DESIGN OF SYSTEM — NORTH ATLANTIC LINK OO 



the same alarm. The alarm relay channel circuit is arranged so that if it 

 fails it does not give the major alarm. 



A special alarm system for the cable current supply is described in the 

 companion paper on that equipment.^" 



Fault Location 



Length and inaccessibility have always imposed difficult requirements 

 on fault locating techniques for undersea cable systems. Before the ad- 

 vent of repeatered systems, much effort was expended on use of imped- 

 ance methods — i.e., those involving resistance and electrostatic capaci- 

 tance measurements to locate a fault to ground, or a break.* Some work 

 has been done also on magnetic pickup devices towed along the route by 

 a ship. 



With the advent of undersea repeaters, the problem has become even 

 more difficult, both because of the effect of the repeater elements on 

 impedance measurements, and because a fault may interrupt trans- 

 mission without affecting the dc loop. 



It has been necessary, therefore, to reassess the whole problem of 

 fault location. 



Location of Faults Affecting the DC Power Loop 



Three types of measurement are made on submarine cables at present : 



(a) Center conductor resistance, assuming a short circuit or a sea 

 water exposure at some point, which might be either a fault or a break. 



(b) Dielectric resistance, assuming a pinhole leak through the dielec- 

 tric and a sea water exposure. 



(c) Electrostatic capacitance, assuming a center conductor break insu- 

 lated from sea water. 



By these methods, faults and breaks in non-repeatered cables can be 

 located although the accuracy of determination, and indeed the practical 

 success of the operation, is dependent on the nature of the fault, the 

 situation with respect to earth potential, the skill of the craftsman and 

 many other factors. 



The presence of cold repeaters in the circuit adds considerably to the 

 difficulty, both because of the resistance/temperature characteristic of 

 the electron tube heaters (153 in series with each cable) and because of 

 polarization effects exhibited by the castor oil capacitors in the trans- 



* In the literature, "fault location" is a generalized term encompassing the 

 field. A "fault" is an exposure of the cable conductor to the sea without a break 

 in the conductor. A "break" is an interruption of the conductor with or without 

 exposure to the sea. 



