IEEE 1234:2019 pdf free download – IEEE Guide for Fault-Locating Techniqueson Shielded Power Cable Systems
5.2 Precautions for ac cable systems
Many cable fault locators and troubleshooters use high-voltage dc as part of their standard fault-locatingprocedure. It is known that dc testing may generate additional cable defects in service-aged ac cables withextruded dielectric insulation, in particular，XLPE.Therefore, dc proof testing of service-aged cables withextruded dielectric insulation is not recommended. This precaution does not apply to dc cable systems orpaper-insulated cables.
Failure to completely discharge a cable after it has been charged with voltage may result in failure upon re-energization. Due to trapped charges, which have a long discharge time constant, the residual dc voltage maycause a failure upon re-energization. See the IEEE Std 400TM omnibus.
Some utility or industrial standards do not yet take into account the potentially damaging effects of dc testingon extruded cables used in ac systems. If dc testing should become necessary, the cable manufacturer shouldbe consulted for the maximum dc test value. This restriction does not apply to cables with laminated, oil-impregnated-paper-type insulation.
There is a key difference between what is considered a de cable test and an applied dc voltage during faultlocating.As stated, it is not recommended to dc test an extruded cable. Typically when fault locating, a dcvoltage is applied when using the thumper method to determine the breakdown voltage of the fault.Thisinformation is then used to determine the proper thump voltage level to use.This process of setting the thumpvoltage takes just a few seconds, as opposed to dc cable testing which is typically a minimum of 15 minutes induration.Reducing the thump voltage and the number of times a cable is thumped can also be achieved by oneof the arc reflection methods discussed in Clause 7.
DC voltage sources are also used as part of some fault-locating techniques including the bridge methods,current tracing methods,metallic shield/sheath, and jacket (oversheath) fault locating.Again, these methodstypically apply a dc voltage less than the peak of the ac operating voltage for a short time duration.
Connected equipment and the cable terminal voltage ratings should be considered before applying a dc testvoltage.Special consideration should be given if there are gas-insulated cable terminations.
When testing a faulted cable, be cautious of induced voltage from parallel transmission circuits. Please referto IEEE Std 1727IM
When fault locating on a defective cable, installation, or system, a single system ground at the test site isrecommended, see Figure 3.The metallic shield(s) of the faulted cable or cable being tested shall be connectedto system ground with a mechanically sound connection. High current magnitudes with fast rise times canflow during cable fault locating.Short and solid connections are required to reduce the creation of voltagepotential differences between grounding points and the test set. If the metallic shield connection is missing,deteriorated, or has been removed, it shall be replaced at this time. A safety ground cable shall connectthe instrument case with system ground.All ground connections should be as short as possible. If the testinstrument produces high voltages, the safety ground cable shall be at least a braided or stranded #2 AWGcopper cable.Only after the safety ground cable is in place shall the test cables be connected to the phaseconductor and the metallic shield. The phase conductor-to-ground connection for the cable to be tested maynow be removed.
Should a local ground be advisable or required for the test equipment, the case ground of the test instrumentshall remain connected to system ground in order to maintain an acceptable single ground potential.
All ground connections shall be screw-type connections that cannot accidentally be disconnected.