In some countries, AC-immune DC track circuits are used on AC electrified lines. This is the predominant method of track circuiting on overhead electrified parts of the UK rail network. One method provides 5 V DC to the rails, one of the rails being the traction return and the other being the signal rail. When a relay is energised and attached to the track, normal voltage is 5 V DC. When there is a break in the circuit and there is no train, the voltage rises to 9 V DC which provides a very good means for fault finding. This system filters out the voltage induced in the rails from the overhead lines. These track circuits are limited in length to about 300m.