396 



ALTERNATING CURRENTS 



should be practically an open circuit when the line is at normal 

 voltage. They should provide an easy path to ground for the 

 discharge. They should be able to absorb the energy of the 

 discharge. They should be able to suppress the dynamic arc 

 which follows the transient discharge and which the power of 

 the system tends to maintain. 



One type of arrester for low voltage is shown in Fig. 360. A 

 number of cylinders made of non-arcing metal are connected 

 between the line and ground. There is a small air-gap between 

 adjacent cylinders. A carbon rod of high resistance is shunted 

 from the conductor across approximately three-fourths of these 

 cylinders, a medium resistance across approximately one-half the 



FIG. 360. General Electric multigap lightning arrester. 



cylinders, and a comparatively low resistance across a little over 

 one-quarter of the cylinders. The cylinder spacing is such that 

 the full line pressure which exists across the last five cylinders 

 cannot jump the series gaps. However, any considerable 

 increase of line voltage causes a discharge through the resistance, 

 across these five gaps and thence to ground. If the discharge 

 becomes sufficiently heavy, the voltage drop through the high 

 resistance becomes excessive and increases the voltage across 

 the next four gaps which then break down and assist in the 

 discharge. In a very heavy discharge, the voltage drops across 

 all the resistances become large and the discharge passes to 

 ground through the entire series of gaps. When the line returns 

 to normal voltage, the cooling effect of the large number of 

 cylinders, combined with the rectifying property of their metallic 

 vapors, tends to prevent the dynamic arc being sustained. Such 

 arresters are suited only for low voltages (up to 5,000 volts), and 

 can absorb only small amounts of energy. 



