FIG. 20.

It is very important that a thorough metallic connection be made between the pipe to 'be protected and the shielding pipe. Otherwise, the service pipe is likely to corrode where current leaves it to flow through earth to the shielding pipe. Unless the shield is in the form of a pipe completely surrounding the structure to be protected, this method of protection is uncertain and should be used only in very special cases. When applying this method it has been found necessary to take care that the auxiliary shielding conductor does not merely increase the electrode area from which the current leaves, because in this case the current will continue to leave from the structure which is to be protected unless an insulating covering is applied to the pipe beyond the protecting shield. This has been found to be the practical result where a shielding conductor of the same or less contact area was placed in the earth near the structure to be protected and where the stray current has left both structures.

III. MEASURES INVOLVING INTER-CONNECTION of AFFECTED STRUCTURES AND RAILWAY RETURN CIRCUIT

A. ELECTRICAL DRAINAGE OF CABLE AND PIPE SYSTEMS Electrical drainage consists in connecting the affected structure to the railway return circuit by insulated conductors in such a manner that the current leaves the structure through these connections instead of flowing to earth. This prevents corrosion in the neighborhood of the drainage connections, but increases the current flowing on the structure and the voltage drop along it, which latter results are generally undesirable for reasons discussed in detail in subsequent paragraphs.

Drainage connections are usually made by running copper cables either to the busbar of the railway supply station or to negative return feeders. Connections to tracks should be avoided because the failure of rail bonds might cause dangerous currents to flow over the drainage connection and also because of the possibility of getting a current reversal, particularly when the adjacent substation shuts down during the light load period. However, when insulated negative feeders are used, the drainage connections may be made to the rail terminals of the feedConnections to rails are sometimes installed where a conduit line or a pipe crosses a railway track at a considerable distance from the power supply station and other means of draining would be awkward and expensive, but they should be made with considerable discretion and should be carefully recorded and regularly inspected.

ers.

Where used, drainage should be reduced to a minimum consistent with the protection of the drained structure in order to reduce the hazard to other adjacent underground systems.

The drainage of one system tends to establish differences of potential between the various underground systems, resulting in interchange of current with consequent injury to the system at the higher potential. In order to avoid this condition, it is desirable to interconnect the various systems and drain them over common conductors. As structures owned by different interests cannot be bonded together except by

an agreement between the owners this has frequently of itself made it impossible to apply a comprehensive drainage system to all structures because of the impossibility of obtaining an agreement of all owners to allow connections to their structures, except on condition that other interests assume liability for any injury which may result from such interconnections.

If, however, the foregoing method of unified drainage is carried out so that the drained structures are at all times negative to earth, no electrolytic corrosion of such structures will result. Just how difficult it may be to maintain pipes negative to earth at all points and at all times by means of drainage is a question which cannot be answered until investigations have been carried further.

The objections to electrical drainage apply most forcibly to pipe networks, particularly to gas and oil pipes on account of the inflammable substances carried. Drainage should be considered only as a supplementary measure to the improvement of the railway return circuit or as a temporary measure in cases where acute electrolytic corrosion has resulted. It can never take the place of an adequate railway return circuit.

Notwithstanding its numerous disadvantages and limitations, there are engineers who believe that pipe drainage has a definite field of usefulness. The Committee, through its Research Sub-Committee, is still actively engaged in investigating the magnitude and importance of the technical factors involved and until further information shall have been acquired, the Committee will not be in a position to reach a conclusion on this subject.

1. Drainage of Cable Sheaths.

(a) Method of Draining Cable Sheaths. In order to afford complete protection to cable systems, it has been found that they should be interconnected and have drainage conductors of sufficient conductivity located so that the lead sheath of the cable network is everywhere lower in potential than the adjacent earth. Cable systems are usually installed in vitrified clay, creosoted wood, or fibre ducts, and if kept free from water, the tendency to collect current is much less than if they were in direct contact with the earth. Owing to the higher resistance thus introduced between cables and earth and the continuous character of the cable sheaths, it is usually possible to lower the potential of the system below that of the adjacent earth in all localities by draining relatively small currents at one or more points.

In order to prevent the interchange of current through earth between the several cable sheaths in any conduit system, it is necessary to bond the sheaths together at frequent intervals. Some companies make a practice of bonding at every manhole and good practice requires such bonding at intervals not to exceed five hundred feet. Bonding is usually accomplished by sweating a flat copper strip or a copper cable to all cables within any system which may properly be bonded together. Foreign cables which enter any duct system are also bonded to the system they parallel. It is often necessary to interconnect signal cables

with lighting and power cables so as to avoid differences of potential which might otherwise occur, but where this is done, a fuse should be installed in the bond connection to the signal cable so as to eliminate the possibility of high voltage current getting on the signal cable sheaths.

It is desirable to provide means for measuring all drainage currents and where the drainage feeder is extended to the supply station, an ammeter or shunt is usually installed for that purpose within the station. Where the drainage cable does not enter the supply station, measurement can be made within a manhole or on a pole, or wherever the drainage cable is accessible.

Where a cable system tends to become positive in regions remote from the railway supply station, it is necessary either to use a long copper cable for drainage at a considerable expense or to resort to some other method of protection. Aerial telephone cables are sometimes used for this purpose, but are not employed except where other conductors are not available or would be unduly expensive.

Cables are sometimes found to be positive only during certain periods of the day or their potential may reverse from time to time due to fluctuations in the railway load. Where this condition is considered dangerous from the electrolysis standpoint an automatic switch is sometimes installed which is closed during the period the cable is positive and automatically opens when the cable becomes negative, the object being to prevent the cable from taking on current while in a negative condition. The cost of automatic switches and the fact that they add an objectionable complication to the plant are reasons why their use should be restricted as much as possible.

Automatic or manually operated switches should be provided in all drainage cables terminating in railway supply stations in order that they may be opened during the period when the station is not in operation. Automatic substations which start and stop without attendants should be provided with facilities for accomplishing this result.

(b) Heating Effect of Stray Current on Cable Sheaths. Stray current on the sheaths of lead covered cables causes a heating effect which impairs the carrying capacity of power cables. In some cases this effect may be objectionable.

The following formulae have been developed for single conductor and three conductor cables to give their current carrying capacity when sheath currents flow. The values obtained give the conductor the same temperature rise above surrounding structures as produced by their normal current when no sheath currents are present.

The formulae have been developed on the following basis:

1. That the watts dissipated in the sheath are effective in raising the sheath temperature but that they do not affect the rise of the conductor over the sheath.

2. Resistivity of lead 12 times that of copper. This assumption. while not strictly correct, will give results within an accuracy obtained by considering other factors as constants, such as the radiation constants of the lead sheath.

Definitions

A = temperature rise of conductor over sheath for a given conductor current.

B = temperature rise of sheath over cable surroundings for the same conductor current as for A.

C = temperature rise of conductor over cable surroundings for the same conductor current as for A and B.

D= outer diameter of lead sheath in inches. d = inner diameter of lead sheath in inches.

a = area of conductor in circular inches I,= amperes flowing in sheath.

I。= normal current rating of cable.

=

area in circular mils 1,000,000

I = conductor current with (XI) sheath currents.

The values of A, B, and C can be found for single and three conductor cables by referring to Atkinson's article on "Carrying Capacity of Cables" in the September, 1920, issue of the Journal of the A. I. E. E.

Examples

1. Single conductor cable, 250,000 C. M., 1/8 inch lead sheath, 4/32 inch paper insulation. Normal current 510 amperes. What is resultant carrying capacity with 100 amperes sheath current?

2. Round Three Conductor No. 4/0,

32

paper insulation,