GROUND RADIO BULLETIN NO. 18 - OCTOBER 31, 1949 COMMS DEPT. L.A.D.
LOCATING FAULTS IN POWER CABLES
This note presents a description of procedure to be followed in locating short circuits and grounds in underground power cables using available test units or improvised equipment.
In order to obtain conclusive information, the fault should be of low resistance, i.e., “dead short“, rather than one which only breaks down under high voltage.
If the latter condition is occurring, the voltage should be maintained on the cable or even raised until the fault breaks down if possible without damage to transformers, etc.
The type of fault, either ground or short, and the conductors affected may easily be determined by use of a test lamp, megger or ohmmeter.
The model OBB-225 cable detecting set is available by wiring Miami and is preferable if time is available to await shipment. This set consists of a noise generator which is connected to the cable at one end and a portable pick-up device for fault location. If the fault consists of a short between two conductors of the cable, the noise generator should be connected to those two conductors to form a loop from the noise source out to the fault and back to the noise generator. If the fault consists of a grounded conductor, the noise generator should be connected between the bad conductor and ground to form the aforementioned loop. The pick-up set is then carried along the cable route until the noise dies out or drops in volume over the fault location.
If the model OBB-225 test set is not available, an improvised set-up may be made as outlined below using the same procedure as for the OBB-225 unit. A spark coil may be substituted for the noise generator and a pick-up unit made by winding approximately 4500 feet of #36 insulated magnet wire on a wood bicycle wheel or similar framework. This pick-up coil is fed into a grid leak detector and its output fed into headphones. The detector and batteries may be mounted on the coil frame to facilitate handling. (See Dwg. GEN-5A-1152)
In some cases, fairly accurate results may be had by connecting the defective cable conductor to one of the spark plugs of an automobile which is left running and using a portable battery radio which contains a loop antenna as the exploring medium.
FORM 9211—2107
