402 



ALTERNATING CURRENTS 



Porcelain has excellent mechanical and electrical character- 

 istics, but is more expensive than glass. Internal flaws are not 

 readily detected and cracks in the porcelain cause rapid deteriora- 

 tion of the insulator. Porcelain is practically the only material 

 used for insulators on high-voltage power lines. 



Patented compounds have good mechanical characteristics 

 and are readily moulded to any desired form. They cannot 

 withstand the severe mechanical stresses combined with the 

 electrical stresses and weathering encountered in power lines. 



In the larger sizes of pin-type insulator, the insulator is made 

 up in sections cemented together, Fig. 366. Pin-type insulators 



FIG. 366. Typical 77,000-volt, pin-type insulator. 



can be safely used for voltages up to about 66,000 volts, but for 

 these high voltages they are large, expensive, and produce ex- 

 cessive torsion in the cross-arms. 



171. Suspension-type Insulator. It seemed at one time as 

 if the insulator would limit transmission voltages, as the pin- 

 type had practically reached its limit in size, weight, and cost. 

 The introduction of the suspension-type insulator, however, has 

 raised the limit of transmission voltages to more than double the 

 value possible with the pin-type insulator. With the suspension 

 type of insulator, the conductor is suspended instead of being 

 rigidly supported. A string of suspension insulators is made up 

 of several units in series, the number of units depending on the 

 voltage. A single unit can safely operate at from 16,000 to 

 25,000 volts, depending on local conditions. Under normal 



