The style ST & SZ Impedance Bonds are designed to allow return DC propulsion current to flow around insulated rail joints without interfering with the functioning of adjacent track circuits. A tuned circuit can be incorporated to enhance the impedance characteristics of the Bond for AC track circuits that are operated at the resonant frequency of this circuit. A Bond is typically installed on both sides of the insulated joints (see figure 1) and connected together through the units center tap. Signal and propulsion cables are connected to the taps on either side of the Bond.

Signal currents transmitted on the tracks are effectively blocked, rail to rail, by the inductive impedance of the bond coils. Propulsion currents, however, pass through the coils in a manner that offers little impedance to the propulsion current. Signal current passes through the windings in series, producing an unopposed magnetic flux and thus the required impedance.