Electrical Apparatus magazine, July 2002 'Finding Fault'--Locating Cable Defects

Minimizing process downtime dictates prompt repair of defects in power, control, and communications cables. First, however, defects must be located. When conductors are hidden within raceways, or buried underground, fault location requires electrical measurements made at one or both ends of the circuit. Many tests have been developed for that purpose.

Cable faults, which most often involve insulation failure, can be of four types: open circuit; high series resistance (a bad connection, for example); short circuit; and ground fault. Locating them involves three steps:

1. Characterize--determine the nature of the fault.

2. Localize--use "terminal" methods to find the particular section of cable involved.

3. Locate the fault, using "tracer" methods.

In a terminal test, some property of the deenergized cable is measured, such as insulation resistance or capacitance (both of which can locate an open circuit). In a tracer method, an electrical signal (such as a d-c current, or a capacitor discharge pulse) is injected onto the faulted circuit. How the circuit responds can indicate where the trouble is.

Most test methods work only for certain types of faults. Voltage drop measurements (based on current flow through a loop including both faulted and undamaged conductors) can locate either short circuits or grounds. The "earth gradient" test, monitoring earth return current, will locate a ground fault.

The most versatile, most often used tracer tests include one of the simplest (the impulse or capacitive discharge test that sends a high energy pulse down the defective conductor where it produces an audible sound at the fault) and the far more sophisticated "pulse echo" or time domain reflectometry (TDR) test. Similar to radar, the TDR method sends a pulse along the cable that is reflected back from the fault to an oscilloscope screen. Those not only locates the fault, but can identify its probable nature.

Each test method has some disadvantage. For example, the capacitive discharge test risks damage to good insulation). No single test is effective in all situations. Some engineers contend that cable fault location remains more art than science.

Industry news...

Featured Technical Article
July 2002

On this page is a summary of the Electrical Apparatus July 2002 featured technical article, by Richard L. Nailen, P.E. , "'Finding Fault'--Locating Cable Defects " Was bestimmt das Axialspiel eines Motors? ... Fehlerfindung bei defekten Kabeln ... Recherche des pannes - Comment rechercher les défauts de câble ... Buscafaltas - Encontrando Defectos en los Cables...

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