OVER-VOLTAGE PROTECTION 623 



line insulation until the phone or power lines burn off. Where 

 there is a cross but no paralleling, it is only necessary to use the 

 fused switch on either side of the cross to isolate this section in 

 case of a break. 



From the standpoint of protection, telephone circuits can be 

 classified as follows: 



Class 1. Telephone circuits which do not cross or parallel 

 power lines. 



Class 2. Telephone circuits which cross but do not parallel 

 power lines. 



Class 3. Telephone circuits which parallel power lines but 

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

 Class 4. Telephone circuits which are on the towers or poles 

 with the power lines. 



This classification covers every possible case, from a telephone 

 line far removed from the power circuit to one mounted on the 

 transmission towers themselves. Classes 3 and 4 are the most 

 common. The sources of trouble vary from lightning only in 

 Class 1, to lightning, crosses, and induction in Class 4. 



The recommendations for the protection of the telephone 

 circuit according to the classification of the circuit into which it 

 falls are as follows: 



Class 1. Telephone circuits which do not parallel or cross 

 power lines. 



Disturbances : Lightning. 



Recommendations: Vacuum-tube lightning arresters from 



each line to ground at all telephone stations. 

 Class 2. Telephone circuits which cross but do not parallel 

 power lines. 



Disturbances: These circuits are subject to lightning dis- 

 turbances and to contact with high-voltage power lines 

 through broken wires, etc. They are not subject, to any 

 extent, to electro-magnetic or electrostatic induction. 

 Recommendations : 



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



lightning arrester in series with the main telephone 

 line on both sides of crossing at nearest telephone 

 stations. 



2. Combined vacuum-tube and air-gap lightning arresters 



at all other stations. 



