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How do you calculate microhms from millivolt drop test?

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    How do you calculate microhms from millivolt drop test?

    You have performed a millivolt drop test on a 1600-ampere, low voltage circuit breaker. The result is 25 millivolts at 100 amperes. What is the contact resistance, in micro-ohms, and is the value acceptable?

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    MilliVolt Drop

    Quote Originally Posted by adamwilson View Post
    You have performed a millivolt drop test on a 1600-ampere, low voltage circuit breaker. The result is 25 millivolts at 100 amperes. What is the contact resistance, in micro-ohms, and is the value acceptable?
    It is simply ohms law, E(voltage)=I(amperage) x R (Resistance in ohms)

    Algebraically manipulate the equation to: R = E/I
    R=.025/100
    .0025 ohms which equals 250 micro-ohm, which is not an acceptable contact resistance number for a 1600 amp low voltage circuit breaker

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    millivolt drop test procedure

    If you are using AC current to measure the millivolt drop test you are doing it wrong. You should use DC because you want to find out the resistance. If you use AC the capacitive and inductive components of the impedance might give you an erroneous result.

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    Quote Originally Posted by DetroitLions View Post
    It is simply ohms law, E(voltage)=I(amperage) x R (Resistance in ohms)

    Algebraically manipulate the equation to: R = E/I
    R=.025/100
    .0025 ohms which equals 250 micro-ohm, which is not an acceptable contact resistance number for a 1600 amp low voltage circuit breaker
    Actually .025/100=0.00025

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    Micro-Ohm

    Quote Originally Posted by adamwilson View Post
    You have performed a millivolt drop test on a 1600-ampere, low voltage circuit breaker. The result is 25 millivolts at 100 amperes. What is the contact resistance, in micro-ohms, and is the value acceptable?
    R=E/I
    =0.025/100
    =0.00025 ohm
    =250 micro-ohms, not acceptable

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  11. STXERS17 is offline Junior Member Pro Subscriber
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    Quote Originally Posted by Skyehaven40 View Post
    R=E/I
    =0.025/100
    =0.00025 ohm
    =250 micro-ohms, not acceptable
    Hi Skyehaven40,

    What would be the acceptable contact resistance for the LV Circuit Breaker?

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    Quote Originally Posted by STXERS17 View Post
    Hi Skyehaven40,

    What would be the acceptable contact resistance for the LV Circuit Breaker?
    According to NETA, 50% rule is applied.

    "Compare bolted connection resistance values to values of similar connections. Investigate
    values which deviate from those of similar bolted connections by more than 50 percent of the
    lowest value."

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    Quote Originally Posted by Taranus View Post
    According to NETA, 50% rule is applied.

    "Compare bolted connection resistance values to values of similar connections. Investigate
    values which deviate from those of similar bolted connections by more than 50 percent of the
    lowest value."
    The question only give us one test result so how would the 50% rule work in this situation. I know 250 micro-ohm is high for LV CB contact resistance because I usually get around 20 micro-ohm or less when DLRO from Line to load or between connections. I haven't find any standard that give an acceptable limit yet.

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    Quote Originally Posted by STXERS17 View Post
    The question only give us one test result so how would the 50% rule work in this situation. I know 250 micro-ohm is high for LV CB contact resistance because I usually get around 20 micro-ohm or less when DLRO from Line to load or between connections. I haven't find any standard that give an acceptable limit yet.
    I'm with you on this. You can answer it only from your testing experience. In my opinion this question should be removed.
    But is just my opinion...

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    I've seen this exact question. They are basically testing you on two fronts:

    1.) Can you calculate the value using ohms law?
    2.) Based on your experience, is this acceptable?

    250 micro-ohms is way to high for a 1600A breaker of any type. Test enough breakers and you will understand.

    You can usually find recommended values in circuit breaker instruction books, and if you compare them, you will find that they are all pretty similar. Here is an example from Square D:

    Click image for larger version. 

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    NW08 = 800A, NW12 = 1200A, etc.

    Contact Resistance Test

    Circuit breaker pole resistance tests are not reliable indicators of circuit breaker performance because the resistance values are influenced by a number of transient factors including contact surface oxidation, foreign material between the contacts, and testing methods. NEMA AB 4 paragraph 6.4.1 states: “The millivolt drop of a circuit breaker pole can vary significantly due to inherent variability in the extreme low resistance of the electrical contacts and connectors. Such variations do not necessarily predict unacceptable performance and shall not be used as the sole criteria for determination of acceptability.”

    High pole resistance may also be caused by eroded contacts, low contact force, and loose termination. The only one of these factors likely to be present on a new circuit breaker is a loose termination, since the contacts are new and there has been no opportunity for contact pressure to have drifted from the factory setting. A loose termination can be corrected in the field.

    If a contact resistance test is done, it is important to do it after the contacts have been conditioned by instantaneous primary injection testing to ensure the contacts are clear of resistive films, oxidation and foreign material. If the circuit breaker has been in service with no performance issues, (overheating or nuisance tripping), contact resistance measurements are redundant and of little value.

    Square D™ recommends that a Digital Low Resistance Ohmmeter (DLRO) be used, using a 10 A dc test current for circuit breaker ratings below 100 A, and using 100 A dc for circuit breakers rated 100 A and above. the median (middle) value of three readings (toggling the circuit breaker between each reading) should be recorded for each pole tested.

    If this value is equal to or less than the value listed in Table 26, the pole is acceptable. If the reading is higher, the cause should be investigated and corrected if possible. Contact your local field office for more information.

    http://www2.schneider-electric.com/r...202%202015.pdf

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