OVER-VOLTAGE PROTECTION 603 



combination, in which case the voltage at the front of both the 

 reflected and transmitted waves would be zero. Both these 

 devices are patented. 



The above has dealt with the excess voltages which could 

 occur when a line is connected to a source of energy. Dangerous 

 voltages are, however, also liable to be set up when a loaded or 

 short-circuited line is suddenly broken. In this case the voltage 

 rise depends on the value of the interrupted current, and the rapid- 

 ity with which the circuit is broken, and again on the natural 

 impedance of the circuit. 



It was previously shown that the energy of a circuit was stored 

 in both the magnetic and dielectric fields, corresponding to the 

 current and voltage values. At a certain instant, therefore, the 

 two stored quantities are equal, while if the current is zero all the 

 energy must, of course, be stored in the dielectric field and vice 

 versa. We thus had: 



2 2 ' 

 and the relation between voltage and current 



IE. 



-\E- 



For transmission work the ratio 



-^ = 138 log ohms, 



C T 



and this value generally falls between 400 and 200 ohms. For 

 transformers, however, it is considerably higher, being around 

 3000, while an underground cable has a much lower natural im- 

 pedance than an overhead circuit. 



For example, if in a circuit having a natural impedance of 

 400 ohms, a current with a maximum value of 200 amperes is 

 suddenly broken, the surge pressure cannot exceed 200X400 = 

 80,000 volts, because this is the maximum value of the voltage 

 wave which is necessary for storing in the dielectric field the whole 

 amount of energy which was previously stored in the electro- 

 magnetic field. 



Traveling waves similar to the above are also set up by atmos- 

 pheric lightning phenomena. The gradual accumulation of static 

 charge on a line from the neighboring atmosphere increases its 



