The relay, therefore,somehow, has to compute the impedance as seen from its location and compare it whit set value to take the trip decision. Because of the simple series model of the faulted line, the line impedance is directly proportional to the distance to fault. Hence the name distance reply. Such a reply is called under-impedance reply. In practice, however, the word under is dropped and the reply is simply called impedance relay. It is possible to synthesize several more complicated distance relays. To distinguish this relay from the other distance relays, we will call it simple impedance relay.