CABLE TESTING 687 



Irregularities in joint resistances, etc., render it necessary to 

 supplement the loop tests by exploration tests which will show 

 definitely the particular length of cable in which the fault exists 

 Also the cable may be burned off, in which case the loop tests 

 are not applicable. 



(D) The idea of the exploration tests is to send some char- 

 acteristic signal into the cable and to find by means of a suitable 

 detector the point at which the signal ceases to be heard as the 

 exploring device is moved along the cable. 



Taking the case shown in Fig. 428, when the sheaths of the 

 various lengths of cable are bonded, to prevent electrolysis, a 



FIG, 428. Pertaining to locating a ground by exploration tests. 



diminishing portion of the current will flow in the sheath to 

 points beyond the ground, as indicated. Currents will also flow 

 in the sheath, which are due to inequalities of ground potential, 

 as produced, for example, by stray currents from street-car lines. 



If the detector is a simple coil of wire connected to a telephone 

 and held with its plane parallel to the length of the cable, the 

 sheath currents, from whatever cause, will affect it in the same 

 manner as if they flowed in the conductor and an exact location 

 of the trouble is not possible. 



When dealing with three-phase cables, it is possible to use a 

 longitudinal exploring coil, devised by W. A. Durgin, which is 

 not affected by the sheath currents. 



It depends for its effectiveness on the fact that the conductors 

 in the cable are spiralled, the lay, or length of a complete spiral, 

 being about 20 inches, for a No. 2-0 three-phase paper-insulated 

 cable. 



The exploring coil consists of a laminated iron core, of a length 

 determined by the lay of the cable, over which is wound a coil of 

 insulated wire with its. terminals attached to a telephone. The 

 core is placed parallel to the axis of the cable. Any current 



