevs systems know have a isolation fault detection ... it gives code 76 as the alarm .... car still drives everything works ... except you are stuck with a code 76 alarm on the display glass ..... I know because I caused the fault by wrongly reading the Chinese instructions on my dc dc converter  lol its a new code error. I had to call hpevs directly to get told what it meant because its not yet in there manuals  ...


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