Three-wire operation was adopted in Winnipeg as a means of meeting the requirements of a law passed by the Manitoba Legislature, prescribing certain limitations in track voltage drops. Two substation districts were changed over in 1919, and since that time practically the entire system has been converted to three-wire operation.

In 1920, after considerable experimenting, a three-wire system was substantially completed in Wilmington, Delaware, and a complete electrolysis survey made under both two-wire and three-wire operation. With the latter, a considerable improvement in car operation due to higher average voltage was reported, and also better electrolysis conditions on water and gas pipes. Stray currents and overall potentials were reduced to about one-half their values with two-wire operation. Reversing potentials were found on the telephone cables in some areas and some adjustment of the drainage of this system will be necessary before it can be said to be entirely satisfactory.

3. Reversed Polarity Trolley System.

This method of railway operation involves using the running tracks as the positive conductor instead of the trolley wire. It has at various times been suggested as a means of electrolysis mitigation, and in at least one case it has received an extended trial. Fundamentally, however, it is not a mitigation method, because it merely reverses the direction of the stray current and in no way affects the magnitude thereof. With reversed polarity the same amount of corrosion will result as with normal operation and the only difference will be the localities in which the damage will occur. Under normal operation using the running tracks as the negative conductor, the electrolytic damage will generally be confined to the area immediately surrounding the direct current power station or the track feeder connection points. With reversed polarity, the electrolytic corrosion will be scattered over the outlying districts which with normal polarity would constitute a negative area. If the trolley system is operated with reversed polarity, it is extremely difficult to effectively drain the lead sheaths of underground cable systems, because there is no definite point of low potential to which to drain.

In 1912 the polarity of the electric street railway system in New Haven, Connecticut, was reversed, making the running tracks the positive conductor. This method of operation was adopted by the railway company in order to afford immediate relief to the gas works, and to the water and gas piping systems in the central part of New Haven, where very serious damage was occurring. It was then thought that in the outlying sections the damage would be less concentrated, and also failures would be less serious and more easily repaired, than in the central business district. It soon became evident that it was practically impossible to adequately drain the underground telephone cable system, and that even with reversed polarity the general electrolysis conditions of the water and gas piping systems were still far from satisfactory, and after a trial of eight years, this method of operation was abandoned.

The New Haven experiment, therefore, indicates that the reversal of railway polarity to rails positive is merely a means of relieving dangerous electrolysis conditions in the vicinity of the power station, at the expense of the cable and piping systems at some distance from the sta

tion. When no underground cable systems are involved, reversed polarity is useful as a temporary means of immediate relief to an endangered piping system in the interval immediately preceding the installation of effective electrolysis mitigation.

4. Periodic Reversal of Trolley Polarity.

If the polarity of the trolley is reversed daily, at a time when the load on the system is a minimum, few operating difficulties will be encountered and some improvements in electrolysis conditions will result. It is obvious that pipes in any locality will be in a positive condition only half as long as with normal operation, and there may also be a further reduction of electrolysis due to redeposition of the corroded metal during the period when the pipes are negative. Laboratory experiments made by the Bureau of Standards, the results of which are shown in Fig. 16, indicate that with a daily reversal of polarity, the corrosion of iron pipes at any point will be about twenty-five per cent. as great as will result without such reversals. A similar relation, though not precisely the same as that shown in Fig. 16, exists with respect to lead when subjected to periodically reversed currents.

This method of operation has been employed by the Pacific Electric Railroad Company of Los Angeles since 1918 in Pomona, Redlands, San Bernardino, Riverside, and Corona. In general it is not applicable to cities where lead cable systems are installed underground, as it would greatly complicate and sometimes render impracticable the drainage of such systems. However, where the cable system is small and confined to the vicinity of the power supply station it may be drained satisfactorily through an automatic switch which permits current to flow from the cables, but automatically prevents the reversal of such flow.

Some of the operating difficulties discussed in connection with threewire systems will be encountered with this system. The operating difficulties attending a more frequent reversal of the trolley potential would be considerably greater, and no attempt so far has been made to do this. 5. Double Contact Conductor Systems.

The double overhead trolley system of electric traction as at present used in Cincinnati, and the corresponding underground conduit systems as used in Washington and in parts of New York City, if properly maintained, eliminate the danger of electrolysis. This system has in past years been strongly urged by some pipe owning companies and engineers who believed it to be the only method by which complete immunity from electrolysis could be obtained. It is now generally recognized, however, that a substantial degree of protection can be obtained by less expensive and objectionable methods and the demand for the double contact conductor system is, therefore, not being pressed at the present time. The chief objections to its use are the cost of installation and the increased operating difficulties which it involves, as well as an unsightly appearance of the streets in the case of the double overhead trolley. The double contact underground system, as used in New York and Washington, not only removes the source of stray current, but requires no overhead wiring or poles and in rare cases may be justified or required for that reason alone. Merely as a means of electrolysis mitigation, the increased cost of the double contact conductor system does not appear to be justified.

II. UNDERGROUND STRUCTURES SUBJECT TO INJURY BY STRAY CURRENTS

A. LOCATION WITH RESPECT TO TRACKS

In general, the problem of protection from stray currents has to do with conditions under which the affected structures and the tracks are already in place, that is, where their respective locations are fixed. In the great majority of instances, therefore, a discussion of the most favorable relative location of underground structures and rails can have but little more than an academic interest. However, in laying new underground structures or replacing old ones, it is in the interest of safety to locate them at as great a distance from the rails as possible. Uusually conditions other than electrolysis determine the location of mains, but where it is possible to locate mains on both sides of a street having car tracks, such construction prevents the crossing of service pipes under tracks and is in the interest of good electrolysis conditions. Where mains or services must cross under tracks there is a considerable advantage in having them as deep as possible, but a depth of more than 4 or 5 feet is ordinarily not justified.

B. CABLE SYSTEMS

1. Avoidance of Accidental Contacts with Other Structures.

From an electrolysis standpoint, it is usually necessary to treat lead sheath cables as distinct from other underground structures due to the fact that lead is appreciably more susceptible to corrosion from stray current than iron, and also because different measures are usually applied to the protection of lead sheath cables than to other underground metallic structures. One ampere flowing steadily for a year will carry into solution about 20 pounds of iron or about 74 pounds of lead. This high electro-chemical equivalent of lead and the thin walls ordinarily used for cable sheaths require that unusual care be exercised in their protection.

In the Bell Telephone System precautions are taken to avoid contact between its lead sheathed cables and other underground structures, such as foreign cables, rails, steel bridges, gas or water piping system and the metallic structure of steel-frame buildings. Where it is necessary that cables cross a bridge structure, this is frequently accomplished in creosoted wood duct. Occasionally, however, iron pipes are used to conduct cables across a steel bridge, but where this is done, these pipes are supported so that they are insulated from the metal work of the bridge.

2. Conduit Construction.

Cable sheaths cannot be said to be insulated from earth even when installed in non-conducting duct material, but as compared with pipes which are laid directly in the earth, their resistance to ground is generally very high. Unless surrounded with mud or water, cable sheaths usually make a line contact with the duct walls, whereas pipes make a surface contact of much greater area.

The study of the insulation of cable sheaths from earth therefore resolves itself into a study of suitable conduit construction methods since experience has demonstrated the failure of any sort of wrappings, dips, or coatings to afford protection of any value from electrolysis. Indeed, wrappings, dips, and coatings have been shown to be distinctly harmful where pipes or cables are positive to the earth since they tend to localize the discharge of current and thus to accelerate failures.

(a) Signal Cables. The experience of the Bell Telephone System has demonstrated that multiple and single vitrified clay duct and creosoted wood duct are all equally good as duct material from the standpoint of electrolysis, their choice in specific cases being a question of supply and cost. Iron pipe is occasionally used, but, due to its cost, only when necessary in avoiding obstructions.

When iron pipe is used, it is so laid that there will be no contact between it and the trolley rails, steel bridges, water pipes, gas pipes or other underground structures or the metal work of buildings. When iron pipes must be laid as conduit so close to rails or other grounded metallic structures that a separation of at least one foot of earth cannot be obtained, the pipes are separated from the rails or other grounded metallic structures by a layer of concrete or creosoted plank. Three inch vitrified sewer tile with cement joints is now being commonly used for laterals to poles or building connections.

In good conduit construction the necessity is recognized of rendering the joints between lengths of duct material, sufficiently tight to prevent the infiltration of dirt and silt and also to maintain a sufficient slope to the conduit to insure good drainage toward manholes, the manholes in turn being drained by sewer connections or to sumps. Particular care is exercised to prevent dips or pockets in conduit runs where moisture might collect. It is the practice to rack cables in manholes, a free space of twelve inches being maintained between the lowest cable and the manhole floor. The cables are in metallic contact with the metal hanger which, in turn, may be in contact with or built into the manhole wall, experience having indicated that no appreciable increase in cable resistance to earth is obtained by insulating the cables at these points with porcelain or other insulating material.

Where lateral cables enter buildings, it is the usual practice in the Bell System to avoid all contact between the cable and the metal structures of buildings, and wherever this is impracticable, the continuity of sheaths on the entering cables is broken by an insulating joint.

Occasionally conduit runs must be built through swampy ground or along sections of the coast where the conduit is permanently below sea level. Where such conditions are encountered, no method is practically possible for insulating cable sheaths from earth and such insulation is not attempted. Such locations are frequently extremely troublesome from the electrolysis standpoint, and therefore special precautions have to be taken.