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In some cases it is common to see a core balance current transformer with four output wires, i.e., the secondary winding and the test winding as shown below. I'm wondering how I test it. What kind of signal should I inject? Should I inject a current signal that it is induced in the actual secondary?

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Source of the picture: T. Novak, L. A. Morley and F. C. Trutt, "Sensitive ground-fault relaying," in IEEE Transactions on Industry Applications, vol. 24, no. 5, pp. 853-861, Sept.-Oct. 1988.

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2 Answers 2

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If you take a look at a typical UL943-compliant GFCI interrupter circuit, based on this Onsemi chip:

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The IC produces a test signal for the UL943 mandatory periodic self-test operation. The test current of 6-8mA is 1/2 of a mains cycle and is below the trip threshold for an actual fault, but high enough that it can be detected so that the circuit does not trip out on test failure.

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  • \$\begingroup\$ Thanks. If I have a microcontroller in my application, can I perform the test using some kind of voltage to current converter then? Or can I just feed the test winding directly from the micro ports? \$\endgroup\$
    – user115094
    Mar 14, 2020 at 14:08
  • \$\begingroup\$ Worst case, a transistor and a resistor or two. Maybe direct from the port, depending on drive capability. \$\endgroup\$ Mar 14, 2020 at 15:14
  • \$\begingroup\$ Could it be a square wave from the micro to a filter to generate like a sine wave? \$\endgroup\$
    – user115094
    Mar 14, 2020 at 15:43
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If all 3-phase lines go through the core the secondary should get nothing because the magnetic fields cancel each other. But if there's one phase OFF or for some other reason the currents are not equal then the secondary outputs something which depend on the current in the remaining phase lines. That can be used to detect non-symmetry which can be a sign of a fault. It can be a ground fault or something else. Neutral can be also in the same bunch, it doesn't harm ground fault detection.

As well if 1-phase line and neutral go through the core an output from the secondary shows there's current leaking to somewhere, maybe to the ground.

Automatic fault current breakers use this principle.

You should input to the test winding mains AC through the carefully calculated current limiter (resistor capacitor or low voltage source) to test the detection and breaking or alarming capability of the final system. It's up to you to calculate the needed test current starting from the detection specs and transformer data.

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