To open the contacts, the relay coil is energized by applying the nominal voltage to the coil. The nominal voltage is usually 5, 12 or 24 volts. This is usually selected by the customer depending upon their application’s DC power supply. When the coil of the relay is energized, it produces a magnetic field that is at least equal and opposite to that of the permanent magnet. This electro-magnetic field produced by the coil effectively nulls the magnetic field present from the permanent magnet. This in turn opens the reed switch.