Ground Fault Interrupter
Ground fault interrupters are designed to protect people from electrical shock by interrupting a household circuit when there is a difference between the currents through the hot and neutral wires, shown as red and black in the graphic. Such a difference might indicate an abnormal leakage of current from the hot wire. The hot and neutral wires pass through a sensing coil so that the two currents, traveling in opposite directions at any instant, give a net zero current through the coil. Since a current-carrying wire produces an AC magnetic field, a slight imbalance, even of the order of milliamperes, induces a voltage in the sensing coil. This triggers an electromagnet that trips the circuit breaker, shown as a moving rectangle, in about 1 millisecond. Assuming the appliance is not damaged, the reset button on the electrical outlet restores normal operation. This is activated by pressing the third button |⊲ on the ground fault trigger. (Wait a couple of seconds before you do this.)
Reference: Ground Fault Interrupter.