Cable Earth-Gradient Detector Auxiliary Device

Figure 1 is an illustration of an earth-gradient detector. 

This detector has two spikes that are driven into the earth above a buried cable. The current that flows through the earth in the vicinity of a cable when the cable’s insulation breaks down under the stress of the thumper’s high-voltage pulse causes a difference in potential
between these spikes. When connected to the spikes, the microammeter of the detector deflects to the left or to the right according to the direction of the current flowing from one spike to the other.

Earth-Gradient Localization

Figures 2a and 2b illustrate the basic method of using an earth-gradient detector to locate a fault in a buried cable. Current flows in several paths through the earth from the point of the damaged insulation to the driven rod. These paths are represented by broken-line curves in Figure 7. These currents produce a voltage gradient between any two points at the surface of the earth. The technician locates the fault by placing the spikes of the earth-gradient detector at different surface locations along a straight line between the ends of the cable. At successive locations (1, 2, and 3) the technician reads the deflection of the detector until he reads a reversal of deflection (location 4). The technician backtracks until he finds a location (5) where there is a null deflection.

The technician then reads deflections (6, 7, and 8) along a line that crosses the first line at a right angle. The location of the fault is at the second null deflection (location 8). Through the use of this method, the technician does not need to know the route of the buried cable in order to locate the cable fault.