« PreviousContinue »
lation. More especially is this the case with single wires uf small gauge insulated with india-rubber. Such breaks are very easily located by treating the two severed portions as separate condensers, charging each of them in turn for a definite period with a battery of sufficient E.M.F. to give a suitable discharge deflection, and then noting the discharge deflection, which we will call d and dl respectively. Let L be the total length of the circuit, and I the length of that portion giving, say, the deflection d, then the distance of the break from that extremity which was connected to the galvanometer when
Ld d was obtained is given by the formula, 1
d+ dl The connections for the test will be the same as those employed in the measurement of electrostatic capacity by the direct deflection method, the two extremities of the broken circuit taking the place of the standard and condenser under test respectively, and a good earth is necessary for satisfactory working.
Most of the foregoing methods for the localisation of faults on electric circuits apply more especially to those cases where the cable or line has been laid or fixed in position. If, on the other hand, the cable be wound on a drum or bobbin at the factory, and a fault have developed, say, in process of manufacture, several very efficient methods are available for localisation by running the cable over from one drum to another or into a tank of water, and the fault may thus be found and repaired on the spot, where everything required for the purpose is at hand, thus simplifying the matter considerably.
We will proceed to deal with these methods in turn by considering the manner of their conduct in practice.
A very simple method of localising faults of appreciable magnitude in a length of core when available for manipulation in the manner described above, is represented in Fig. 80.
The coil of wire under test is wound over from the drum D on to the drum Di through water in an insulated trough T. One extremity of the wire is connected by a suitable well-insulated commutator and brush on the spindle of drum D with one pole of a battery E, the remaining pole of which is connected through the gal
vanometer G with the water in the trough T. When the fault passes into the water in the trough, it affords a direct path for the current from E, and a deflection is, in consequence, produced on G. It is necessary for this test that the outer surface of the core under test
should be primarily dry, as otherwise the leakage through the fault and over the surface of the wet tape or other external covering will give rise to a permanent deflection on the galvanometer, and so disturb the definition of the test.
Warren's Method, as performed with a sensitive galvanometer, is indicated in Fig. 81, where D and Di, as before, indicate the two drums, which should be of metal, or metal sheathed, in order to provide a ready path for the leakage current from the wet surface of the coil.
The core is wound on the two drums, as indicated, and a portion, a b, some 6 ins. in length, between the two drums, is cleaned and dried. The metal or sheathing of each drum is then put to earth in turn, by means of the keys K and K1, and the respective deflections, with due allowance for electrification, and the length of core on each drum, are noted; the larger deflection indicates, as a rule, the drum on which the fault lies, it being that drum which was earthed at the time. The core is then wound off that drum for a certain distance and the test repeated, until the fault lies between the drums. The whole portion from D to D1 is then cleaned, and an earth wire passed slowly along (a moist rag or sponge connected to earth answers very well), until the deflection is again obtained, thus indicating the exact location of the fault. Needless to state, the drums D and D1 require to be well insulated in the first instance, a very good plan being to mount them on wooden platforms provided with four feet in the shape of porcelain telegraph insulators, in
verted, and secured to the platform, or, better still, special oil insulators may be used, and form a very effective mode of insulation.
Jacob's Method is illustrated in Fig. 82, where D and D1 represent two metal sheathed drums, one of which, D, is directly connected to earth, and the other, Di, is
insulated in itself, but connected to earth through the galvanometer G. E is the battery, one pole of which is connected to the extremity of the conductor as shown, and the other to earth. The core is wound over from
D to D1, and, while the fault is on D, all the leakage current from it passes to earth viâ the sheathing of D, but, as soon as it arrives on Di, the leakage current, or at least part of it, passes
through the galvanometer G, and a deflection results. The exact position of the fault can, as in the previous case, be located by passing the galvanometer connection, in the form of a moist rag, over the surface of the core in the immediate vicinity of the fault, it being arranged between the drums for that purpose.
A similar method is depicted in Fig. 83, where D and Dl represent two insulated drums as before, E the battery connected between the conductor under test and earth, and G the galvanometer connected between the drum Dl and earth. R is a moist rag or sponge connected directly to earth. The core is wound over from D to D1, all leakage from the fault on D going to earth viâ R, until the fault passes over, when part of the current passes to earth through G, producing a deflection; the winding is then stopped, and the fault located by gliding R slowly along the surface of the insulation.
Another method is illustrated in Fig. 84, and consists, as usual, in winding the core on the two metal or sheathed drums D and Di, both of which are insulated and connected with a switch S, by means of which they can be put to earth, either independently or in conjunction with one another, as required. T is a tank of water through which the core is passed, its position during working being maintained by passing it under a smooth grooved pulley (a shackle insulator answers the purpose admirably) at the centre of the tank. vanometer G and the battery E are connected between the tank and the extremity of the conductor. The mode of procedure is' as follows: D and D1 are earthed inde pendently in turn, and the resulting deflection from one or the other is an indication of the drum on which the fault lies. Thus, if Di be earthed, and a deflection results, it is caused by a leakage current from the fault on D, which, travelling by way of the moist surface of the tape or other external coating, returns to the battery viâ the tank and galvanometer, its only course, since D is insulated, and vice versâ. The choice of drums having been made in this manner, both D and D1 are earthed,