Testing impedance bonds is important because they affect both traction return current and railroad signal operation. A failed or poorly connected impedance bond can cause traction power problems, track circuit problems, or unsafe signal conditions.
One common test is a DC resistance measurement. This test verifies that the impedance bond provides a low-resistance path for traction return current through the rails.
Low resistance is important because traction return current can be high. If the resistance is too high, the bond or rail connection may heat up, create voltage drop, or fail to carry current properly.
A low-resistance ohmmeter is commonly used to check the resistance of the bond and its rail connections. Loose, dirty, corroded, or damaged connections can cause poor readings.
Inductance may also be measured to verify that the impedance bond has the proper electrical characteristics. An LCR meter or frequency-capable impedance tester may be used for this type of test.
The bond must have the correct impedance at the frequencies used by the railroad system. It should allow traction return current to pass while helping block or limit the signaling current used by the track circuit.
Insulation resistance testing may also be required. This helps verify that there are no unwanted leakage paths, shorts, or insulation failures between windings, terminals, frame, or ground.
Visual inspection is just as important as electrical testing. The technician should look for corrosion, broken cables, loose hardware, burned terminals, overheating, water damage, damaged insulation, or signs that the bond has been physically struck.
Proper documentation should be completed after testing. Test results help compare present readings to previous readings and identify problems before they become service-affecting failures.