In the diagram, [fig. 617], assume the length of the cable AE to be 550 yards of 25,000 cir. mil., EF, 500 yards of 40,000 cir. mil., and FC, 1,050 yards of 30,000 cir. mil. These lengths must be reduced by calculation to equivalent lengths of one size, and for this purpose it is best to select the largest size. The results of this calculation are as follows:
550 yds. of 25,000 cir. mil. = 880 yds. of 40,000 cir. mil.
500 " " 40,000 " " = 500 " " 40,000 " "
1,050 " " 30,000 " " = 1,400 " " 40,000 " "
This makes the total resistance of the loop equivalent to 2,780 yards of 40,000 cir. mil. If the contact show a balance for a reading of 372.5, this indicates that the fault is at a distance of 372.5/1,000 of 2,780 = 1,035.5 equivalent yards. Of this, 880 are in the stretch A E. Consequently the fault is:
1,035.5 - 880 = 155.5 yards from E.
Fig. 618.--The Fischer portable cable testing set, designed for locating crosses, grounds and breaks in cables, also for conductor and liquid resistance measure. The distinguishing feature of the set is the master switch. By means of this switch, connections can be made for the various tests by a single movement, thus avoiding the labor and time which have to be expended in interchanging the connections and memorizing the rather complicated scheme of connections.
CHAPTER XXVIII
AMMETERS, VOLTMETERS AND WATTMETERS.
An ammeter or ampere meter is simply a commercial form of galvanometer so constructed that the deflection of the needle indicates directly the strength of current in amperes. A good ammeter should have a very low resistance so that very little of the energy of the current will be absorbed; the needle should be dead beat, and sufficiently sensitive to respond to minute variations of current.
According to the principle of operation, ammeters and voltmeters are classified as: