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Last Post 09/04/2009 9:07 AM by  Tony Hodges
CR8401-1000-N
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Tony Hodges
CRM Staff
Basic Member
Basic Member
Posts:154


--
09/04/2009 9:05 AM
    I am interesting in purchasing one of these ground fault current
    transformers:

    CR8401-1000-N
    CR8410-1000-N
    CR8420-1000-N

    I have a couple questions though.

    1. We want to measure the current imbalance of 60Hz line pair. So I
    think if we used one of your transformers we would simply put the wire
    pair through the core and then what we measure out would be proportional
    to the current difference. I am not sure if the transformers above are
    all that you have or if they are the best solution for what we need, can
    you recommend or suggest which one to use please? Here are
    specifications:

    1.a. Sensitivity: imbalance of 100uA with output voltage from the
    transformer in the 0.1 to 4 millivolt range. The greatest sensitivity
    the better. Maximum imbalance we would measure would be less than one
    Amp. In our case if our output railed at 0.1 amp, that would be OK.
    The output voltage will be going to an analog-to-digital converter for
    further processing.

    1.b. maximum differential currents that the 60Hz line pair would be
    delivering could be upwards of 100Amps. Not sure if there are any
    limitations other than inner diameter of the core? It would be great to
    be able to use the same coil to measure either ground-fault currents or
    the current down one side of the line. For both kinds of measurements
    we would be changing the connections to our system in order to use the
    correct resistor values.


    R = Vs * Te / Ip

    Ip = 100uA
    Te = 1005
    Rmax = 0.1mV * 1005 / 100uA = 1K.
    Rmin = 4 mV * 1005 / 100uA = 40K

    Not sure if this will work, not sure what the maximum current
    measurement would be.

    2. Can you explain the "Ir" shown in the specification? I am assuming
    that it is the current net current in the primary side (in our case it
    would be the imbalance current between the 60Hz line pair)?

    3. Could I use a CR8449-2000 (or similar) for both line imbalance
    measurement as well as current through one side of the line?

    4. What is the difference between the ground fault current transformers
    as compared to the general purpose transformers with high turns ratio?
    Tony Hodges
    CRM Staff
    Basic Member
    Basic Member
    Posts:154


    --
    09/04/2009 9:07 AM
    Dear Sir,

    Thank you for your interest. The designs you want are the -G units for
    ground fault.

    However, the design for 100 uA of current sensing would probably not be
    something that can work for these products. 100 uA is extremely small and
    generates very small magnetic field. Typically you can expect the ground
    fault CTs to measure 3-4 mA of imbalance. The reason for this is every CT
    with a core must have a certain amount of energy to excite the core. The
    design of the ground fault CTs uses 80% nickel, 20% iron type cores. This
    type of core has relatively a small amount of iron that is very easy to
    excite, and therefore a 3-4 mA signal will be able to produce a measurable
    output. But 100 uA of current would probably produce little if any of
    output signal as the majority of the energy would go to exciting the core.

    Another issue is at even 3-4 mA, the output at best will be in the 50 mV or
    less range, irregardless of burden resistor. Again the reason for this is
    that the core at these levels is only partially excited, and the percent of
    signal to excitation is very low. As the signal gets larger, the core
    becomes fully excited, and the output capability increases dramatically.
    The result is that the transfer function is very non-linear until the core
    becomes fully saturated. Typically the ground fault CTs are applied such
    that a single point of sensing is desired. For human protection, that is
    3-4 mA.

    This leads directly to your question on using the same CT for ground fault
    levels and 100 Amp sensing. You will probably need 2 different CTs for the
    two different applications. I have attached an updated spec sheet that
    fully explains this. CTs can sense currents up to the point of saturation.
    A CT secondary can be shorted, and the current flowing through the secondary will be the primary divided by the turns ratio. This is the most accurate point of a CT as losses are minimal. A resistance is then added to the secondary. As this resistance is increased, the voltage that the CT must
    develop to keep the current flowing according to the turns ratio also increases. This results in higher magnetic field densities in the core,
    which also results in more losses. You can keep increasing the resistance
    until you reach saturation, at which the magnetic field strength in the core
    can no longer be increased. This saturation point is a function of core
    size and the number of turns on the secondary.

    A ground fault CT has minimal iron in it so it excites fairly easily.
    However this also has the effect of decreasing dramatically the saturation
    point. A general purpose CT uses silicon steel core and is 90% iron. Its
    excitation is significantly higher, but its magnetic saturation is much
    higher as well. These are typically used to measure above 500 mA.

    This is the main difference between ground fault and general purpose CTs.

    Irregardless of CT chosen, the smaller the output voltage required, the more
    accurate the part.

    You can see on the spec sheet that the output saturation level is very much
    lower on the ground faults than on the general purpose.

    My recommendation is to use the CR8420-1000-G for your ground fault, and if possible, wrap the wire pair through the hole more than once. This will
    give you the best chance at sensing below 3 mA. Please note again that you
    will need to empirically determine the burden resistor on this application
    as the transfer function of a CT at this low is non-linear. You may need an
    amplifier to get it to a level that can be read by your A to D.

    I would use the CR8449-2000 for the 100 amp sensing. If you keep the
    voltage output low enough, it will provide a linear response at 100 amps
    input. (You should be ok to run it at 100 Amps, because all the CTS are
    designed to handle 4 X Ir continuously.)

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