Leakage of stray currents into the ballast bed and earth can be a significant problem if the cables running from the rails are electrically connected to the impedance bond housing case and the case is in contact with the earth. This can occur if the cases are mounted on reinforced concrete where the mounting bolts contact the re-bar, if the bottom of the case is resting on concrete, or if dirt and debris accumulate between the bottom of the case or signal equipment and the concrete.
An accumulation of ballast, dirt, or other debris around the locations where the cases or signal equipment are installed along the right-of-way can also provide a path for current leakage. This type of installation can result in a continuous maintenance problem if an effectively high rail-toearth resistance cannot be achieved.
Some impedance bonds are located outside the tracks on timber ties to eliminate points of possible contact with earth. The center taps of the impedance bonds should be insulated from the mounting case. Switch machines also must be electrically isolated from the running rails.
Any embedded switch machines and junction boxes need to be insulated from the rail and insulated from concrete or other material.
Dissimilar metals shall be isolated or separated for corrosion control. Materials selection needs to consider the environmental conditions where the items are being installed.
Yard tracks should be isolated from the main line tracks to reduce corrosion.