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tricity in being forced through an air-space meets a high resistance, and the energy required to force the current through this resistance is transformed into intense light and heat. The heated air and volatilized substances make of the space a conductor to some extent, but one with a comparatively high resistance.
Lightning discharge currents do not act the same as the currents from dynamos. They do not always take the path that would be taken by a dynamo current, but frequently jump across spaces of air rather than go round corners and bends in conductors. It is consequently important that the path from the lightning-arrester to the earth be straight, so that the lightning discharge will not cause damage on its way to the earth. Though arresters are made to break automatically the connection with the earth as soon as the discharge current has passed, a heavy discharge may prove too much for the arrester and the arc be long continued. For this reason, everything about the arrester should be non-combustible. Since parts of the arrester have good connection with the earth, special care must be taken to prevent leakage, and consequently the base on which the parts are mounted should be of a material that will not absorb moisture.
The two earths are required to assure good and sufficient connection with the earth. It is bad practice to connect to pipes in a building, because there is then no certainty that the discharge current will go to the earth without damage in some unlooked-for place.
7. Testing :
a. All series and alternating circuits must be tested every two hours while in operation, to discover any leakage to earth, abnormal in view of the potential and method of operation.
b. All multiple-arc low-potential systems (300 volts or less) must be provided with an indicating or detecting device, readily attachable, to afford easy means of testing where the station operates continuously.
C. Data obtained from all tests must be preserved for examination by insurance inspectors.
These rules on testing to be applied at such places as may be designated by the association having jurisdiction.
A “ series circuit ” is one in which everything in the circuit, — dynamo, wire, lamps, etc., — comes one after the other, “ in series”; the same current going through each. (Fig. 19.)
The electrical pressure necessary to force the current through such a circuit is usually high, because each device requires a certain pressure, and the total pressure on the circuit must be the sum of the pressures required by the different devices. The high pressure, of course, makes leaks more probable and more disastrous. (Page 31.)
In “alternating circuits” the current alternates in direction. Such a circuit has peculiar properties that make it possible, where branches are taken off, to change
Fig. 19. - Series Circuit, showing Same Current passing
through Lamps and Motor.
the pressure with transformers to anything desired. With alternating circuits the pressure between the wires that leave the station is high, and this system is consequently classed with series systems. (Page 154.)
FIG. 20.- Multiple Circuit, showing Lamps and Motor working
with Constant Pressure.
In the “multiple-arc" system, or, better, the multiple system, there is a constant pressure between the main wires, and the different devices are connected-in between the wires, each device operating at the full press
ure of the system and taking only such current as the resistance of the device allows. (Fig. 20.)
The pressure between the mains of a multiple system - except alternating circuits—is usually below 300 volts, and is consequently “low," but there are often multiple circuits on which motors are operated, that have a pressure of 500 volts. (Page 36.)
“Detecting” or “indicating devices” are arranged to operate by the leakage current. A simple form that will serve as an illustration, is as follows:
Suppose that in Figure 21, A and D are the main wires of a multiple system, and that I and l' are two incandescent lamps connected-in between the mains, but in series. They will not burn brightly, because instead of each lamp's getting the full pressure, each gets only one-half. At B suppose there occurs an accidental connection with the earth. If now there be a permanent ground-connection at C, the current that passes through I does not all go through l', but can partly, at least, leak through the earth, up to B and thence to D. As a result, less current than before the accidental connection occurred will go through l' and more than before through l. This will, of course, make 1 brighter and l' less bright. Thus when the left-hand lamp burns brighter, there is a leakage to the ground on the right-hand wire, and when the right-hand lamp burns brighter, there is a leakage on the left-hand wire. A lamp may also be put in the wire leading to the ground. This lamp will then light up more or less, according to the amount of the leakage.
MOTORS. 8. Motors :
a. Must be wired under the same precautions as with a current of the same volume and potential for lighting. The motor and resistance-box must be protected by a doublepole cut-out and controlled by a double-pole switch, except in cases where one-quarter horse-power or less is used on low-tension circuit, a single-pole switch will be accepted.
6. Must be thoroughly insulated, mounted on filled dry wood, be raised at least eight inches above the surrounding floor, be provided with pans to prevent oil from soaking into the floor, and must be kept clean.
C. Must be covered with a waterproof cover when not in use, and, if deemed necessary by the Inspector, be enclosed in an approved case.