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  1. #21
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    Re: Bias current sensor for floating earth

    Please find attached again the LTspice file. I hope now is ok.
    Don't think so. U1 LED is driven with wrong polarity and has negative transistor supply.

    Click image for larger version. 

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    Many previously mentioned problems are still present, ignoring balanced ground fault, huge circuit power dissipation, about kV voltage drop at current limiting transistors.

    I have to be in compliance with the IEC 61557-8. But I will ask to the standards department to be sure.
    Please check particularly sub clauses 4.1 General requirements and 4.2 Types of IMDs.



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  2. #22
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    Re: Bias current sensor for floating earth

    Quote Originally Posted by FvM View Post
    Don't think so. U1 LED is driven with wrong polarity and has negative transistor supply.

    Click image for larger version. 

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    Many previously mentioned problems are still present, ignoring balanced ground fault, huge circuit power dissipation, about kV voltage drop at current limiting transistors.


    Please check particularly sub clauses 4.1 General requirements and 4.2 Types of IMDs.
    According to standars department, it is NOT necessary to be in compliance with the IEC 61557-8.

    On the other hand, the last design is not working properly, So I will say that the best idea I have right now in order to solve the earth fault issue is the first one. I mean the next attached simulation schematic.

    ISS.txt

    Of course it has very high current leakages y and other issues. But maybe someone in the forum can help me to improve it. I am open to all the comments.

    Thanks in advance.



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  3. #23
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    Re: Bias current sensor for floating earth

    U2 LED reversed, but I see what you mean.

    it is NOT necessary to be in compliance with the IEC 61557-8
    You should know. If circuit isolation is a safety feature, your present design may also conflict with essential standards, e.g. IEC 61010 or other other applicable safety regulations.

    But apart from this point, I still believe that the properties of true insulation monitoring circuit are superior to your design, no matter if it's required by safety rules or not. I must confess however that I'm biased by applications that use full featured IMDs.

    For a technical assessment of your design, you should fix some specifications:
    - acceptable circuit current consumption respectively power dissipation
    - detectable insulation resistance range and accuracy
    - minimal leakage resistance generated by the measurement circuit
    - necessity to detect all kinds of leakage (symmetrical and asymmetrical)
    - testability without creating an actual ground fault



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  4. #24
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    Re: Bias current sensor for floating earth

    Hi!

    Thanks again for the replies.

    I have to be in compliance with the next standards:

    - IEC 62109-1
    - IEC 62109-2
    - UL1741


    Furthermore, regarding to the technical assesment this are the specs:

    - acceptable circuit current consumption respectively power dissipation
    Re: As low as possible but it has not critical to have high power consumption. the only thing to take into account is to choose the right components to withstand such a high voltages and currents.
    - detectable insulation resistance range and accuracy
    Re: It should be in the range from 10K to 80K with steps of 10K. For example if the threshold is configure at 20K the output should be enabled when the impedance of the short circuit is <= 20K
    - minimal leakage resistance generated by the measurement circuit
    Re: This is also not important for the current design. But I would say that the Rce_ON of the transistors is ok.
    - necessity to detect all kinds of leakage (symmetrical and asymmetrical)
    Re: It is not neccesary to detect symmetrical leakages, Just only asymetrical.

    Greetings



  5. #25
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    Re: Bias current sensor for floating earth

    Hi people!

    I thought a new design and I think this time is going to work fine...

    Click image for larger version. 

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    You can also find the LTSPIce simulation here:

    SCH5.txt

    Two comparators are going to be able to detect a short circuit with earth. With R1 and R4 you can adjust the Threshold(RT) of the R_Leakage. For example:

    if RT = 740 Ohm => RL+ or RL- <=25KOhm => Comparator output is high. Otherwise comparator output is low.

    you can adjust RT at maximum of 80K.

    What do you think? Is it a good solution?

    Greetings

    Note: SCH4.txt is wrong. Please check the previous SCH5.txt
    Last edited by flote21; 20th February 2020 at 21:03.



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