Differential CT is used to sense earth leakage current. Out put current of Differiantial CT will be difference of Phase current and Neutral current divided by turns ratio. This current is fed to current to voltage converter. One variable resistor is provided in the fee back path to audjest sensitivity. This signal is full wave rectified and given to relay driving circuit with a small delay.
SCR is used as relay drive for latched operation. Push to on switch is used for resetting the circuit. Test switch is used to check the working of the circuit by creating a artificial leackage current of 20mA. Another good referance design from Fairchild Semiconductor http://www.fairchildsemi.com/ds/RV%2FRV4141A.pdf
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