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VCB with high DLRO reading

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    VCB with high DLRO reading

    Good day everyone. I was task to check a 12kV 800A vacuum circuit breaker for 4.16kV application. As part of pre-requisite prior to accepting the item as spare, we conducted insulation and contact resistance. Insulation resistance is acceptable. What concerns me is the big difference in reading when we conduct the contact test.

    L1-L1 = 56.4 micro-ohms
    L2-L2 = 34.7 micro-ohms
    L3-L3 = 242.0 micro-ohms

    The 3rd contact is so high compared to the other two (2). I read somewhere in this forum that we can not just take a 10A injection test as basis for judging if the contact resistance is good or bad. By the way, I use a Megger DLRO10HD with 10A 2.5 milliohms setting.

    I manage to secure a copy of Factory Acceptance Test Report of similar VCB and it says the acceptance criteria is 0-90 micro-ohms. I don't know how they conduct their test at the factory but I think they use higher amp.

    For me personally I will not accept it and something must be done. Badly need some technical and legal (if there is) feedbacks from you guys. I already inform our supplier and waiting for their feedback.

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    Quote Originally Posted by RoyLabis View Post
    Good day everyone. I was task to check a 12kV 800A vacuum circuit breaker for 4.16kV application. As part of pre-requisite prior to accepting the item as spare, we conducted insulation and contact resistance. Insulation resistance is acceptable. What concerns me is the big difference in reading when we conduct the contact test.

    L1-L1 = 56.4 micro-ohms
    L2-L2 = 34.7 micro-ohms
    L3-L3 = 242.0 micro-ohms

    The 3rd contact is so high compared to the other two (2). I read somewhere in this forum that we can not just take a 10A injection test as basis for judging if the contact resistance is good or bad. By the way, I use a Megger DLRO10HD with 10A 2.5 milliohms setting.

    I manage to secure a copy of Factory Acceptance Test Report of similar VCB and it says the acceptance criteria is 0-90 micro-ohms. I don't know how they conduct their test at the factory but I think they use higher amp.

    For me personally I will not accept it and something must be done. Badly need some technical and legal (if there is) feedbacks from you guys. I already inform our supplier and waiting for their feedback.
    I would retest with a 100A ducter. Also make sure you are testing on the pole bus and not the fingers themselves, this can result in higher and inconsistent readings.

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    Quote Originally Posted by SecondGen View Post
    I would retest with a 100A ducter. Also make sure you are testing on the pole bus and not the fingers themselves, this can result in higher and inconsistent readings.
    Yes the test was on the pole bus. I tried to manually crank and close/open the VCB 4 times and the contact resistance for L3 has reduced to 174 micro-ohms. I will include the 100A Ducter for our next CAPEX.

    Regardless of the test method, I still believe the big difference of reading is something to be corrected though.

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    NETA MTS for reference:

    7. INSPECTION AND TEST PROCEDURES
    7.6.3 Circuit Breakers, Vacuum, Medium-Voltage (continued)
    * Optional
    Page 85
    ANSI/NETA MTS-2015
    4. Microhm or dc millivolt drop values shall not exceed the high levels of the normal range as
    indicated in the manufacturer’s published data. If manufacturer’s data is not available,
    investigate values that deviate from adjacent poles or similar breakers by more than 50 percent
    of the lowest value.


    When in doubt always try to find the manufacturers Instruction book for absolute values. This is from the GE PowerVac manual just for an example.

    9.3—Primary Circuit Resistance
    A resistance check of the primary circuit may be made
    with the breaker closed. Use a low resistance measuring
    instrument rated 100 amperes which measures in microhms.
    The 100 ampere reading should be 30 to 60 microhms for a
    1200 amp. Breaker, 25 to 50 for a 2000 ampere breaker. and
    5 to 25 microhms for a 3000 ampere breaker when connected
    across the primary bars on the breaker side of the disconnect
    fingers. Do not connect directly to the disconnect fingers as
    errors may occur due to finger spring pressure.

    The main idea in my opinion is are your test procedures in accordance with NETA and the manufacturers instructions?

    Are the test results repeatable?

    If the test results fall outside of acceptable values after a reasonable effort to make sure you are testing correctly, then that is why "Defective" stickers are made.

    The whole reason for testing is to fail equipment that does not meet minimum standards. Just make sure you are testing correctly and have as much documentation as possible to back up your findings.

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