624 ELECTRICAL EQUIPMENT 



Class 3. Telephone circuits which parallel power lines, but 

 are not on the same towers or poles and do not cross power lines. 

 Disturbances: These circuits are subject to lightning disturb- 

 ances, and electro-magnetic and electrostatic induction. 

 They are not subject to contact with the power lines. 

 Recommendations : 



1. Insulating transformers at all telephone stations. 



2. Combined double-pole fused switch and vacuum-tube 



lightning arrester at all telephone stations on the line 

 side of the insulating transformer. 



3. Drainage coils, preferably one at each end of line. 



A diagram of connections for the apparatus used on 

 this class of telephone circuits is shown in Fig. 397. The 



Double-pole Sv/ilch Transposition of 



with Expulsion Fuses Telephone Line.s 



Telephone Line Insulating Transformer 



FIG. 397. Diagram of Connections for Protective Apparatus Recommended 

 for Telephone Lines, Classes 3 and 4. 



double-pole horn gap shown on the diagram is not used 

 on this class of circuit, but on circuits coming under 

 Class 4. 



Class 4. Telephone circuits which are carried on the towers 

 or poles with the power lines. 



Disturbances: These circuits are subject to lightning dis- 

 turbances, electrostatic and electro-magnetic induction, 

 and to crosses with the power lines. 

 Recommendations : 



1. Insulating transformers at all telephone stations. 



2. Combined double-pole fused switch and vacuum-tube 



lightning arrester at all telephone stations on the 

 line side of the insulating transformer. 



3. Double-pole horn gap across line at each station on 



line side of all other apparatus for the protection of 



