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gradients in the tracks and the differences of potential between underground metallic structures and rails, thereby reducing the flow of current in underground metallic structures. (See Chapter 2, Insulated Negative Feeder System.)

NOTE. The insulated negative feeders may run separately from the negative bus to various points in the track network, or a smaller number of cables may be used with suitable resistance taps made to tracks at various places.

With this system the drop of potential in the track feeders is independent of the drop of potential in the tracks.

DESIGN, CONSTRUCTION, OPERATION AND

MAINTENANCE

The practical electrolysis problem is due to stray current from electric railways. Instances of stray direct currents from other sources sometimes occur, but such cases are not specifically considered in this report.

Currents straying to earth from electric railway tracks frequently find their way to water and gas pipes, telephone and power cables, and other underground structures. When this current leaves these structures through earth, corrosion results. Thus not only are the structures of many different companies subject to injury, but by reason of the different public services dependent on such structures, the public as a whole has a direct interest in this type of electrical interference. The problem, therefore, is one which is pre-eminently adapted to co-operative treatment.

In many cities it has been found advantageous to form joint committees, composed of technical representatives of the several utilities concerned, to investigate the local electrolysis situation and determine by agreement a course of procedure to be followed. Such committees should attack the problem in an open and fair-minded manner with the object of effecting, in the most economical way, mitigation of all the troubles resulting from the presence of stray currents in the earth, including corrosion, fire and explosion hazards, heating of power cables, and operating losses and difficulties. To this end, they should be composed of men, or have mer associated with them, who are trained in the technique of electrolysis. Active committees of the kind described are now existent in Chicago, Kansas City, Omaha, St. Paul, New Haven, Milwaukee, and Syracuse. The principle of co-operation has been recognized by the Railroad Commission of Wisconsin in an order authorizing an Electrolysis Committee in the City of Milwaukee. Such committees act as clearing houses of information and keep all the interested companies informed as to changes in their systems which may affect the electrolysis situation. Under the direction of such a committee joint electrolysis surveys may be conducted and unified methods of mitigation installed and maintained.

The magnitude of stray currents is determined by the design, construction, maintenance, and operation of the railway system. In general, the same factors that determine the amount of stray currents are those that have a direct bearing on the economy of railway operation. A good example is that of an insufficient number of substations, which results both in large stray currents and poor railway economy. Similar results follow from defective bonding, rails of inadequate size, or failure to interconnect tracks. For this reason, it is believed that many existing railway systems can be modified in such a way as to increase their own economy of operation, while at the same time securing important reduction in stray current. Measures of this character, which are essential to the most economic operation of the railway, should be regarded as a prerequisite of the application

either to the railway or to the affected structures, of measures specifically for electrolysis mitigation.

Prior to the consideration of measures for electrolysis mitigation, the following features should be given due attention:

1.

Measures Tending Both to Railway Economy and the Reduction of Stray Current.

(a) The return system, including track bonding, should be put in proper condition.

(b) The number of substations should be made a maximum consistent with railway economy.

2. Measures Employed Solely for Electrolysis Prevention.

Where necessary to effect a still further reduction in electrolysis below that provided by the most economic railway system one or more of the following measures should be taken:

(a) Applicable to Railways. (1) Additional substations, (2) Insulated negative feeders, (3) A modified system of power distribution such as a three-wire system.

(b) Applicable to Affected Structures. (1) Insulating joints in pipes. and cables, (2) Insulating coverings for pipes.

(c) Interconnection of Affected Structures and Railway Return Circuit. (1) Electrical drainage of cable sheaths, (2) Electrical drainage of pipes.

I. RAILWAYS

A. FEATURES WHICH AFFECT ELECTROLYSIS CONDITIONS 1. Track Construction and Bonding.

(a) Importance of Rail Circuit. Stray current is increased by insufficient rail weights and imperfectly bonded track joints. While the major portion of the current of a grounded return railway generally returns through the tracks and return feeders to the power station, a portion finds a parallel path through the earth and its buried metallic structures. As the current flowing in each path is inversely proportional to the resistance of that path, it is of prime importance to make the resistance of the track circuit as low as possible by the use of rails of adequate weight and proper bonding.

(b) Rail Bond Resistance and Tests. The contact resistance of the bond terminal connection to the rail may be a considerable part of the resistance of the joint if the bond is not properly installed and maintained, and it is therefore essential in selecting the type of bond to be used, that special consideration be given this feature.

It is the usual practice to measure the resistance of the bonded joint including three feet of rail in terms of a length of continuous rail. The equivalent length of a properly bonded joint including three feet of rail, varies from 3 to 6 feet, depending upon the size of the rail, and the type, length and cross sectional area of the bonds. On some electrified steam roads it is the practice to bond so that the joint alone will have an equiv

alent resistance of 20 inches of continuous rail and to rebond when this resistance increases to 42 inches. On street railway systems bonding to an equivalent length of 3 to 6 feet is common practice where short bonds are used, rebonding when the joint resistance including three feet of rail increases to that of 10 feet of rail. A single No. 0000 long bond installed around the splice plates will have, with three feet of rail, a resistance equivalent to from 8 to 15 feet of continuous rail, depending upon the size of the rail.

Practice varies widely as to the frequency of testing rail bonds, but most railway companies make complete tests of all bonds at least once each year and more frequent tests on tracks subject to excessive traffic or deterioration. Good practice would require annual tests of all bonds, and semi-annual on tracks in which the bond failures exceed five per cent annually.

(c) Types of Bonds. Bonds may be classified according to the method of fastening them to the rails as follows:

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There is a further distinction between exposed and concealed bonds, the latter being used where the prevention of theft is a serious consideration, in which case the bonds are installed underneath the splice plates.

Local conditions will largely determine the type of bonding to be used. Consideration should be given to the economy of construction, maintenance costs, facilities for using bonding equipment, tools, etc. In recent years there has been a marked tendency toward the more general use of all types of welded bonds with almost complete abandonment of soldered bonds and those mechanically applied to the head of the rail. Pin-terminal and compressed-terminal bonds are still extensively used for application to the web of the rail, but even here the welded type is finding favor with many companies. One reason for the increasing use of oxy-acetylene and electric alloy welded bonds is to be found in the lighter, cheaper, and more portable tools for their application, some of the newer methods and apparatus which have been developed for this class of work being far superior to those formerly employed.

Soldered Bonds are applied to the head, base or web of the rail by means of solder, a blow torch being used to heat the rail to a soldering temperature. The difficulty of securing a permanent and low resistance contact has caused practically all railway companies to abandon this type of bond.

Brazed or Welded Bonds are applied either by the use of the heating effect of an electric current or arc or an oxy-acetylene gas flame.

The Resistance Weld of bond to rail is accomplished by clamping a carbon block against the head of the bond and heating this block to a high temperature by the passage of a large electric current or by drawing an arc on the face of the block.

In the Electric Arc process the arc is drawn directly on the rail and bond terminal. In both the resistance and arc methods of welding or brazing the rail and bond terminals are brought to a welding or brazing heat and united in a solid mass by filling in metal, thus forming a mechanical and electrical union. The filling in metal may be a copper or iron wire used as an electrode. When the bond terminal is steel, the latter metal is used. Several methods, differing somewhat in the equipment used and the methods of applying the heat to the bond and rail, are in use, and the selection of the most suitable of these will depend upon a number of factors and often upon local conditions.

The Oxy-Acetylene process is similar to arc welding except that the heating is accomplished by means of an oxy-acetylene gas flame from a blow torch.

These methods give a connection of low resistance and short bonds can be applied to the head of the rail without much danger of theft due to the small amount of copper involved and the tenacious contact between bond and rail.

Pin Expanded Terminal Bonds have a hole in each terminal through which a tapered drift pin is driven to expand it into a hole drilled in the web of the rail, after which a pin, slightly larger than the drift pin, is driven into the hole and left there to prevent contraction. This type of bond requires great care and accuracy in manufacture and in installation, but when properly installed makes a very efficient and satisfactory construction. The essential features are a carefully and accurately milled terminal and a perfectly clean, circular-drilled hole, reamed to proper diameter, in the rail. Care should be used to brighten the terminal with emery paper just before installing and to avoid contact with the fingers, which will cause corrosion between the terminal and the rail. Holes should be drilled dry and bonding should not be done except in fair weather so there will be no moisture to induce corrosion. This type of bond is usually applied to the web of the rail. As it requires only small portable tools it has been found to be particularly well adapted to main line tracks under operating conditions.

Compressed Terminal Bonds are of two kinds, one being a single solid terminal bond applied to the web of the rail in a manner similar to the Pin Expanded Terminal bonds described above except that contact with the rail is secured by means of a heavy screw or hydraulic compressor applied to each end of the terminal, causing it to compress longitudinally and expand laterally, bringing the copper into firm contact with the steel. The screw compressors used for compressed terminal bonds are objectionable where

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