water systems and to a limited extent on gas systems. While the success of protecting cable systems by drainage is generally recognized, there are important differences in the application of drainage to cables and to piping systems which make the application of drainage to the latter difficult and uncertain. Among the important differences between the drainage of cable and piping systems are:

1. Cables are electrically continuous and uniform conductors, while pipes are not uniform conductors and are sometimes discontinuous conductors due to the joints in them. Experience indicates that in mains having cement joints a large percentage of these joints are of high resistance, and in mains having lead joints, occasional joints of very high resistance are found and many of the joints have resistances higher than several lengths of pipe. Therefore, drainage will lower the potential of the pipe for relatively short distances from the drainage taps, so that to be effective a greater number of drainage taps must be installed than for a cable system of the same extent. The number and location of taps will depend upon the extent and physical layout of the pipe network, and the expense involved will depend upon the number and locations of the taps required.

2. Under certain conditions there is a tendency for current flowing on a pipe to leave it on the positive side of a high resistance joint, returning to the joint on the negative side, or else to flow to another structure. As a result of this, joint corrosion may occur at high resistance joints unless both sides of the joint are maintained negative or neutral to the adjacent earth at all points and under all conditions; and conversely, no electrolytic corrosion will occur on either side of a high resistance joint if the entire surface of both the adjacent pipe lengths is permanently negative to the surrounding earth. The difficulty of keeping a complicated network of pipe negative to the adjacent earth by means of drainage is much greater than in the case of cable systems. 3. Cable systems are placed in ducts with manholes conveniently spaced so that the effect of the application of drainage to a cable system may be adjusted so as to produce the results desired, whereas with pipes buried in the ground, and in large cities beneath improved pavements, it is more difficult to make the necessary measurements to ascertain the effects of drainage.

4. Cables are relatively small and contained in ducts so that unless they are in wet or marshy ground, they are but partially in contact with the earth, whereas, gas or water pipes are buried directly in the earth. Because of this condition, the drainage of an underground piping system with but few high resistance joints results in the flow of larger amounts of current than does the drainage of a cable system.

5. Currents flowing in piping systems conveying inflammable substances, such as gas or oil, constitute a fire and explosion hazard and many cases have been reported where stray currents have caused arcs which have ignited the gas or oil when the continuity of the pipe was broken. One of the objections to the presence of excessive currents

on gas or oil pipes is the necessity for bonding around a cut in the pipe whenever a pipe is opened for repairs. Under such conditions a copper wire cable is connected around the point on the pipe to be opened. Jumper cables, terminating with adjustable clamps, are used by some companies for this purpose.

Under certain conditions there is also danger of increasing potential differences between service pipes in confined air spaces which may result in causing arcs due to the intermittent contact between pipes which will puncture the gas pipes and ignite the escaping gas.

3. Application of Drainage to Pipes.

(a) Maintaining Pipes Negative to Earth. Investigations of the Research Sub-Committee show that when electrical drainage feeders are connected to a jointed piping system the drained pipe is maintained negative to the soil for only a few hundred feet from the point of connection. In such cases it is necessary to extend the drainage feeder along the principal pipes in the positive area, which extends theoretically about 40 per cent. of the distance from the supply station to the end of the feeding district, and connect to the pipes at frequent intervals.

(b) Effect of Pipe Drainage on Current Interchange. Various conditions exist in piping systems which tend to affect the interchange of current between them, and these should be fully recognized in the consideration or employment of pipe drainage.

If a single pipe system exists, as for example, a water system in a small town, the drainage of that system will not as a rule result in objectionable interchange between various parts of the network. However, there are usually several piping systems present, such as a lead calked water pipe system and a lead calked gas pipe system. If these piping systems are not interconnected at many points through appliances, or are not otherwise connected together, the drainage of one or both systems might result in serious interchange of current.

The application of drainage to one piping system in a territory where another piping system exists may result in an interchange of current between the drained and undrained systems, so it is necessary to resort to the common drainage of all of the piping systems to be protected, as the potential inequalities created by separate drainage cause electrolysis at points where the current leaves the undrained system to find its path to the drained system. Even with the most carefully installed and maintained unified system of drainage, it cannot be expected that all danger from current interchange will be eliminated.

Pipe systems laid with cement joints, Dresser Joints, or other high resistance joints and not interconnected with other systems, will usually need no other form of protection against electrolysis. If, however, such a system exists in a territory also occupied by a piping system with lead calked joints and connected to it at many points through appliances or otherwise, the service pipes of the system with the high resistance joints and the sections of the mains to which they are connected, will be electrically connected to the more continuous system and so far

as electrolysis is concerned should be considered as a part of that system. Any electrolysis condition existing on the continuous system will therefore be experienced by such service pipes and the sections of the mains of the discontinuous system as connect directly with it and any measure which tends to protect the continuous piping system will also affect the services of the discontinuous system. This condition is illustrated in Fig. 18, where a continuous water piping system is connected through appliances to gas services. Although the gas mains are laid with cement joints, they are being damaged by current brought to them over the water mains.

The application of pipe drainage under conditions here described may afford protection to some portions of the piping system and increase the damage to others. In some areas gas services and water services are connected with each other through appliances so that at these locations the two piping systems are maintained at practically the same potential. In most piping networks, however, there will be extensive areas where the gas and water systems are not interconnected by such appliances and even where they do exist they cannot always be relied upon to maintain the two systems at practically the same potential.

(c) Effects of Different Kinds of Pipe and Joints. A fundamental difficulty in applying electrical drainage to piping systems is usually present and this is the great variation of conductivity of different kinds of pipes and of different joints. In any cast iron piping system the resistance of the joints varies through wide limits. In many cities there are a number of different kinds of pipes in use: steel mains with welded or screw joints have a low resistance; steel mains with gaskets made of rubber are high in resistance, while cast iron mains with cement joints are unusually high in resistance. With electrical drainage the current on the pipes is increased and the potential drop along these pipes and over the joints is increased in like proportions.

Because of these conditions it is difficult to apply drainage without increasing the potential differences between the different piping systems at some points.

SUMMARY OF GOOD PRACTICE

This summary is intended only as an annotated index or guide to the contents of Chapter 2 of this report, not as a substitute. Before forming an opinion or taking even preliminary action on any subject treated in the report the full text should be studied.

A. RAILWAYS

1. Track Construction and Bonding.

(a) The use of heavy rails with joints properly bonded and well maintained is the first requirement for good track conductivity and the minimizing of stray currents.

(b) In paved streets welded rail joints are regarded as the best and most permanent form of bonding.

(c) Rail joints including 3 feet of rail which have a resistance in excess of 10 feet of adjacent rail should be rebonded, except joints bonded with long bonds, which should be renewed when the resistance exceeds that of 15 feet of adjacent rail.

(d) Bonded joints should be tested at least once each year and such tracks as show bond failures in excess of 5 per cent. annually should be tested every six months. A failure is here defined as exceeding the resistance specified in paragraph (c).

(e) Cross bonds, connecting the two rails on single track, and the four rails on double track should be installed at intervals not to exceed 500 feet in city systems and from 1,000 to 2,000 feet on interurban lines. (f) Jumpers of one or more conductors should be used around all special work, and should connect to all rails on both sides of the special work. The size of such jumpers should be proportioned to the current on the rails, but in no case should they be smaller than No. 0000 for one track. In addition, where practicable, all special work should be bonded and maintained as other track rails.

2. Track Insulation.

(a) In the construction of electric railway tracks and roadbeds the electrolysis problem should be given consideration with economy of construction, maintenance, and operation.

(b) Roadbeds should be constructed with as high electrical resistance to earth as consistent with other considerations, special attention being given to keeping them dry by drainage. Where practicable, rails should be kept out of contact with the earth.

(c) Clean crushed stone ballast offers a much greater electrical resistance to stray current than does solid concrete as a foundation under ties.

(d) Where crushed stone or gravel ballast is used it should be kept clean. If earth, sand, or street dirt is permitted to filter into ballast of this character its insulating property is greatly impaired. Vegetation should be kept down, as this tends to make the roadbed moist and to fill the ballast with foreign material.

(e) Salts, which are often used to prevent freezing at switches and frogs, greatly reduce the resistance of roadbeds and should be avoided as much as possible.

(f) Zinc chloride and similar chemical tie preservatives reduce, while creosote and gas oil increase the electrical resistance of ties.

3. Reinforcement of Rail Conductivity.

Copper is not economically employed when connected in parallel with tracks, and therefore subjected to the same voltage drop as exists on the tracks, as it cannot be loaded to capacity with track voltage drops ordinarily permissible.

Buried copper conductors or old rails used to supplement the track return also increase the contact area between the return circuit and the earth and thereby tend to augment stray currents. For these reasons the use of such supplementary conductors should be avoided.

4. Power Supply.

(a) Power supply stations for electric railways should be located with consideration to their effect on overall potentials and potential gradients in the tracks.

(b) In selecting locations for substations, particularly for interurban lines, consideration should be given to the extent and character of the underground metallic structures in their immediate vicinities.

(c) Connections to tracks in wet locations or the installation of bare track feeders in earth or in water courses should be avoided.

(d) Numerous independent connections to the track for the return of current afford the most effective means of reducing high potential gradients and overall voltages and thereby limiting stray currents, and as many should be provided as consistent with good engineering and economic considerations.

This can be accomplished by the use of additional power supply stations, by the installation of insulated negative return feeders, or by the three-wire system wherein each car on the negative trolley becomes a point of return. Combinations of these may also be employed.

(e) The most generally satisfactory method of increasing the number of independent return points on a track system is by the use of additional substations and the tendency of railway practice is now in this direction.

(f) Considerable progress has been made in recent years in the development of automatic, semi-automatic, and remote control substations and these are now being used both on interurban lines and for city service. The economies attending such substations make possible a greater number of feeding points than can economically be supplied through manually operated stations.

(g) By employing the maximum number of substations consistent with economy, rather than the minimum number, stray currents will be greatly reduced.

5. Interconnection of Tracks.

As a rule, interconnection of tracks will improve general electrolysis conditions, but may be detrimental in one locality while improving conditions in another.

6. Insulated Negative Feeder System.

(a) Track gradients and overall potentials can be limited to any desired extent by the use of insulated negative feeders but the cost of such installations, the additional power loss accompanying their use and the reduction in operating voltage at the cars may make their use uneconomical except in connection with frequent power supply stations.