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 bal last 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.

(b) In general in the application of insulated negative feeders, the negative bus should be connected to the track at more than one point, that is, negative feeders should be extended along the track to nearby intersections. Small stations of 300 to 500 k.w. capacity in city networks may usually be connected directly to the track at one point only and preferably to the nearest track intersection.

(c) Insulated negative feeders should be run from the negative bus to the rails in such a manner as to insulate them thoroughly from the earth and from each other. The tying together of any of these feeders should be avoided. In some cases, however, it may be allowable to tie a single feeder to the rail at two or more points through resistances to adjust the currents drawn from the tracks at the various points of connection.

(d) Connections to tracks should preferably be made in dry rather than in wet locations.

(e) Means should be provided on all negative feeders and feeder taps for conveniently measuring the current flow thereon and where practicable these means should be installed within the power supply station.

(f) Insulated negative feeders are not as well adapted to reducing stray currents from interurban lines as from city networks.

7. Three-Wire System.

(a) The three-wire method of railway power supply will greatly reduce stray currents when properly applied and also give better operating voltage at the cars.

(b) Where a few large supply stations are used the first cost of converting an existing railway system for three-wire operation is usually smaller than the first cost of any other measure which will give the same degree of protection from electrolysis.

(c) There are difficulties to be encountered in connection with three-wire operation which should be carefully considered before adopting that system.

8. Reversed Polarity Trolley System.

(a) With reversed polarity the amount of stray current is not reduced but the electrolytic corrosion will be scattered over the outlying districts instead of being confined to the vicinity of the power supply station. With reversed polarity the drainage of cable sheaths is rendered impracticable.

(b) This measure should not be considered except as a temporary means of relieving dangerous conditions in the vicinity of the power supply station at the expense of the cables and piping systems at a distance from the station, pending the installation of an effective method of electrolysis mitigation.

9.

Periodic Reversal of Trolley Polarity.

(a) If the polarity of the trolley system is reversed daily, electrolytic corrosion will be materially reduced although the drainage of cable sheaths will be rendered complicated or impracticable.

(b) Some of the difficulties attending three-wire operation will also be encountered with the periodic reversal of the trolley.

10.

Double Contact Conductor Systems.

(a) Practically complete immunity from electrolysis can be had by the use of a properly maintained double contact conductor system either underground or overhead, but the expense and difficulties involved in such installation are not justified merely as a means of electrolysis protection.

11. Alternating Current Systems.

(a) Electrolysis resulting from the use of alternating current by street railways is negligible.

B. AFFECTED STRUCTURES

1. Location with Respect to Tracks.

(a) The close approach of piping systems to railway tracks and the laying of shallow service pipes under tracks should be avoided as far as practicable.

(b) On streets in positive areas where car tracks exist gas and water mains are sometimes installed on both sides of the streets. Such construction permits the use of shorter services and obviates the necessity for placing service pipes under tracks.

2. Avoidance of Contact of Cables with Pipes and Other Structures.

(a) In the installation and maintenance of cable systems precautions should be taken to avoid contact between lead sheaths and other underground structures, such as foreign cables, rails, steel bridges, gas or water pipes and the steel frames of buildings, except as such contacts may be required for specific reasons.

3. Conduit Construction.

(a) Cable sheaths should be kept out of intimate contact with the earth by the use of suitable duct materials, proper conduit construction, and adequate conduit drainage. Dips in the conduit where moisture might collect should be avoided wherever practicable.

(b) The use of iron pipe for laterals to poles and buildings should be confined to conditions where no other form of conduit is suitable or permissible.

(c) Wherever long laterals to poles are installed the horizontal portion should be of vitrified tile, fiber, stone or some similar duct material, using iron pipe only for the bend at the base of the pole and for the vertical portion up the pole.

(d) Unless necessary as a protective measure for isolated sections, cable sheaths should not be artificially grounded. Grounds in negative areas through which stray current might be picked up should be avoided whenever practicable.