For example. Each open arc lamp requires about 50 volts. In the system shown in [fig. 783], the pressure measured across the brushes of the dynamo is assumed to be 1,000 volts. As this current flows through the circuit 45 volts will be actually lost in each lamp, and as the drop on the line wire is usually about 10 per cent. of the total voltage, there will be a drop of 5 volts on the conductor between any two lamps. In the circuit shown, there are twenty lamps, therefore, the difference in pressure between either terminal of the dynamo and middle point A of the circuit will be 10 lamps × 50 volts = 500 volts. Likewise, the difference in pressure between any two points on the circuit will be equal to 50 volts multiplied by the number of lamps included between them.
Fig. 783.—Series system of distribution. This is a constant current system, so called because the current remains practically constant. It is used chiefly for arc lighting.
Ques. Describe the danger in a series arc light system?
Ans. Since the total voltage of the system is equal to the sum of the volts consumed in all of the lamps, it is high enough to be dangerous to personal safety.
This is illustrated in [fig. 783]. If the line be grounded at B owing to defective insulation, the pressure of the circuit at that point will be zero, and therefore, a man standing on the ground could touch that point without receiving a shock, but if he should touch the line at the point C, he will receive a slight shock of 150 volts, as there are three lamps between the point C, and the ground connection B. Therefore, the danger of touching the circuit increases directly with the resistance between the point touched and the ground connection, so that if a man touch the circuit at the point D, he will receive a dangerous shock of 16 × 50 = 800 volts. In practice, sixty lamps are often placed on a single arc lighting circuit, so that its total pressure is about 3,000 volts, thus greatly increasing the danger of the system.
Ques. What is a constant current system?
Ans. The series system is a constant current system, and is so called because the current remains practically constant, while the voltage falls throughout the circuit in direct proportion to the resistance.
Ques. What are the principal applications of the series system?