Some of the best preventatives that may be provided to guard against false proceed signals due to track relays failing to release with a train in the circuit, are:
- Use as much resistance as practicable between battery and track.
- Use low resistance bond wires, and maintain bonding in good shape.
- Keep ballast well cleared from contact with rails.
- Maintain insulation in insulated track joints in good condition.
Aside from these simple remedies no definite rule can be given to combat foreign current. If it is so troublesome that these methods do not overcome it, the circuit affected must be carefully studied to determine the source of the foreign current and its path to the rails, then special means can usually be provided to overcome it.
Ballast Resistance and Leakage
The importance of ballast resistance has long been recognized, and this always has been considered the great variable, whereas, investigations show that the ballast resistance is at least no more variable than the rail resistance, and that of the two it is more important to reduce the rail resistance to a minimum, and especially to establish it as a constant.
When the ballast leakage problem was first taken up (on the L. & N.), various kinds of ballast were measured in both wet and dry weather, the intention being to determine the lowest possible resistance per 1000 ft. for each kind of ballast. It was proposed in this way to establish a standard minimum resistance per 1000 ft. for each kind of ballast. For instance, if a number of measurements in wet weather showed 8 ohms per 1000 ft. as a minimum for track circuits with crushed rock ballast, it was the intention to adopt 8 ohms as the standard minimum ballast resistance per 1000 ft. for all track circuits where crushed rock ballast was in use. If a number of wet weather measurements showed 4 ohms per 1000 ft. as a minimum for cinder ballast, it was the intention to adopt 4 ohms as the standard minimum ballast resistance per 1000 ft. for all track circuits where cinder ballast was used. It was the intention to follow out the same process and establish a standard for all kinds of ballast in use. This was soon found to be impracticable.
After making many ballast resistance measurements, it was noticed that the variation of the resistance on any track circuit, as between wet and dry weather, generally followed quite a definite rule. For instance: If the resistance per 1000 ft. of dry ballast was found to be 28 ohms or more, it would be not less than 8 ohms per 1000 ft. when wet; or if resistance of dry ballast was found to be between 22 and 28 ohms per 1000 ft., it would be not less than 6 ohms per 1000 ft. when wet.
Once a relay is picked up or energized, but a small amount of current is required to maintain it in that condition. This is one reason why it is important to keep the ballast clear of the rails and it is because of the condition which may cause a relay to remain energized that rules are in force requiring the signalmen to disconnect a track relay when track forces are changing out rails.
Combined Rail and Bond Wire Resistance
On circuits newly bonded with two 46-in. galvanized iron wires a joint, the combined rail and bond wire resistance was found (on the L. & N.) to vary from .02 ohm per 1000 ft. of track on some circuits to .265 ohm on others, a difference of over 1300 per cent. This was rather puzzling. After a great many measurements had been made on different circuits it was found that no two measurements gave the same results, notwithstanding the fact that in many circuits the size of rail, length of bond wires, and age of bonding were exactly the same. On account of the bonding being new and the channel pins well driven, the contact between the bond wire and rail was above suspicion. The only other part of a track circuit that could possibly be the cause of this difference was in the contact between the angle bars and rails, and this later proved to be the case. Actual measurements made in the field proved that when the rail is new and the joint bolts tight, nearly all of the current flowing from rail to rail passes through the angle bars, whereas when the rails get old a coating of rust and dirt forms between the rail and angle bars, forcing practically all of the current through the bond wires. On most of the circuits measured on the L. & N. the combined rail and bond wire resistance was found to be less than .1 ohm per 1000 ft. of track, although many were found to be between .10 and .30 ohm per 1000 ft. It is interesting to note that two circuits were found bonded with two 52-in. iron wires, for which the combined rail and bond wire resistance measured .410 ohm, and that by adding two 40 per cent. copper clad bond wires to each joint the combined resistance was reduced to .144 ohm.