Relays are electromechanical device, which permit a small amount of current to control a much larger amount of current. Relays generally contain a coil and set of contacts. Current flowing through the coil of the relay creates a magnetic field, which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw switches. Spectrum applications of relays include electric vehicles and rapid transit, undersea cable branching systems, scientific & test equipment, silicon wafer processing equipment, oil well down hole applications, battery backup and UPS systems. High voltage relays transfer power in high voltage, in medical equipment such as defibrillators, in aerospace applications, like high frequency antenna couplers and radar and in commercial applications like electric vehicles and rapid transit. Electromechanical relays have applications in automated test equipment (ATE). Reed relays and solid-state relays have applications in matrix and multiplexes modules. Time delay relays and solid-state timers include input controls and displays. Output specifications include the switch type, number of contacts, contact configuration, and pole and throw specifications. Protective relays come in a variety of types. Distance relays are very popular devices for transmission line protection.