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Sine wave sensing isolation in NEC

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    #16
    To the OP: the NEC doesn't really address this directly. The NEC requires that various connected components be 'listed', or 'suitable' or whatever. Your measurement tool is like a toaster; the NEC won't tell you how to make it; instead the NEC will require that it be 'listed' and then you would need to look at the various UL standards or engineering standards to figure out how to build a suitable tool.

    It looks like paulengr pointed you to a possible UL standard for such an instrument...though if you are only building a few of these I'd bet that buying the standard book would be more expensive than simply buying appropriately rated/listed instruments.

    The AMC1311 is an 'isolation amplifier', designed to take a 2V input signal but with several KV of isolation between input and output. This means, for example, that you could measure the millivolt signal across a current shunt resistor where that resistor is 'floating' at 120V to ground. IMHO the biggest problem with something like the AMC1311 is that you need to provide an isolated power supply for the measurement side. If the goal is to measure _current_, then I generally use something like a 'closed loop hall effect current transducer'; something such as https://www.digikey.com/product-deta...1002-ND/409815 The benefit here is that you are measuring the magnetic field developed around an insulated conductor, so you don't have any direct galvanic connection to the circuit being measured. You don't need a floating power supply, and the output is power limited by your instrument power supply. Similar but cheaper (and lower accuracy) systems work with open loop hall effect sensing or magneto-resistive sensing.

    -Jon

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      #17
      Interesting that the OP hasn't come back with more detail.

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        #18
        Originally posted by electroman00 View Post
        [B]Again...Suggest all above to re-read the OP !!
        Originally posted by hbiss
        All I can say is that some engineers don't express themselves in a way that makes you confident that they know what they are talking about.
        No engineer is going to say "I need to sense the sine waves..." So either he's dumbing it down for us or he doesn't know himself.

        And no, I'm not an engineer. Are you??

        -Hal

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          #19
          Originally posted by hbiss View Post



          No engineer is going to say "I need to sense the sine waves..." So either he's dumbing it down for us or he doesn't know himself.

          And no, I'm not an engineer. Are you??

          -Hal
          Yes.

          And if your background is programming you’d go towards the “obvious” solution because whatever you are building is already 95% software. Hardware is “simple”. But such a solution could be doing spectrograms and looking at harmonics too. There are a bunch of problems though. The raw analog interfacing. Buffering data too since the ADC is going to generate a sample at regular intervals but the CPU might be off doing other things. And building it to meet Listing requirements and sending samples to the Listing agency for testing. It is a huge job for what seems very simple to do if you work on the software side not hardware.

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            #20
            There are a few articles that might apply but article 647 is for sensitive electronic equipment derived from a separately derived system. It may not apply IDK.

            647.1 Scope. This article covers the installation and wiring of
            separately derived systems operating at 120 volts line-to-line
            and 60 volts to ground for sensitive electronic equipment.
            They say I shot a man named Gray and took his wife to Italy
            She inherited a million bucks and when she died it came to me
            I can't help it if I'm lucky

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              #21
              (Answer to Winnie's comment)

              Good suggestion.

              But, doesn’t “closed loop hall effect current transducer” require power. . . be it separate or dependent upon the system being monitored?
              We can install a non-invasive monitoring system. This will do away with breaking the line and installing shunts, but OP already indicated that he is looking at Texas Instrument AMC 1311.

              Non-invasive approach works like a clamp-on transducer. No disconnecting of existing wires. Similar to a current transformer. . . works through induction.

              I understand OP’s concern about wading into NEC’s “realm of influence”. If you need to provide power to an instrument (in this case from the MCC) it would certainly involve safety regulation.
              Can be manipulated because it is very low voltage (2 volts) and then amplified to a manageable level.

              Of course you can feed it power from the PLC.

              OP is still conducting feasibility studies. As such, it is in a very early stage of development . . . nothing is existing except the MCC.

              So, it is only fitting for an engineer to do research that may impact the project as he goes forward and ultimately, any factor that will affect the finish product.
              This is exactly what he is asking for...not how to build this thing. People that would go into tangents are really just name-dropping. . . making it look like they know something.

              As the saying goes: Let’s cut to the chase. (as Hollywood directors go)

              As others have NOT been kind enough (not you) I think, OP should not be subjected to unfriendly attitude.

              We all benefit from questions being raised—not just answers and opinions.

              There is no such thing as stupid question. . . only stupid answers.
              Last edited by myspark; 11-04-19, 12:19 PM. Reason: Missing text

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                #22
                Originally posted by myspark View Post

                But, doesn’t “closed loop hall effect current transducer” require power. . . be it separate or dependent upon the system being monitored?
                We can install a non-invasive monitoring system. This will do away with breaking the line and installing shunts, but OP already indicated that he is looking at Texas Instrument AMC 1311.
                Yes, it does. The systems that I usually work with require a +-15V power supply to run the instrument. The output is referenced to the mid point (0) of the 3 terminal power supply. Some such units work with a single 5V power supply, but they all require some power.

                The big benefit over something like the AMC1311 (which does have its place!) is that you don't need a power supply for the 'hot' side of the transducer system. You just need the power supply on the galvanically isolated 'instrument' side.

                -Jon

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                  #23
                  Thank you,

                  Since ISOLATION is the "name of the game" would opto- coupling work in this situation? I've worked on several telemetering instruments that are often super sensitive to transients that affect accuracy. . . not on high energy distribution systems like this.

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                    #24
                    Originally posted by electroman00 View Post

                    NEC says "shall be listed"...



                    What is the requirement??!!??
                    Repectfully, Where does the NEC state this has to be listed?
                    The NEC does not require all products used to be listed, some are only required to be approved. This application may be required to be listed by state or OSHA requirements. This is kindof an NEC application, but keep in mind the NEC is for premise wiring
                    Moderator-Washington State
                    Ancora Imparo

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