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Cable Fault Locators

Cable Fault Locators

When at some local point in a cable, insulation has deteriorated to a degree that a breakdown occurs allowing a surge of current to ground, the cable is referred to as a faulted cable and the position of maximum leakage may be considered a catastrophic insulation failure. After all clearances have been obtained and the cable has been isolated in preparation for cable fault locating, it is strongly recommended that a fixed plan of attack be followed for locating the fault. As in diagnosing any complex problem, following a set step-by-step procedure will help in arriving at the solution or, in this case, pinpointing the fault efficiently. At the very start, it is a good idea to gather as much information as possible about the cable under test. Information that will help in the fault locating process is:


■ Cable type — is it lead covered, concentric neutral (bare or jacketed), tape shield?
■ Insulation type — is it XLPE, EPR, Paper?
■ Conductor and size — is it CU, AL, stranded, solid, 2/0, 350 MCM?
■ Length of the run — how long is it?
■ Splices — are there splices, are the locations known?
■ T-taps or wye splices — are there any taps, are the locations known, how long are branches?

Once intial anlysis and testing are complete, there are two types of cable fault locators available:

Time Domain Reflectometry (TDR)

The pulse reflection method, pulse echo method or time domain reflectometry are terms applied to what is referred to as cable radar or a TDR. The technique, developed in the late 1940’s, makes it possible to connect to one end of a cable, actually see into the cable and measure distance to changes in the cable. The original acronym, RADAR (RAdio Detection And Ranging), was applied to the method of detecting distant aircraft and determining their range and velocity by analyzing reflections of radio waves. This technique is used by airport radar systems and police radar guns where a portion of the transmitted radio waves are reflected from an aircraft or ground vehicle back to a receiving antenna.

Thumper (Surge Generator)

These devices are basically high voltage impulse generators consisting of a dc power supply, a high voltage capacitor, and some type of high voltage switch. The power supply is used to charge the capacitor to a high voltage and then a contact closure discharges the capacitor into the cable under test. If the voltage is high enough to break down the fault, the energy stored in the capacitor is rapidly discharged through a flashover at the fault creating a detectable sound or “thump” at ground level. The important specifications of a thumper are the maximum voltage it can develop and how much energy it delivers to the fault.

A few years after polyethylene cable began to be installed underground, evidence began to surface that due to “treeing” in the insulation, high-voltage thumping of this plastic cable for long periods of time was doing more harm than good. The same is not true for PILC cables where typicallyhigher voltage and more energy is required to locate faults with no damage to the cable. There is mixed opinion as to the treeing situation in EPR. Due to this treeing situation, many utilities issued work rules reducing the maximum allowable voltage to be used for fault locating.

The energy output of any surge generator measured in Joules (Watt-Seconds) is calculated as follows: E = V2 C2 where E = Energy in Joules, C = capacitance in μf, V = voltage in kV To increase the “bang” at the fault the only two options are to increase the voltage which can be done by the operator or increase the capacitance which must be done by the manufacturer. Figure 34 shows the output energy curve of a typical four microfarad surge generator that generates 1250 Joules at a maximum voltage of 25 kV. If the fault locating crew is told that the output voltage of the thumper must be limited to 12.5 kV (one half of 25 kV), the output energy of their thumper is reduced by a factor of four down to 312 Joules.

In a practical world, 300 to 400 Joules is the threshold for hearing a thump at ground level with no acoustic amplification and with very little background noise. If the thump at the fault can not be heard, the only option is to increase voltage in order to find the fault, make a repair and get the lights back on.

 

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