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 neces sity 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.

4. Insulating Joints in Cable Sheaths.

(a) Insulating joints are sometimes used in cable sheaths under special conditions to prevent electrolytic injury which might result from contact with steel bridges or buildings. They are also occasionally used to prevent the flow of current on cable sheaths where drainage is undesirable or impracticable.

5. Surface Insulation of Pipes and Cables.

(a) Surface insulation in the form of dips, paints and wrappings cannot be depended upon as a permanent method of preventing electrolysis. (b) Thick coatings in the form of pitch or parolite poured into a containing box built around the pipe are occasionally used in preventing electrolysis under special conditions where the expense is warranted. 6. Insulating Joints in Pipes.

(a) Insulating or high resistance joints, such as those of the Dresser type or cement joints, if used throughout a pipe line at frequent intervals, or at specially selected locations, may afford substantial protection against electrolysis. This practice relates particularly to gas and oil pipes.

(b) It is sometimes permissible to use a comparatively few insulating joints if care is taken to see that the flow of current on the pipe is practically eliminated.

(c) Insulating joints are often installed in service pipes for the purpose of preventing the interchange of current between pipe systems.

7. Shielding.

In special locations a pipe may be protected from electrolysis by a metal shield, wholly or partly surrounding it and electrically connected thereto.

C. INTERCONNECTION OF AFFECTED STRUCTURES AND

RAILWAY RETURN CIRCUIT

1. Cable Drainage.

(a) Lead sheath cables in urban districts or where parallel with interurban railways commonly require some form of electrolysis protection and this is usually accomplished by drainage.

(b) In some cases the heating effect of stray current on the sheaths of power cables may reduce the current carrying capacity of the cable.

(c) Drainage connections should be made to the negative bus of the railway supply station or to the rail terminal of insulated negative feeders. Connections to the rails should in general be avoided.

(d) Cable sheaths when drained are made negative to the surrounding earth at practically all times. Drainage should be reduced to a minimum consistent with the protection of the cables.

(e) In general, all signal cable sheaths in any conduit system should be bonded together at every manhole. Where advisable and permissible all power cable sheaths should be similarly bonded.

(f) So far as practicable or advisable, all cable systems should be drained at the same locations or by the same drainage feeders and differ

ences of potential between adjacent or intersecting cable systems should be eliminated by cross bonding.

(g) Fuses should be installed in all connections between signal and power cable sheaths, and should be so proportioned as to protect the sheaths from dangerous currents.

(h) Means should be provided for conveniently measuring all drainage currents and maintaining close supervision on all drainage systems, and where possible this should be accomplished by installing meters within the power supply station and making them accessible to the cable owning companies.

(i) Means and operating regulations should be provided for opening all drainage cables during periods when reverse currents would otherwise flow over them, if the magnitude and duration of such reverse currents is objectionable.

2. Pipe Drainage.

(a) There are wide differences of opinion among competent engineers who have studied pipe drainage as to its adaptability to various conditions. Numerous questions are involved in regard to which there is not sufficient information available at the present time to permit the drawing of accurate conclusions, and for this reason this subject is being investigated by the Research Sub-Committee of the American Committee on Electrolysis. There are, however, certain objections to the use of pipe drainage which are discussed in this report and which should be carefully considered before employing it.

(b) Pipe drainage is in use more or less on water systems and to a limited extent on gas systems.

(c) As a method of mitigation, drainage is not so well adapted to pipes as to cable sheaths.

(d) High resistance joints are prevalent in all jointed pipe lines and they greatly complicate the application of drainage.

(e) To lower the potential of a jointed piping system below that of the surrounding earth, it is usually necessary to extend the drainage conductors over a considerable area and connect to the pipes at numerous locations.

(f) Corrosion at high resistance joints in pipe lines carrying current may occur unless the pipe on both sides of the joint is maintained negative or neutral to the adjacent earth, in which case no corrosion will

occur.

(g) Drainage generally increases the current flow on pipes and such current increases the hazard from oil and gas ignitions and explosions.

(h) In small towns on interurban railways pipe drainage is less objectionable than in urban districts where complicated pipe networks exist.

(i) Drainage of a large network of pipes should be used only as an auxiliary to a railway system, properly designed and maintained from the electrolysis standpoint. When so used it should be installed and maintained under competent supervision.

ELECTROLYSIS SURVEYS

I. Introduction

A. PURPOSE AND SCOPE OF ELECTROLYSIS SURVEYS

1. Purpose of Electrolysis Survey.*

Electrolysis surveys deal with the various methods and classes of measurement employed to determine the hazard to underground metallic structures due to stray electric currents, the extent of existing damage already produced, and the mitigative measures that may best be employed for reducing the danger of future trouble. There are discussed below the methods of determining electrolysis conditions, of collecting data upon which the design of mitigating systems may be based, the types and kinds of instruments that should be used, the procedure to be followed in the working up of the data, and the interpretation of the results of the

survey.

2. Difficulty of Standardizing Survey Procedure.

It should be emphasized that in general, no two electrolysis surveys will be conducted in precisely the same manner, so that specific rules of procedure cannot be laid down that will be applicable to all cases. The procedure set forth below is intended to cover the measurements that experience has shown are most frequently required, and to describe the best methods of taking such measurements. The number of readings taken and the procedure to be followed will vary so much with local conditions that reliance must be placed on the judgment of the person making the test. In fact, the proper procedure during a large part of the survey will depend in large measure on the results obtained in preliminary tests. It is important, therefore, that electrolysis investigations of any importance be made under the direction of a competent engineer very familiar with methods of procedure and the interpretation of electrolysis test data. 3. Information Obtainable by Electrolysis Surveys.

By means of proper measurements, it is possible to determine with a fair degree of definiteness, the extent and location of the areas in which pipes and other structures are endangered by stray currents and, with sufficient accuracy for most purposes, the degree of seriousness of the trouble. The cause of any damage that may be in progress at the time of the survey, whether due to stray currents or corrosion by the soil, cinders, or other natural causes, can generally be ascertained, and in the case of stray current corrosion, the source of the current can generally be determined. The various factors connected with pipe systems, such as high resistance joints, very low soil resistances, and the use of improper mitigative measures, can also be detected. Defects in the railway return system, such as poorly bonded rail joints, infrequent cross bonds, insuf

*For a definition of the term electrolysis survey and other terms used in this chapter, see Chapter 1, on Principles and Definitions.

ficient conductance in the negative return, improper use of such conductance, excessive feeding distances and other causes of electrolysis trouble can usually be definitely determined.

B. TYPES OF SURVEYS

In the following discussion several types of surveys must be recognized. The first is that which may be called a complete electrolysis survey which is made for the purpose of determining the extent and location of the danger areas, and with a view of determining the proper procedure to be followed for the mitigation of any trouble which may be found to exist.

The second type of survey, known as a maintenance survey, embraces such surveys as would usually be made by a pipe or cable owning company solely for the purpose of determining whether previously existing conditions have changed, and differs from the more complete survey mainly in that most of the information with respect to the railway power distribution system is not required and fewer electrical measurements are taken, the number and character of such measurements depending on the thoroughness with which the survey is to be carried out.

A third type of survey which needs little discussion, except as to methods of making tests, is one made to determine whether ordinances or regulations governing electrolysis conditions in a municipality are being complied with. Such surveys are usually made periodically, in periods varying from three months to a year. In general, only those quantities are measured which are specifically defined by the ordinances or regulations which are in effect in the locality in question.

C. GENERAL PRELIMINARY DATA

1. Data on Underground Structures.

In making electrolysis surveys, a considerable amount of preliminary data are usually desirable. It is important first to gather all evidence regarding the character, extent and location of known damage to underground structures. This evidence is usually obtained from the utility companies concerned, but even though these companies can give no direct testimony as to the injury to underground structures, this should not be taken to indicate that no damage exists. The data on the underground systems should include the relative location of the mains, the railway tracks and underground cable systems. The size and kinds of pipe and the types of pipe joints used are usually important. Numerous questions relating to the interconnection of gas, water, and cable systems are also of importance.

2. Data on Railway Systems.

As regards the data on the railway systems, the following should be determined: (1) Location and capacities of direct current railway supply stations; (2) Location of railway lines and character of service, whether city, suburban, or interurban, and the car schedules on different parts of