1.  Magnetic Metals offers wound GFCI differential transformers to those customers who do not want to wind and assemble their own components. The cores used in differential transformers reliably measure ground faults of 5mA, l0mA, 30mA or more over the temperature range from -35°C to +80°C as required by UL Standard 943. This is accomplished through the use of a high permeability core material in ring form. The high permeability is maintained after low resistance faults which could drive the core into saturation and leave it in the remanent state.

The GFCI differential transformers are normally wound with 1000 turns of 38 AWG to 40 AWG magnet wire and are placed in pinned core holders for easy circuit board mounting. The pins can be bent for horizontal or vertical mounting depending on the construction of the GFCI device.

The table below is a list of our standard GFCI differential transformers.

Select a part number above for a detailed drawing.

2.  ALCI sensor transformers with a temperature stability of 0°C to 80°C are available in all of the above parts.  Please indicate this requirement upon ordering.
 
3. A Ground to Neutral Short Detection Coil is required by UL Standard 943 to meet the tripping of the GFCI device when the resistance between the ground and neutral wire drops below a minimum value. The generally accepted method to accomplish this is to induce an isolated voltage in the neutral wire, which when shorted to the ground will produce a signal in the sensor core and trip the device. Either 60 Hz inductors or the dormant oscillator approach as described in the IC manufacturer’s circuit information are used. For the dormant oscillator approach we recommend any of the above part numbers utilizing the ferrite core series F3XXX.

4. Some design hints:

As can be seen from the design curves for the 5029 core below, it is important to lay out the fault detection circuit so that the differential transformer works as a true current transformer. The error of a current transformer is minimized when the burden resistor is made small and the number of turns and the inductance of the core are made high. We recommend using, for instance, the 5029 core with 1000 turns and a burden resistor of 500 Ohms or less at 5mA and l0mA and a burden resistor of 250 to 350 Ohm for 30mA fault currents. For 100 to 300 mA devices we recommend a separate winding with a low burden resistor in addition to the 1000 turns sensing coil.
Additional information and computer generated design curves for specific applications are available upon request.

For further assistance please contact your local sales representative.