398 



ALTERNATING CURRENTS 



transient discharge. This results either in a permanent arcing- 

 ground or the destruction of the gap. 



169. The Aluminum Cell Arrester. The aluminum cell 

 arrester has proved to be the most reliable type of arrester and 

 is now universally used on large power systems. It is based on 

 the following principle: If aluminum be immersed in certain 

 electrolytes and a direct-current voltage be impressed on the 

 aluminum and the electrolyte, no appreciable current flows, 

 except for an instant. This is due to the fact that the current 

 builds up a very thin film of aluminum oxide on the plate, which 

 acts as an insulator. This film builds up with alternating 

 current as well as with direct current. The oxide, however, 

 constitutes an insulator and also a dielectric of almost infinitesi- 

 mal thickness. Therefore, considerable capacitance exists be- 

 tween the plate and the electrolyte. This would result in a 

 considerable charging current through the arrester, with alter- 

 nating current, if it were connected directly across the line. 



This film is an excellent insulator 

 up to approximately 340 volts effect- 

 ive. When the voltage exceeds this 

 critical value, the film breaks down 

 and allows a large current to pass. 

 When the voltage again drops below 

 this critical value, the film re-forms 

 and stops the current flow. Hence, 

 such a device is an electrical safety 

 valve. Its characteristics are there- 

 fore ideal for a lightning arrester. 

 Several cells are always connected in 

 series, the number of cells depending 

 on the voltage. The oxide films pre- 

 vent any discharge so long as the 

 line voltage is normal. If the line 

 voltage becomes abnormally high, the 

 aluminum films are broken down and 

 the discharge passes readily to ground. When conditions again 

 become normal, the films re-form and no power arc can follow 

 the discharge. 



In practice, the aluminum is in the form of cones, the proper 



P'IG. 362. Cross-section of 

 aluminum cell arrester.. 



