LOW-FREQUENCY INDUCTION 



575 



Fundamentals of Problem — Magnetic Induction 

 Induction arises due to the fact that any wire transmitting electricity 

 is surrounded by electric and magnetic fields which may cause voltages 

 to appear on other wires in these fields. The relative strengths of 

 the electric and magnetic fields depend on the characteristics of the 

 circuit, the former being a function of the voltage on the circuit and 

 the latter a function of the current in it. Induction due to electric 

 fields is commonly called "electric induction" while that due to 

 magnetic fields is called "magnetic induction." 



When a ground occurs on a power line there are two factors which 

 influence the induction into neighboring telephone circuits: 



(a) The residual voltage is increased, which increases the electric 



induction. 

 ih) The residual current is increased, which increases the magnetic 



induction. 



Both from theoretical analyses and experience it is known that mag- 

 netic induction is more important than electric induction in most cases 

 of low-frequency induction. Consequently, the demonstration is con- 

 cerned only with magnetic induction, i.e., induction due to the power 

 system currents. 



The magnetic field about a wire faulted to ground and carrying fault 

 current is shown in Fig. 1. This magnetic field varies in proportion 



Fig. 1 — Process of induction from currents. 



to the current in the wire causing it. If other conductors, such as 

 the pair of telephone wires shown in Fig. 1, lie within this field there 

 are induced along them voltages proportional at every instant to the 

 time rate of change of the magnetic flux which links the wires. 



