CARRIER TEl.EPllOXr OX IIIC.II rOl.T.ICF. US'ES 



171 



faiU, the arc ostaMislud williiii llic ixirilaiii tube causes tlic liiliu 

 to l)rcak ami [K-rmils the wires to fall apart. In power line c.irrier 

 telephone practice this fuse is so installed that a clear droj) of at least 

 20 ft. is obtained. The tliird element of (protection is the shunt coil 

 with the mid-point grounded. In many respects this elcnuni i^ I lie 



l.iiK- Cirritr Telephone System Using Wire 

 Coupling 



most important one, since it provides a low impedance path to ground 

 for power frequencies, thereby draining ofT the 60 cycle potentials 

 which are ccjUected by either the coupling wires or the condensers 

 in normal operation. 



As will be noted from Fig. 27 the line series inductances and this 

 shunt inductance coil comprise a unit (the upper panel) which is 

 known as the filter coil unit. The coils on this unit are insulated 

 for 20,000 volts on the line terminals and are constructed of edgewise 

 wound copper ribbon large enough to carry heavy momentary currents 

 without damage. The fourth element of protection is a fused switch 

 and surge arrester such as is commonly emplo\ed for the protection of 

 private telephone lines exposed to power lines. This device consists of 



