An impedance bond is a specially designed electrical component used in railway signaling systems, especially in electrified territory where the running rails carry traction return current.
The impedance bond is primarily an inductor, which means it uses a coil of wire to create inductance. This inductance allows the bond to react differently to different frequencies of electrical current.
Impedance bonds are connected across the two running rails at track circuit boundaries, insulated rail joints, or other locations where traction return current must continue to flow while the track circuit must remain separated.
The main job of an impedance bond is to provide a low-resistance path for high-magnitude traction return current while presenting higher impedance to the signaling current used by the track circuit.
In simple terms, the impedance bond works like a selective gate for electrical current. It allows traction current to pass but helps keep signaling current within its own track circuit.
The reason this works is because traction current and signaling current operate differently. The difference in frequency allows the impedance bond to pass one type of current while opposing another.
The construction of an impedance bond usually includes a coil wound around an iron core. This core improves the inductive properties of the bond and helps it perform its electrical function.
Each impedance bond is designed for a specific railway system. Its current carrying capacity, impedance value, and frequency response must match the traction power system and the type of track circuit being used.