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 (Very Low Frequency (VLF) AC Cable Testing)

Most of the world has quit testing cables with DC voltage. It is well known that testing cables at the
historically applied 4 – 5 times normal voltage causes solid dielectric cables to fail prematurely. Also, DC leakage currents tell little about the true condition of a cable’s insulation and even less so about its accessories.
With DC out of favor, something else had to be found. Enter VLF withstand, VLF Tan Delta, and VLF
Partial Discharge testing. A VLF AC test set is just an AC hipot but with an output frequency of 0.1 Hz or lower, rather than 50/60Hz. The lower the frequency the lower the current and power required to apply an AC voltage to a high capacitance load like cable. It requires 600 times less power to AC hipot a cable at 0.1 Hz as at 60Hz. With the present day VLF products available from HVI, it is now possible to AC hipot a cable to verify its integrity. The VLF hipot enables users to AC test long cables in the field with relatively portable and affordable equipment. There is no better way to verify the integrity of a cable following repair than to apply 2 – 3 times normal AC voltage for a period of time. The other methods used don’t get the job done: DC hipoting, a 5kV megohmmeter test, hot stick adaptors that apply far less than the cables operating voltage, and/or a soak test. Don’t be back in the same neighborhood a week after a repair because of additional damage resulting from the original in-service failure, damage to adjacent cables, damage inflicted to the repaired cable by over voltage thumping for hours, or because of a faulty repair. VLF it before re-energizing for the maximum assurance that the cable is healthy.

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