Eaton 380 VCP-Wind Instructions For The Use, Operation And Maintenance

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Page 28 I.B. 70A2580H02Effective November 2012 Style70A2580H02Printed in USA6-4 VACUUM INTERRUPTER INTEGRITY TESTVacuum interrupters used in Type VCP-Wind 38kVVacuum Circuit Breakers are highly reliable interruptingelements. Satisfactory performance of these devices isdependent upon the integrity of the vacuum in theinterrupter and the internal dielectric strength. Both ofthese parameters can be readily checked by a oneminute ac high potential test. Refer to Table 6.2 for theappropriate test voltage. During this test, the followingwarning must be observed:WARNINGAPPLYING ABNORMALLY HIGH VOLTAGE ACROSSA PAIR OF CONTACTS IN VACUUM MAY PRODUCEX-RADIATION. THE RADIATION MAY INCREASEWITH THE INCREASE IN VOLTAGE AND/OR DE-CREASE IN CONTACT SPACING. X-RADIATIONPRODUCED DURING THIS TEST WITHRECOMMENDED VOLTAGE AND NORMAL CONTACTSPACING IS EXTREMELY LOW AND WELL BELOWMAXIMUM PERMITTED BY STANDARDS. HOWEVER,AS A PRECAUTIONARY MEASURE AGAINSTPOSSIBILITY OF APPLICATION OF HIGHER THANRECOMMENDED VOLTAGE AND/OR BELOWNORMAL CONTACT SPACING, IT IS RECOMMENDEDTHAT ALL OPERATING PERSONNEL STAND ATLEAST FOUR METERS AWAY IN FRONT OF THECIRCUIT BREAKER.With the circuit breaker open and sitting solidly on thefloor or secured in a fixed position, connect all topprimary studs (bars) together and to the high potentialmachine lead. Connect all bottom studs together andground them along with the circuit breaker frame. Startthe machine at zero potential, increase to appropriatetest voltage and maintain for one minute.A successful withstand indicates that all interrupters havea satisfactory vacuum level. If there is a break-down, thedefective interrupter or interrupters should be identifiedby an individual test and replaced before placing thecircuit breaker in service.After the high potential is removed, discharge anyelectrical charge that may be retained. To avoid anyambiguity in the ac high potential test due to leakage ordisplacement (capacitive) current, the test unit shouldhave sufficient volt-ampere capacity. It is recommendedthat the equipment be capable of delivering 25milliamperes for one minute.The current delivery capability of 25 mA AC and 5 mADC apply when all three VIs are tested in parallel. Ifindividual VIs are tested, current capability may be onethird of these values.A DC test may only be performed if a DC test unit is theonly equipment available. Issues identified through DCtesting must be verified with an AC test beforetaking any corrective actions. When a DC test isperformed, a high field emission current from amicroscopic sharp spot on one contact can bemisinterpreted as a sign of a vacuum failure. Tominimize misinterpreted test results, the open interruptershould always be subjected to both voltage polarities.That is, apply the DC Voltage first, so that one contact ofthe interrupter is the cathode, and second, so that theother contact of the interrupter is the cathode.A bad interrupter filled with air will have a similarly highleakage current in both polarities. However, a goodinterrupter with a good vacuum level may still have highleakage currents. Such an interrupter usually has a tinysharp spot on one or both contacts that produces a highfield emission current. Such an interrupter wouldnormally withstand the required AC voltage which is thedefinitive test of its vacuum integrity.CAUTIONSOME DC HIGH POTENTIAL UNITS, OPERATING ASUNFILTERED HALF-WAVE RECTIFIERS, ARE NOTSUITABLE FOR USE TO TEST VACUUMINTERRUPTERS BECAUSE THE PEAK VOLTAGEAPPEARING ACROSS THE INTERRUPTERS CAN BESUBSTANTIALLY GREATER THAN THE VALUEREAD ON THE METER.Table 6.2 Test VoltageVacuum Interrupter Integrity TestVoltageCircuit Breaker RatedMaximum Voltage AC 60 Hz DC38 kV rms 60 kV rms 85 kV