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Vacuum Interrupter Integrity

Vacuum contactor reliability is dependent on the integrity of its sealed vacuum. The vacuum interrupter provides excellent dielectric strength to interrupt high currents to the vacuum contactor. While vacuum interrupters are known for being low maintenance with long life, it is important to ensure the interrupter is completely sealed for proper vacuum contactor performance. Check the dielectric strength of a vacuum interrupter using a High Potential test set.

High Potential Test

High Potential, or Hi-Pot, tests should be performed using a 50/60 hertz test set with a continuously variable voltage up to 20kV RMS. X-radiation is negligible at this level; however, personnel should not be closer than 6 feet to the interrupter under test to avoid electrical high voltage shock hazards. The contactor should be free of dust and other contaminants before conducting this test.

Vacuum Contactors & Starters | Joslyn Clark

Connect output leads of the test set across the interrupter terminals with the contactor in the OPEN position. Slowly raise the test voltage from 0 to 10kV RMS and hold for 15 seconds. During this time, ignore any discharges or test set tripping unless it becomes impossible to reach test voltage and keep for 15 seconds. Throughout the test, leakage current should not exceed five milliamps. If the auto ramp Hi-Pot tester cannot raise voltage slowly, repeat the test 10 times or until it reaches the desired voltage level.

A successful withstand of the Hi-Pot test indicates that the interrupter is properly vacuum sealed. If a break-down occurs during the Hi-Pot test, remove the vacuum contactor housing a defective interrupter immediately.

A defective or unsealed vacuum in a vacuum contactor causes improper and unreliable motor starting operation, creating an unsafe environment for operators.

Contact Resistance

With the switch closed, use a micro-ohmmeter to measure the resistance across the terminals. The contact resistance should be less than 200 micro ohms. If higher contact resistance values are measured, then the high potential test should be performed. If the contactor passes Hi-Pot test, recondition the interrupters by cycling the contactor on/off up to 20 operations. If the resistance still exceeds 200 micro ohms post cycling, replace the contactor.

Contactor Welds

Joslyn Clark Vacuum Contactor designs eliminated the possibility of contact bounce and contact oxidation, both of which contribute to contact weld. Under normal operating conditions within the ratings listed, the contactor will not weld. However, if normal operating conditions are exceeded, measure the contact resistance.

A low resistance reading while the contactor is in the open position indicates the contacts may have welded together and requires replacement. For the 1140 and CVs series contactor, check for contact wear while the contactor is energized by inserting a 50 thousandths shim into the actuation arm. The 50 thousandths shim should fill the overtravel gap on a new contactor. If the gap measurements under 10 thousandths, replace the contactor immediately. If the gap is between 10-20 thousandths, the contactor should be replaced soon.

How to Adjust Contacts

The Joslyn Clark maintenance manuals describe how to unpack, install, and maintain the device. It also lists recommended spare parts with quantities based on routine wear and breakdown situations. The contactors, however, should not be readjusted. They are self-adjusting. 

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