CABLE THUMPING

CABLE THUMPING: EQUIPMENT CONSTRUCTIONS, OPERATIONAL
PRINCIPLES, AND BASIC LOCALIZATION TECHNIQUES

A cable thumper is an electrical test set that generates repetitive high-voltage high-energy pulses. A cable thumper transmits these pulses into a power cable in order to cause a fault in the cable to break down and, consequently, produce an audible sound and a strong current in the earth surrounding the fault. The sound reveals the location of the fault. If the sound is not easily heard at the surface of the earth, an acoustical detector is used to locate the cable fault. Alternatively, an earth-gradient detector can be used to locate the fault by sensing the earth currents that flow near the fault.
 

Thumper Constructions

There are two basic types of cable thumpers: the series-gap type and the pulse type. There are two basic types of detectors: the acoustical detector and the earth-gradient detector. 

Either type of detector can be used with either type of thumper. The constructions of thumpers and detectors are explained in the next four subject headings.

                          Illustration of a Cable Thumper

Series-Gap Type
 

Figure 1 is an illustration of a series-gap type of cable thumper. The illustration shows the following:
· A knob-controlled variable transformer. This transformer controls the magnitude of high-voltage output
pulses.
· A kilovoltmeter. This meter indicates the voltage of the thumper’s built-in impulse capacitor.
· A primary ammeter that indicates the input current.
· A microammeter that indicates the output current.
· A power cord.
· An output test lead.
· Jacks for connecting the battery leads.
· An impulse-control gap handle. This handle adjustments the dimension of the series gap.

Pulse Type

 
Pulse-type cable thumpers have the same general construction as series-gap cable thumpers. The important difference in construction is that a set of additional components allow the rate that output pulses are generated to be adjusted independently from the output voltage adjustment.


Thumper Operational Principles

Generating a High Energy Pulse in a Series-Gap Thumper
Figure 2a is a simplified schematic diagram of a series-gap thumper. The thumper’s high-voltage power supply is similar to the power supply of a DC applied potential test set. This power supply charges an impulse capacitor. A kilovoltmeter indicates the magnitude of the impulse capacitor’s voltage. A variable transformer is used to control the maximum voltage that charges the impulse capacitor.

Before a test, the series gap is adjusted to a maximum dimension. The voltage of the capacitor is adjusted to the level that is appropriate for testing the cable, and the dimension of the series gap is subsequently adjusted until it flashes over. This flashover causes a pulse of high voltage to be transmitted into the cable under test and also discharges the capacitor. A short interval of time (approximately one to 30 seconds) elapses before the capacitor charges to a voltage level high enough to again cause the series gap to flash over. Pulses of high voltage are repeatedly transmitted into the cable. The interval of time between pulses can be shortened by adjusting the gap to a smaller dimension. Making the gap smaller consequently reduces the peak voltage magnitude of the output pulses.

Generating a High Energy Pulse in a Pulse-Type Thumper

Figure 2b is a simplified schematic diagram of a pulse-type thumper. The operational principle of a pulse-type thumper is the same as that of a series-gap thumper except that the time interval between pulses is controlled by a high-voltage contactor and a timing circuit. For a pulse-type thumper, the time interval between pulses can be adjusted without affecting the voltage magnitude of the output pulses.