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DC “Hi-Pot” Testing Guidelines for MV Cables

DC High Potential (HI-POT) Testing of Medium-Voltage Power Cable

OverviewThis procedure is intended to providegeneral guidelines for high potentialdc testing of power cables.

All tests made after cable installationand during the guarantee period shallbe made in accordance with applica-ble specifications.

All safety precautions must beobserved during testing athigh voltage.

Read and understand and follow theOperator’s Manual for the particulartest set being used!

Test EquipmentDirect current test equipment isavailable commercially with awide range of voltages.Accessory equipment necessaryto safely conduct high voltagetests—such as safetybarriers, rubber gloves and nonconducting hard hats—mustbe used; consult appropriatesafety officer.

Test ProceduresSee IEEE Standard 400 (page 105).Acceptable procedures, althoughvarying slightly in technique, havemore or less been standardizedas either a “withstand test” or a“time-leaking current test.”

Before performing any dcoverpotential tests:

• All equipment must be discon-nected from the cable circuit, i.e.disconnect transformers, switchtaps, motors, circuit breakers,surge arrestors, etc. This will pre-clude damage to such equipmentand will prevent test interruptionsdue to flashovers and/or trip-outsresulting from excessiveleakage current.

• Establish adequate clearancebetween the circuit test ends andany grounded object, and toother equipment not under test(about 2.5 feet).

• Ground all circuit conductors notunder test with all cable shieldsincluding nearby equipment.

• Consult termination manufactur-ers for maximum test voltage recommendations andtime limitations.

The direct current test voltagemay be applied either continu-ously or in predetermined stepsto the maximum value inaccordance with applicablespecifications.

• Continuous Method – Apply testvoltage at an approximate riserate of 1 kV per second or 75%of the rated current output of theequipment, whichever is less.Some equipment will take longerto reach the maximum test volt-age because of the amount ofcharging current.

• Step Method – Apply test volt-age slowly in 5 to 7 increments ofequal value to the maximumspecified. Allow sufficient time ateach stop for the leakage currentto stabilize.

HI-POT TESTING PROCEDURESNormally this requires only a few seconds unless cable circuits ofhigh capacitance are involved.Record leakage current at eachstep.

Maintain the test voltage at theprescribed value for the time des-ignated in applicable specifica-tions. The following times areusually considered adequate: atthe end of the test period, set thetest set voltage control to zero,allow the residual voltage on thecircuit to decay, then ground theconductor just tested.

CAUTIONIt should be recognized that dccharges on cable can build up topotentially dangerous levels ifgrounds are removed too quickly.Maintain solid grounds after the teston the cable for at least 4 times theduration of the test. On exceptionallylong cable lengths, it may be neces-sary to increase the grounding time. Itis advantageous to maintain thesegrounds longer and while reconnect-ing circuit components.

• Acceptance Testing – Afterinstallation and before the cable isplaced in regular service, thespecified test voltage shall beapplied for 15 consecutive min-utes.

• Proof Testing – At any time dur-ing the period of guarantee, thecable circuit may be removedfrom service and tested at areduced voltage (normally 65 per-cent of the original acceptancevalue) for 5 consecutive minutes.

• Record the leakage current atone minute intervals for the dura-tion of the test time involved.

SPEC F025December, 2005

Phone: 888-593-3355www.generalcable.com

DC “Hi-Pot” Testing Guidelines for MV Cable

DC High Potential (HI-POT) Testing of Medium-Voltage Power Cable

CommentsDC overpotential testing of medi-um-voltage power cables is usuallyperformed with negative polarityconnected to theconductor.

DC overpotential testing is a toolonly for determining insulationresistance at higher voltages.Effective insulation resistance of thecable system may be calculated bymeans of Ohms Law:R = V/I. The relation is:

Megohms = Kilovolts x 1000

Microamperes

Insulation resistance may also bemeasured with standard instru-ments which give a direct readingat 500 volts (or higher, dependingon the model). IR in general has lit-tle or no direct relationship todielectric or breakdown strength.

The significance of conducting dchigh-voltage tests on nonshielded,nonmetallic-sheathed cable isdependent upon the environment inwhich it is installed because thecharacteristics of the return circuitsare unknown. The environmentmust be carefully considered or testresults may not be significant. Infact, these tests can result in dam-age to the cable insulation.

Humidity, condensation and actualprecipitation on the surface of acable termination can increase theleakage current by several orders ofmagnitude. Humidity also increasesthe corona current, which indicationis included in the total leakage cur-rent. Wind prevents the accumula-tion of space charges at all bareenergized terminals.

This results in an increase of coro-na. It is most desirable to reduce oreliminate corona current at the baremetal extremities of cable or termi-nations. This may be accomplishedby covering these areas with plasticenvelopes, plastic or glass contain-ers, plastic wrap (e.g. “Saran” or“Handiwrap®”) or suitable electricalputty.

Routine periodic dc maintenancetesting of cable for the evaluation ofthe insulation strength is not a com-mon practice. Some power cableusers have adopted a program oftesting circuits during planned out-ages, preferring possible break-downs during testing rather thanexperiencing a service outage. It isnearly impossible to recommendtest voltage values for those main-tenance tests with the history of thecable circuit. An arbitrary test volt-age level could break down a cablecircuit that would otherwise renderlong trouble-free service at normaloperating ac voltage.

The main usefulness of dc high-voltage testing is to detect con-ducting particles left on the creep-age surface during splicing or ter-mination.

Test equipment should be suppliedfrom a stable, constant voltagesource. Do not use the samesource that is supplying arc weldersor other equipment causing linevoltage fluctuations. The outputvoltage of the test set must be fil-tered and regulated. Consider usinga portable, motor-driven alternatorto energize test set.

The gradual decrease or non-increase of leakage current withrespect to time at maximum testvoltage is the acceptance criteriafor dc hi-pot testing.

Testing Problems

Extra Leakage Current:• Failure to guard against corona• Failure to clean insulation surface• Failure to keep cable ends dry• Failure to provide adequate

clearance to ground• Improper shield termination

Erratic Readings:• Fluctuating voltage to test set• Improper test leads

Environmental influences:• High relative humidity• Dampness, dew, fog• Wind, snow

Results vs. Cable LifeTo date there is no bases forcorrelation between dc testresults and cable life expectancy.

Partial Listing of EquipmentSuppliers

J.G. Biddle CompanyBlue Bell, PA 19422

HipotronicsBrewster, NY 10519

Associated Research Inc.Chicago, IL 60648

Von CorporationBirmingham, AL 35211

SPEC F025December, 2005