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and, the battery and galvanometer being still connected as shown, the core is wound off that drum on which the fault lies until a deflection is obtained on G. This indicates that the fault has just left the drum, and, by cleaning and drying a portion, it can be more exactly located by drawing it slowly through the water until the deflection again occurs. Having been thus located and wound on to the opposite drum, its effects disappear as the leakage current goes to earth, and any further faults
behind can be localised in like manner without removing the coil for repairs.
It will be noted in the diagrams illustrating the fore. going methods of fault localisation between drum and drum, that the opposite extremity of the conductor to that connected to the remainder of the apparatus is marked .“ Free.” This means free, not only in the ordinary acceptation of the term, but also electrically free from surface leakage—i.e., the surface of the insulation to the extent of three or four inches from the point where the conductor is bared must be cleaned, dried, and, if need be, waxed, in order to eliminate any possibility of surface leakage, which latter, if it exist, tends to give rise to secondary deflections which may be regarded as due to the proximity of the fault, and, in this manner, destroy the thoroughness of the test.
For a similar reason the insulation of the drums and tanks, where indicated, should be as perfect as possible.
As regards the battery power to be employed in these tests, there is no hard-and-fast rule, as it depends upon circumstances, such as the resistance of the fault, resistance to leakage current of the surface of the insulation, degree of dampness of the latter, etc., etc., but, as a rule, a voltage almost, if not quite as high as that employed for the measurement of insulation resistance will be required. If a primary battery such as the Leclanché type be used for the purpose, the cells may be protected from short circuit consequent on the passage of dead earth faults by the insertion of a high resistance, such as 10,000 ohms, in series with the battery.
The galvanometer used should be fairly sensitive; a Thomson reflecting instrument, duly provided with a shunt box and short circuit key, answers very well, and an instrument of the D'Arsonval type even better in that its movements are dead beat, and unaffected by the movements of the drums, etc., if of iron, in its vicinity.
To pass on to the subject of fault localisation by the application of a more or less powerful generator current. This is a practice sometimes resorted to in cases of emergency, and usually consists in the actual burning out of the fault by the application of a current of sufficient magnitude for a short period. Thus, an earth or partial earth fault on an electric light or power circuit may often be localised in cases of emergency by passing a current from any convenient generator, such, for instance, as one of the dynamos supplying that particular circuit, through the fault, one pole of the machine being connected to the faulty circuit and the other to the metallic sheathing or armouring of the cable if it be of that description, or, failing this, to an effective earth in the immediate neighbourhood, which, needless to state, should not take the form of a gas pipe. A suitable fuse should be inserted to protect the machine, and prevent undue rush of current on the actual breaking down of the fault, and, above all, the test should not be applied except in cases of emergency, where time is an object, and, even then, only when the faulty section is under the more or less immediate surveillance of a responsible individual competent to check at once any undue combustion of insulation, woodwork, or other inflammable material in the immediate neighbourhood of the fault. The drawbacks to this crude system of fault localisation are several in number, chief amongst which may be mentioned (1) the actual destruction, by the heating effects of the current, of most evidence as to the cause of the fault; (2) the destructive effect of the current upon the immediate surroundings of the cable; and (3) the risk of fire consequent upon the test. The only merit of such a system is its time-saving quality, in that it affords immediate visual evidence of the locality of a fault.
Short-circuits, due to temporary accidental contacts between two cables, may often be localised by this method, the second cable being connected in place of the earth, whilst faults in concentric cables between the inner and outer conductors, or between the outer conductor and earth, may be found in like manner. In the former case, if a partial contact exist between the two conductors, it may often be located by passing a steady current of some ten to twenty ampères, regulated by a suitable resistance, through the fault, the said current being maintained for a definite period, will cause an appreciable rise in the temperature of the cable at the fault owing to the imperfect contact at that point, and this rise will ultimately extend through the whole substance of the cable for a short distance on either side of the fault, which may then be located by passing the hand lightly along the exterior of the cable in the expected vicinity of the fault, when the warmth of the cable at that point will afford sufficient evidence of its immediate locality.
A total or partial disconnection in an insulated cable or wire, where the broken extremities are maintained in imperfect contact by the surrounding insulation, may be localised in like manner by connecting the two extremities of the faulty section through a suitable resistance or cut-out to a generator, when the arc set up at the fault, or heating due to the high resistance offered to the passage of the current at that point, will give sensible evidence of the position of the disconnection.
In all cases cited above, if the resistance of the fault be sufficiently great to oppose the passage of an ordinary low tension current, a higher voltage, varying from one to five thousand volts, according to circumstances, will frequently overcome this obstacle, and similarly assist in the localisation.
I wish it to be distinctly understood that in mentioning the above instances for emergency fault localisation in electric circuits, I do not recommend their general adoption, but have rather included them in this series to illustrate what can be done, and what to a limited extent is done, especially in American and Continental practice, in order to get over the difficulty caused by a temporary breakdown in mains and circuits. In all the cases referred to in the foregoing paragraphs thoroughly reliable safety precautions should be adopted; the tests should only be applied to circuits which are available for immediate inspection; and, above all, they should be carried out under the immediate supervision of experienced men.
The fault or faults having been duly located and repaired, the next proceeding is obviously to test them (the repairs or "joints," as they are commonly termed) in order to ascertain if they have been satisfactorily carried out, and this we will now proceed to discuss under the heading of Joint Testing.
Clark's Accumulation Method is illustrated in Fig. 85, the apparatus required for its conduct being a well-insulated trough T, the insulation of which may be effected by suspending it from a convenient support through the medium of ebonite rods; a sensitive high resistance gal
vanometer G, a testing battery E, capable of yielding from 200 to 300 volts at least, a condenser C, and à Webb's discharge key K. The perfect insulation of the trough must be first ascertained, and, to this end, the pole of the battery which, in the figure, is connected to the extremity of the core containing the joint, is connected to the plate P which is immersed in the water in the trough, and the key K is depressed, thus charging the condenser C. The battery is then disconnected, and,
after an interval of, say, one minute, K is released, and the discharge deflection noted; it should be practically equivalent to the deflection obtained when C is instantaneously discharged at the completion of the charging period.
The insulation of T being thus assured, the joint is immersed, care being taken to clean and dry the insulation on either side for the space of a few inches to prevent surface leakage, and connected with the remainder of the apparatus as indicated in Fig. 85. All being in readiness, the short circuit plug of the condenser C is inserted, and the key K depressed; still keeping K down, the short circuit plug is withdrawn, and the condenser immediately receives a charge from E through the joint. At the end of a stated period of, say, one minute, the key K is released and the discharge deflection noted. This should amount only to one or two degrees, and is compared with that obtained when a perfect piece of core is substituted for that containing the joint. The deflection obtained from the former should not be greater