International projectionist (Jan-Dec 1935)

May 1935 INTERNATIONAL PROJECTIONIST X X Figure 6 the lower fuse and the clips into which it fits are 0. K., at least there is no open circuit, so the leads are touched to E and F. The results of this test are interpreted the same as the test from C to D. Sometimes it is easier to locate the trouble by testing while the load is connected. Assume that the load connected to B in Figure 4 draws 100 amps, when everything is normal. At this particular time the apparatus will not function, so the switch is opened and the test lamp is placed across the end of the fuses at B, and the lamp lights, showing the fuses to be 0. K! Since there are no other fuses between this point and the load, it is evident that something is wrong in some part of the apparatus or the wiring. It is convenient to start at the fuse block, so we close the switch, throwing the load back onto the line. Another test is made across the fuses at the end B, and still the lamp does not light. Let us analyze this condition to see just what happens. Our test lamp lights when the load is not connected; but when the load is thrown on, the lamp goes out. A test is now made across the fuses on the side nearest A and the lamp lights even with the load connected. When the load is hooked onto the circuit, the voltage across the B end of the fuses drops so low that the lamp willnot light, which means that the voltage probably drops to about 20, possibly less. We found the voltage across the other end of the fuses still was 110, so the trouble must be somewhere in the fuses or in the fuse block. Subtracting 20, the voltage at B, from 110, the voltage at the other end of the fuses, gives 90, the voltage-drop in the fuse block. Under these conditions there are about 1800 watts of energy consumed due to the resistance somewhere in the fuse block. This is approximately three times as much as an electric flatiron uses, so it stands to reason that a great deal of heat will be produced. This will be discovered very quickly — in fact, if the circuit is left in such a condition, the fuses will blow due to the heat. The point is that heat is generated and can be found quickly by touching the parts — carefully, of course, as they may be very hot. It should be kept in mind that cartridge fuses having round ends that fit into the clips often run quite warm when 50 to 60 amps, are taken from the line. That is why fuses of larger capacity are made with the knife-type contacts. Since we are discussing principles, and also because we cannot locate the exact point of the trouble by looking for the heated parts, we continue with the testing. Ninety volts is enough to light the test lamp quite well, so we test from C to F, in Figure 4. Nothing happens. This side of the circuit is probably O. K. Then the test is made from E to D and the lamp lights, not to full brilliance, but still quite brightly. Evidently this must be where the trouble is, because normally there should be no drop in voltage across these two points. The test leads can now be placed on the ends of the fuse, as at No. 4 in Fig. 7. The lamp probably will not light, since it is unlikely that a high resistance will develop in the fuse. Such a condition would cause the fuse to burnout almost immediately. Testing as shown at No. 5 (Fig. 7) from the terminal screw to the end of the fuse, quickly shows on which end the trouble is, since the lamp is now across all the parts at this end of the block. If the lamp does not light at one end, the test is repeated at the other as a check; and if it lights, we proceed to dig deeper. Test from the clip to the fuse, No. 6 (Fig. 7). If the lamp lights, we have found the trouble — a poor contact between the fuse and the clip. Continuing, one lead is placed on the part to which the wire is fastened and the other lead on the bare wire, as at No. 7 in Fig. 7. High resistance caused by a loose wire or corroded wire is found by this test. No. 8 (Fig. 7) shows the remaining point to be tested, that is, if high resistance is present between the clip and the part next to it having the screw in it under which the wire is fastened. By this time it should be apparent that the idea is to get the test lamp across the part of the circuit in which FIGURE 7 the high resistance exists. The exact location can be found in this manner. Of course, it is not logical to start the search by testing every small part of the circuit, step by step. In a large and complicated circuit such a procedure would take a long time. A large portion of the wiring, etc., is eliminated in the first few tests. Let us go through some of the steps again, without going into details. The test lamp across B in Figure 4 showed a presence of voltage. Next, the load was thiown on and there was no voltage at tnis point, that is, not enough to light the lamp. Testing across A showed that the full line voltage was present. We have made three tests and have eliminated everything on the line side of the fuses and probably everything from the fuse to the device itself. One more test showed that the upper fuse and the clips were not. the seat of the trouble. We proceed to the lower fuse and it is evident that the trouble is right around this portion of the equipment. It is possible that the exact seat of the trouble has not been found after ail the tests have been made as set forth. In such a case we must test across each piece of metal that was in the circuit when we made the test shown at No. 5 in Fig. 7. It sometimes happens that cracks and seams occur in manufactured articles. Suppose that one of the parts is cracked, possibly on the under side where it cannot be seen. One test lead is put on one end of the part, and the other lead on the opposite end. The lamp lights if the part is defective. It is seldom that all the tests outlined here would have to be made, the trouble being found after a few tests are made. HAVING discussed thoroughly the testing of a line and a fuse block, we now realize that testing apparatus must be suitable for the circuit to be tested. Often the testing device can be adapted to the circuit. An example of (Continued on page 39)