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Switchboards for buildings.
strument to measure the whole current; but as to the voltmeter, the additional complication would not be necessary, since any one of the four voltmeters already connected with the circuit would serve the same purpose. The voltmeter V, located in the center of the top row, is what is commonly called a ground detector, and is simply an instrument provided to test the insulation of the circuits before the current is turned on. The ground detector is not kept in service all the time, but is generally connected permanently either to the ground or to the circuits and is provided with a small switch, by means of which the other side may be connected whenever it is desired to make a test. If the permanent connection is with the ground, then the switch makes the connection with the circuit, and then if there is a leak in the insulation at any point, the fact will be revealed by the movement of the indicator of the instrument.
The switchboards we have discussed so far are arranged for the simple two-wire system, which is the one generally used for wiring buildings provided with a lighting plant; but for distribution from central stations, and also for large isolated plants, where the current is conveyed to a considerable distance, the three-wire system is used, and this will be explained in the next section.
A distributing circuit can be arranged for the threewire system if we have two generators or any multiple of two, but it cannot be obtained with a single machine. If we have only two machines, the switchboard becomes a simple structure; but if there are four, six or more machines, then there will be more or less complication, for we will have to divide the generators into two groups, each one of as nearly the same ampere capacity as pos
sible; and each one of these groups will have to be connected with an independent set of bus bars, so that the machines may be properly equalized. From these two sets of busses, connections will have to lead to the main distributing busses with suitable switches, cut-outs, etc. By explaining simple arrangements, the course of procedure in more complicated cases can be better understood. We will, therefore, start with the consideration of the board presented in Fig. 23, which is arranged with two generators connected so as to feed into one system of distributing mains.
As will be noticed, the generators are not provided with terminals for equalizing wires, none being required. Generator GI connects by means of wires e and f with the circuit breaker c b, which latter connects with the lower contacts of the switch Si. The upper contacts of this switch connect with bus bars 2 and 3. The wires g and h lead to the ammeter Ami. Generator G2 connects with switch S2 and with busses 1 and 2, a connection being made with ammeter Am2 by means of wires g' and h'. As will be noticed, both generators are connected with bus 2, and, while the f wire of the first machine connects with this bus, the f wire of the second one connects with bus 1. In the same way the e wire of the first generator connects with bus 3, but the e wire of the second one connects with bus 2. This latter bus, therefore, is the neutral-wire bus.
Voltmeter Vi connects with busses 3 and 2, and voltmeter V 2 connects with busses 1 and 2. These two instruments indicate the voltage between the neutral wire and the side of the system with which the generators they indicate are connected. The third voltmeter V, is con
Generator for three-wire system.
nected across from ous i to bus 3, and thus serves to show whether the generators are properly connected. If they are, the volts indicated on this instrument will be twice as great as those of the other voltmeters, or to be more accurate, they will equal the sum of the other two readings. If the generators are not properly connected this third instrument will indicate a zero voltage, or the difference between the readings of the other two instruments. By means of the switch S the distributing circuits can be connected with the street supply.
When there are more than two generators they can be connected into a three-wire system by dividing them into two sets of equal ampere capacity. If we have three generators they cannot be connected into a three-wire system, unless one machine has double the capacity of the others, that is, in amperes. If there are four machines, these can be divided into two sets of two machines, and each set can be connected with equalized wires to an independent set of bus bars, and then these two sets of busses can be connected with a third set according to the three-wire system.
In Fig. 23, the lower set of busses can be dispensed with by running the connections from the generators directly to the left side contacts of switch S, making the same connections as are made with the bus bars.
Fig. 24 shows the simplest form of three-wire connections, being arranged for two generators to feed into a single distributing system and without street connection.