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dry atmospheric conditions, there should be no leakage deflection with the testing voltage usually employed. The lever is then brought back to the position marked "Discharge at the left of the front plate, and one side of a standard condenser is connected to terminal A, the other side being earthed.
The short circuit plug being removed from the condenser, the lever is brought to position 1, connecting al and bl, and thus charging the condenser. The charging should last for a definite period, such as 30 seconds, and, no shunt being as a rule required in taking the standard capacity throw, the lever is next brought to a3 and 62, and the deflection noted.
The next step consists in taking the insulation constant, and, to this end the standard condenser is replaced by one terminal of standard high resistance, such
for example, the remaining terminal being earthed before. The lever is then moved over the shunt studs, a4, a5, a6, and a7, until a suitable deflection (the largest possible) is obtained and noted, together with the multiplying power of the shunt used. These two quantities, viz., the deflection and the multiplying power of the shunt, multiplied together and by the value of the standard resistance in megohms, which, in the case cited above, will be unity, constitute the insulation constant into which are divided the respective deflections obtained for the various cables, giving the insulation resistance thereof in megohms.
The constant having been duly ascertained in this manner, the standard resistance is disconnected from A and replaced by the extremity of the cable under test, the opposite extremity being free. The same operations are then repeated to obtain the respective deflections due to the capacity and insulation resistance of the cable, the latter being taken, as usual, at the end of one minute from the time of charging the cable. If earth readings are required, the extended movements of the lever over contacts a9, alo, and all are made as before described, and the ensuing discharge deflections duly noted, having regard to a similar time interval.
This switch forms a compact and useful combination for both stationary and portative purposes, dispensing
with shunt boxes, short circuit, battery, and condenser keys, all of which are included in its design.
The lever is provided with an insulating ebonite shield, as shown in the figure, which prevents the hand from inadvertently touching the upper contacts, and so receiving an unpleasant shock. The whole structure is well insulated on a corrugated ebonite base piece, and in a special design for outside work has been provided with a weather-proof cover, the lever movements being effected by means of an external handle at the centre of the switch front, which is attached to the axis of what is normally the lever handle.
We will pass on now to a consideration of various methods for the
Localisation of Faults on electric circuits, the preliminary treatment of which has already been dealt with in the preceding paragraphs under the heading of Continuity or Circuit Test.
Most of the tests for fault localisation which are to be dealt with in the following paragraphs are the outcome of the requirements of the electrician who has to deal with submarine cables, in that they refer more especially to long cable circuits of which both ends are not available at the testing point, but, nevertheless, there are several methods amongst those to be described, such as the “Loop” tests, for example, which are equally applicable to short lengths of cable or local circuits.
The simplest fault to localise is that due to a complete break in a cable or circuit, and consists of a simple resistance measurement. Thus, the original ohmic resistance of the circuit being known, if we measure the resistance between one extremity and the fault, as represented by earth if the cable be submerged, it only remains to divide the resistance thus obtained by the known resistance of unit length of the circuit, to determine the distance of the break from the available extremity in terms of that unit length.
For example, let us suppose that the original resistance of a uniform circuit 100 ft. in length was 20 ohms, then the resistance of one foot will be .2 ohm. If now, on measuring the resistance between one extremity and earth, we obtain a value of 5 ohms, we know that the distance of the break from that extremity will be
.2, or 25 ft. This method is, of course, only applicable in cases where the fault itself is making good earth, the resistance of which is negligible. As such is seldom the case, we must proceed to deal with those examples in which the earth due to the fault is not perfect, but offers an appreciable resistance to the passage of the current. These are ki wn as
Partial Earth Faults. The various methods for the localisation of such faults are comparatively simple in principle of application, but are rendered somewhat more difficult in practice, especially in the case of submerged circuits, owing to the existence of earth currents, variation in fault resistance owing to chemical and electrolytic action, etc., etc.
We will deal with them in turn, commencing with
Let A B, Fig. 69, be a uniform circuit of which the total resistance R is known, and on which a partial earth fault exists at the point C. The extremity B is first insulated, and a resistance measurement taken between A and earth, the result of which gives us the resistance m of A C plus the resistance of the fault, which total we will call rl. The point B is then earthed, and a second resistance measurement made from the point A, earth being, as before, used as the return, which gives a result r2. Then r = r2 ✓(rl – r2) (R – 12).
Kingsford's Method, which is a a modification of Blavier's, ensures the passage of an equal current through the fault in the second case to that which flows through it in the first instance, when the extremity B is insulated. This fact is ensured by the introduction of a resistance Rl into the circuit, it being connected to that end of the cable which is nearer to the fault C.
Then r = r2 - Vrl - r2 - RI) (R - 12).
The mode of procedure consists in making Rl any value at random, and then obtaining rough values for rl and r2. The value of r resulting from this rough test is then obtained from the above equation, and substituted in the following ; R1 =
r (rl – r)
which, worked out,
R will give an approximate value for Ri. The value thus obtained is reproduced upon the actual resistance, and the test repeated several times until the value obtained for R from the above equation is sufficiently approximate to the actual dimensions of R1 in the test to ensure a fair degree of accuracy in the results. When this point is reached, approximately the same current flows through the fault in both cases.
The Overlap Method is somewhat similar to the foregoing tests, but involves separate measurements from either extremity, A B, Fig. 69, of the cable. The respective extremities A and B are insulated in turn, whilst the tests are being conducted; thus B insulates whilst A is testing, and vice versa. R, rl, and r2 standing for the
R + r1 same quantities as before, r =
2 Fahie's Method has for its object the elimination of polarisation and variation in the resistance of the fault caused by the corrosive action of the sea water in which it is immersed. It involves the employment of two galvanometers consisting of a simple astatic system provided with an index finger suspended by a silk fibre over a horizontal graduated dial, and protected from external disturbing influences such as would be produced by currents of air, by a glass shade. An ordinary reflecting galvanometer is too sensitive for this test. G and Gi, Fig. 70, represent the two galvanometers connected as shown, whilst A, B, and C are the proportional and adjustable arms respectively of an ordinary P. O. bridge. K is a key of the familiar Morse type, the lever of which is connected to the line under test, and the remaining two contacts to the bridge and G1, as shown in the figure. E is the testing battery of from 50 to 60 cells.
The modus operandi is as follows:--The approximate resistance of the line is first ascertained in the usual manner, K being in contact with the bridge, and the
value obtained is left unplugged in C. K is then depressed so as to disconnect the line from the bridge circuit, and connect it instead to earth through Gl. The
resultant deflection is a measure of the cable or polarisation current due to the fault, and. we must proceed to neutralise it in the following manner :-The infinity plug being withdrawn from the bridge, all values in B are plugged up, and a current of opposite polarity to the cable current sent into the line by depressing the battery key, K being on the bridge contact. This current is kept on until a deflection in the opposite direction is obtained on G1, when K is again brought into contact with it, thus showing that the fault has been polarised in the opposite direction. When such is the case, K is kept in contact with Gl until the needle comes to zero, at which instant K is again put to bridge, and the infinity plug having been meanwhile replaced, the galvanometer key is closed, and the resultant displacement from zero noted. C is then readjusted, and the various operations repeated until a value is obtained for C at which, when the cable current is neutralised, the galvanometer key closed, and the bridge circuits connected, the needle of