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Cable Fault LocatingFault Profiles Guidewww.megger.com/usThe word “Megger” is a registered trademark
TDR LEGENDOpen Conductor/NeutralShorted Conductor/NeutralSpliceWet SpliceHigh Resistance Splicewww.megger.com/usTransformerTee or WyeWater Ingress
Cable Fault Locating Fault Profiles GuideOPEN CONDUCTOR (blow-out)Circuit EquivalentOpen conductor or blow-out occurs due to a high-fault current,a dig-in or a failed splice. An insulation resistance test mayindicate a high resistance value if the fault and ground is dry.If the ground is moist, the resistance measurement may showa moderate to low resistance value at the fault. If the phaseconductor or concentric burns back, the gap distance may beexcessive and may not arc or thump. (Refer to fault positionA.) Testing from the other end of the cable may render betterresults as shown in position B.Example of a typical open conductor trace.Surge GeneratorVoltmeterDuring a surge discharge(thump), the voltmeter will givea weak drop and will neverapproach zero volts (fault A).Fault B will give a strong drop inthe voltmeter, practically reachingzero.An open conductor (or blow-out) will appear as a positivereflection on a time domain reflectometer (TDR) as shownby the solid TDR signature. If the conductor or concentricis burned back and the gap value is excessive, the fault maynot arc (thump). This will result in little or no change in theTDR signature when the arc reflection test is performed. Thesolid signature shown above represents the before surge TDRsignature; the dotted line represents the after surge signature.AmmeterDuring a surge dischargeAs stated previously, an open conductor (or blow-out) will(thump), the ammeter will give aappear as a positive reflection on a TDR. If the gap distanceweak kick forward (fault A). Faultbetween the conductor and concentric has not burned back,B will give a strong kick forward.the fault image will invert or reflect negative when the arcreflection test is performed as seen in the dotted TDR trace.This also indicates good arc or thump condition.1-800-723-2861
Cable Fault Locating Fault Profiles GuideSHORTED CONDUCTOR (bolted fault)A shorted conductor (or bolted fault) occurs when theinsulation burns to a state where the cable conductor andcable neutral (or sheath) is in contact with one another.Insulation resistance is zero to only a few ohms. There is nogap value and therefore this fault will not arc (or thump).Circuit EquivalentExample of a typical shorted conductorlow voltage trace.Surge GeneratorVoltmeterDuring a proof test the kilovoltmeter will read zero and willshow no movementAmmeterDuring a proof test the ammeterwill continue to rise as the outputof the proof tester is raised.www.megger.com/usA shorted conductor (or bolted faults) can be localized byusing time domain reflectometry (TDR). The reflected pulsewill be negative.Since there is no gap value associated with a shortedconductor, the fault will not arc (or thump). Acousticpinpointing will not be an option. A surge generator(thumper) can still be used for pinpointing if anelectromagnetic impulse detector is used along with the surgegenerator.Surge the cable with the surge generator. Track the surgepulse using an electromagnetic detector such as the MeggerMPP2000 Pinpointer. You will detect strong signals up to thetrue fault location. Since the entire surge energy is lost at thefault, weak or no signal will be detected beyond the fault.
Cable Fault Locating Fault Profiles GuideELECTRICAL TREE (pinhole)Circuit EquivalentElectric trees (or pinholes) are the most common fault found inunderground electrical power cable. The fault starts througha process of water ingress or small defects in the dielectric.Through age, these defects begin to grow, taking on thepattern of tree branches. Eventually, one of these branches willbreach the cable insulation and become a fault.Example of a typical pinhole trace.Surge GeneratorVoltmeterAfter discharge from thesurge generator (thumper) thevoltmeter will give a strong kickback, nearly reaching zero volts.The time domain reflectometer signature will first show theend of the cable under test. The cable end will be seen as apositive reflection indicating an open. Refer to the signatureshown with a solid line. After the arc reflection test, a negativereflection will be captured from the point of the fault. Thefault will be shown as a negative reflection indicating the shortcircuit created by the arc (thump).AmmeterAfter discharge from the surgegenerator (thumper) the ammeterwill give a strong kick forward.1-800-723-2861
Cable Fault Locating Fault Profiles GuideSPLICE FAILUREA failed splice can be a difficult fault to identify and locate.Since the physical distance between the phase conductor andconcentric is increased, the gap distance is typically excessive.This creates a fault that may not arc or thump.Example of a typical splice failure trace.Circuit EquivalentSurge GeneratorVoltmeterDuring a surge discharge(thump), the voltmeter will givea weak drop and will neverapproach zero volts. The faultmay not arc or break down onevery discharge cycle.During a proof test, the cablemay hold voltage for a briefperiod before flashing over.AmmeterDuring a surge discharge(thump), the ammeter will give aweak kick forward.Burn current may be necessary toreduce the fault resistance andgap distance.www.megger.com/usA splice failure is very similar to an open conductor (blow-out)in the sense that the resistance and gap value at the fault willtypically be very high. The excessive gap value may make itdifficult to arc or thump the cable. The fault may not arc onevery discharge cycle. Applying burn down current may berequired to improve breakdown performance.The time domain reflectometer pattern for a splice is a smallpositive reflection that is immediately followed by a smallnegative reflection. (Refer to the solid signature shown above.)If water is present within the splice, the negative reflection willtypically be more pronounced. When the arc reflection testis performed, the signature of the splice may change slightly,becoming more pronounced on the negative (as shown in thedotted signature above.) This indicates that during the surge(or thump) the resistance of the splice is becoming lower andthat a thump is occurring. If the gap between the conductorand concentric is too excessive, the arc reflection test will showno change in the TDR signature. This indicates that no thump isoccurring and burn-down may be required.
Cable Fault Locating Fault Profiles GuideCORRODED OR OPEN CONCENTRICCircuit EquivalentSurge GeneratorWARNING: Corroded or open concentric will present severalissues during a fault locate. The concentric is designed to carryfault current. If the integrity of the concentric is jeopardizeddue to open strands or high levels of corrosion, the concentricmay no longer provide the lowest resistance path back to thesurge generator. In this case, the fault current will follow theleast-resistant path, which may be fence posts, phone shields,cable TV shields, gas tracer wires, other utility cables or thedamp earth itself. Humans or animals touching the fencepost, phone, TV, waterline, etc., can be injured or killed. Useof a surge generator (thumper) should be approached withgreat caution. Surge at the lowest possible voltage and for theshortest amount of time.Example of a corroded or open concentric trace.VoltmeterThe meters on the surgegenerator (thumper) will providelittle to no indication of concentriccorrosion. If an arcing faultexists on the cable, the meterswill simply show a sharp dropin voltage and a sharp kick incurrent when the cable is surged(thumped).AmmeterThe time domain reflectometer signature can help identifypossible corroded concentric issues prior to using a surgegenerator or thumper. Small random reflections, typicallypositive in orientation, may be observed indicating areas ofhigh-metallic resistance. It may be difficult to detect the end ofthe cable due to excessive attenuation of the TDR’s transmittedpulse.When the arc reflection test is performed, a negative reflectionwill be captured at the location of the arc or thump. Safetyprecautions should be taken during the pinpointing stage.Corroded concentric presents step-potential hazards.1-800-723-2861
4271 Bronze WayDallas, TX 752371-800-723-2861MEG-762/MIL/1M/V2/6.2012
(thumper) can still be used for pinpointing if an . of the proof tester is raised. Circuit Equivalent. cable Fault Locating Fault Profiles guide 1-800-723-2861 Electric trees (or pinholes) are the most common fault found in underground electrical power cable. The fault starts throu
