How are distance relay zones set for transmission line protection?

Distance relays measure impedance to fault and trip if within set reach. Zone 1 is set to 80-90% of line impedance for instantaneous trip (accounts for CT/VT errors and line parameter uncertainty). Zone 2 reaches beyond line end (100-120% of line + 50% of shortest adjacent line) with time delay (15-30 cycles) for backup. Zone 3 provides remote backup with longer delay. Settings account for load encroachment, arc resistance, and infeed effects. Mho and quadrilateral characteristics are common.