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Contact resistance tester doesn't display any reading

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  1. Kalbi_Rob's Avatar
    Kalbi_Rob is offline Experienced Member
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    Quote Originally Posted by desilvajp View Post
    Eaton doesn't recognize contact resistance on such small breakers because it is unreliable. Yes I know this is a SqD but the designs are so similar.
    Try doing a millivolt drop test when you inject rated current.
    If this is a used breaker, open and close the breaker 5-10 times while dead. This smashes any high spots on the contacts to give a better reading. Under load, the high spots melt until the current is more evenly spread.
    Millivolt drop test is only performed with rated DC current, as discussed in NEMA AB4. Performing millivolt drop test while performing AC testing with load machine is not considered accurate by IEEE, NEMA, or NETA and many white papers have been produced to discuss this subject. I learned this when I changed companies and my old company would just perform it while performing load test and I was proven very wrong at the new company.

    NEMA AB4 Excerpt:

    6.4.2 Equipment
    6.4.2.1 This test should be conducted using a 24 volt, or less, direct current power supply capable of supplying the rated current of the circuit breaker. For circuit breakers rated higher than 500 amperes, the power supply should be capable of delivering no less than 500 amperes.
    6.4.2.2 If the above equipment is not available for field tests, a Digital Low Resistance Ohmmeter (DLRO), or 4-point tester, capable of 10 to 100 amperes (DC), may be used.
    NOTE—Use of a multimeter or low current ohmmeter in place of the power supply will not provide an accurate or reliable measurement of millivolt drop and should not be used.

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    Quote Originally Posted by Kalbi_Rob View Post
    Millivolt drop test is only performed with rated DC current, as discussed in NEMA AB4. Performing millivolt drop test while performing AC testing with load machine is not considered accurate by IEEE, NEMA, or NETA and many white papers have been produced to discuss this subject. I learned this when I changed companies and my old company would just perform it while performing load test and I was proven very wrong at the new company.

    NEMA AB4 Excerpt:

    6.4.2 Equipment
    6.4.2.1 This test should be conducted using a 24 volt, or less, direct current power supply capable of supplying the rated current of the circuit breaker. For circuit breakers rated higher than 500 amperes, the power supply should be capable of delivering no less than 500 amperes.
    6.4.2.2 If the above equipment is not available for field tests, a Digital Low Resistance Ohmmeter (DLRO), or 4-point tester, capable of 10 to 100 amperes (DC), may be used.
    NOTE—Use of a multimeter or low current ohmmeter in place of the power supply will not provide an accurate or reliable measurement of millivolt drop and should not be used.
    Good to know, I too accepted some verbal data instead of reading the up to date spec when I changed companies. Now you have me wondering if the High Current test sets put out AC or DC. Hmmm. I'll have to check it.
    Thanks,
    John

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    Quote Originally Posted by desilvajp View Post
    Good to know, I too accepted some verbal data instead of reading the up to date spec when I changed companies. Now you have me wondering if the High Current test sets put out AC or DC. Hmmm. I'll have to check it.
    Thanks,
    John
    Its AC Wave form! It wouldn't make much sense to test LVCB with High current test sets that are rated for AC currents with a DC source, specially when we trying to look at time curves.

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