The RCB works on the principle of current balance.
    The Incoming conductors (Phase & neutral) are passed through the very
    sensitive torroid transformers from the primary winding and secondary
    electromagnetic relay, which operates the trip mechanism. Normally the
    contracts are held closed by a spring and open when this is released. If
    leakage occurs from either of phases or neutral side of circuit in this
    condition currents are unbalanced by an amount of leakage current and if
    this reached the rated tripping current of RCCB/ELCB then it trips and save
    precious human life.
    Test Button
    Test button is for testing the trip mechanism of
    RCCB. The test should be made periodically to ensure satisfactory working of
    components.