For systems not permanently earthed anywhere, these instruments are nearly all based on a measurement of the pressure difference between each pole and earth, two measurements being required for two wire systems, and three for three wire, whether direct current single phase, or polyphase alternating current. In the case of direct current systems, the insulation, both of the network and of the individual lines, can be calculated from the readings, but with alternating current, the disturbance due to capacity effects is usually too great. In any case, however, the main showing the smallest pressure difference to earth must be taken as being the worst insulated.
For low tension systems moving coil (for alternating current) or moving iron instruments (for direct current) are the most used, while for high tension systems electrostatic voltmeters are to be preferred. On systems having some point permanently earthed at the station, as for instance the neutral wire of direct current system, or the neutral point of a three phase system, an ammeter connected in the earth wire will serve as a rough guide. It should indicate no current so long as the insulation is in a satisfactory state, but on the occurrence of an earth it will at once show a deflection. The indications are, however, often misleading, and serve more as a warning than anything else.
Fig. 2,580.—Westinghouse single phase electrostatic ground detector.
Fig. 2,581.—Westinghouse three phase electrostatic ground detector.
Fig. 2,582.—Wallis-Jones automatic earth leakage cut out. It is an instrument which so protects a direct current circuit that the circuit is broken whenever a leak occurs from either main to earth, and so that the circuit cannot be permanently re-established until the leak has been removed. The instrument and its connections may be explained by the aid of the accompanying diagram, in which T1 and T2 represent the points of connection from the mains, and T3 and T4, the points of connection to the circuit to be protected. So S2, and S3 will preferably be ordinary tumbler switches, but they are diagrammatically represented as plain bar switches, their fixed contacts being diagrammatically represented by dotted circles. When the three switches S1, S2, and S3 are closed, the current passes from T1 to T3 through the small resistance R1, through circuit L to T4, and back through the resistance R2 to T2. In shunt with R1 and R2, are the two moving coils C and C2, working in the magnetic field of the magnets NS, NS, and rigidly fixed on one spindle, which is broken electrically by an ebonite block E. The points of connection to the shunts are adjusted so that when the same current passes out through one and back through the other, the effect on the two coils is equal and opposite, and there is thus no movement. Should, however, any minute portion of the current through R1 leak to earth instead of returning via R2, the balance is disturbed, C becomes stronger than C2, the system is deflected, and a contact is made by the arm A at B, no matter in which direction the coils deflect. The system is similarly deflected for a leak on the other lead. In the diagram these contacts are shown at right angles to their normal plane. As soon as the contact is made, the electromagnet M is energized, the arm of S1 is released and the spring at once pulls it off its contact, at the same time breaking S2. The positions of the blades when the switches are open are shown dotted. The only means the user has of closing the circuit is by putting on S3 by the handle H, which is outside the locked box. The first effect of putting on S3 is to break its circuit; it then by means of the slotted bar P begins to pull on S2 and S1, which can thus be closed again, and held closed by the trigger as before. The circuit, is, however, still broken till S2 is pushed back. Then if the leak be still on, the slot in P allows S1 and S2 to open at once as before. It is therefore impossible to keep the circuit closed while the leak exists. The working condition of the instrument can be tested at any time by switching a lamp on in the circuit and depressing one of the keys K1, K2. This short circuits R1 or R2, throws the coils out of balance, and the switch opens. The contact arm is closed in an inner dust tight case, and it will be noted that it makes contact only; the break occurs at the switches, thus avoiding any sparking. Since the two coils work in the two gaps of one and the same field, changes in the strength of the magnets have no effect, the apparatus is enclosed in a locked metallic box, and the only part to which the user has access is the handle H, and, if desired, the testing keys K1 K2.]
HAWKINS PRACTICAL LIBRARY OF
ELECTRICITY
IN HANDY POCKET FORMPRICE $1 EACH