to the latter diagram, a and b are two fan-shaped sheets of ebonite forming the vertical sides of the switch, and around the inner circumference of which are fixed a series of metallic contacts, al, a2, a3, a4, all, b1, b2, 63, and 64. The (a) contacts are severally connected with the (b) contacts by the successive movements of the lever c, the sole office of which is to thus form a path of negligible resistance between these points; in other words, the lever forms a circuit key severally connecting them.

Of the external connections, A and B are the galvanometer terminals seen to the front of the complete illustration, of which A also provides a means for connecting the standard high resistance for insulation testing, the standard condenser for capacity measurement, and subsequently the extremity of the cable itself under test, respectively. G is the galvanometer, and S a set of shunts, wound with platinoid wire, and contained in the cylindrical ebonite covers seen in the front of the complete switch above. These shunts are arranged on the “Universal" principle, the various points of contact being connected to the studs a4, a5, a6, and a7, yielding 1-10,000th, 1-1,000th, 1-100th, and 1-10th shunts respectively, and, being constructed on the above principle, the switch is available for use with any galvanometer, without altering the respective multiplying powers of these shunts. E and El are the batteries for insulation and capacity measurement respectively, whilst the earth connections are arranged as shown. A subsidiary short-circuiting device upon the switch lever, with corresponding galvanometer contacts upon the inner surface of the front of the switch, serves to short-circuit the galvanometer during the initial charging period in insulation testing, the short circuit being removed as the lever moves forward over successive shunts, thus allowing the electrification readings to be taken.

Taking the various a and b contacts in turn, we will proceed to discover what happens when the connecting lever is moved over the range of the switch. Firstly, when connecting al and b1, the terminal A, and in consequence the cable or condenser connected between it and earth, is charged electrostatically from the battery El; on moving the lever so that it connects a2 and 62, nothing happens unless a shunt be required in taking the

capacity throw, in which case the shunt required is brought into circuit by two small ebonite-handled plugs connected by a short length of flexible wire, one of which is inserted in the hole on the exterior face of the switch, corresponding to the shunt required, and the other in the hole corresponding to contact a2, thus completing the circuit indicated by the dotted line in the diagram of connections.

If no shunt be required in taking the capacity throw, the lever is moved so as to connect a3 and 62, thus discharging the cable or condenser. When the lever is next moved to connect a3 and 63, one pole of the insulation testing battery E is connected to A, and consequently to the cable if connected, the opposite pole being earthed; simultaneously the galvanometer is short-circuited by the device before alluded to, and so protected from injury due to the rush of current, on moving further to positions a4, a5, a6, and a7, connecting them respectively with b3, the 1-10,000th, 1-1,000th, 1-100th, and 1-10th shunts are successively brought into circuit, and the short circuit is at the same time removed from the galvanometer terminals. The connection of a8 and 63 gives the position of "no shunt; " then, the electrification readings having been duly noted, the lever is next moved to connect a9 and 64, when the cable will commence to discharge itself. Then, the initial rush of the discharge current being over, the lever may be moved to connect a10 and 64, when the necessary shunts for taking the "earth readings" may be introduced as in the case previously mentioned, when describing the capacity test with this apparatus, by inserting one of the plugs in the hole corresponding to the shunt required, and the other in that of a10. Finally, with all and b4 connected, the cable discharges itself in the "no shunt" position.

The capacity battery El is connected to the switch contact through a small plug switch at the back, so that it can be disconnected, if desired, by simply withdrawing the plug.

In brief, the mode of using this switch for capacity and insulation testing is as follows:-The testing batteries, galvanometer, and earth connections having been made as shown in the figure, the switch lever is moved over to the "no shunt" insulation position, and the leakage deflection, if any, noted; if used under ordinary

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 b1, 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 а standard high resistance, such as one megohm, for example, the remaining terminal being earthed as 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, al0, 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

5.2, 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 known 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
Blavier's Method.

A≤

г

C

B

FIG. 69.

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 r 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 2. Then rr2 √(rl r2) (R — r2).

of

Kingsford's Method, which is a a modification 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 R1 into the circuit, it being connected to that end of the cable which is nearer to the fault C.