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Contact Resistance testing of BUS, difference between test currents 10A VS 100A

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  1. Static93475 is offline Junior Member Pro Subscriber
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    Contact Resistance testing of BUS, difference between test currents 10A VS 100A

    Just wondering on some theory info about long bus runs on contact resistance testing prior to putting equipment into service. Testing it at 10A VS 100A, does testing it at a higher current make it a more accurate current reading. Thanks

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    Quote Originally Posted by Static93475 View Post
    Just wondering on some theory info about long bus runs on contact resistance testing prior to putting equipment into service. Testing it at 10A VS 100A, does testing it at a higher current make it a more accurate current reading. Thanks
    Here is a link and excerpt from Megger's Guide to Low Resistance Testing
    You will see they highlight that if performing manual test, you want to test at a current low enough not to heat up the metal, thus not changing the resistance properties of the metal:

    https://www.testequipmentdepot.com/m...ce-testing.pdf

    CURRENT SELECTION
    Depending on the instrument chosen, the current selection can be either manual or automatic. The operator should select the highest current suitable for the test to provide the best signal to noise ratio for the measurement. On instruments that offer current levels in excess of 10 amps, care is required to minimize any heating of the sample that would itself cause the resistance of the sample to change.

    Instruments designed for circuit breaker testing have much higher current characteristics and the operator must be careful when setting the test current level. Instruments designed specifically for transformer testing have a special high-voltage power level at the beginning of the test to saturate the winding. These units then switch to a lower constant current mode to measure the winding on the transformer.


    I would highly recommend reading this material to fully understand what you are doing when you are performing DLRO testing.

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    Quote Originally Posted by Kalbi_Rob View Post
    Here is a link and excerpt from Megger's Guide to Low Resistance Testing
    You will see they highlight that if performing manual test, you want to test at a current low enough not to heat up the metal, thus not changing the resistance properties of the metal:

    https://www.testequipmentdepot.com/m...ce-testing.pdf

    CURRENT SELECTION
    Depending on the instrument chosen, the current selection can be either manual or automatic. The operator should select the highest current suitable for the test to provide the best signal to noise ratio for the measurement. On instruments that offer current levels in excess of 10 amps, care is required to minimize any heating of the sample that would itself cause the resistance of the sample to change.

    Instruments designed for circuit breaker testing have much higher current characteristics and the operator must be careful when setting the test current level. Instruments designed specifically for transformer testing have a special high-voltage power level at the beginning of the test to saturate the winding. These units then switch to a lower constant current mode to measure the winding on the transformer.


    I would highly recommend reading this material to fully understand what you are doing when you are performing DLRO testing.
    Thanks for the reply Kalbi, the BUS ducts were long runs (roughly around 500ft) we did the tests at both current levels and the results were almost identical 10A & 100A, NETA states to investigate any deviation from similar connections by more than 50% of the lowest measured value. I was wondering because someone ordered the wrong piece of equipment and we only had a 10A DLRO and we usually use a 100A.

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    Quote Originally Posted by Static93475 View Post
    Just wondering on some theory info about long bus runs on contact resistance testing prior to putting equipment into service. Testing it at 10A VS 100A, does testing it at a higher current make it a more accurate current reading. Thanks
    From what I've picked up from tribal knowledge, I've been taught that a 100A ductor is generally used more for medium voltage gear or low voltage switches/motor starters/breakers that are rated for 100A+ if contact resistance is at the edge of (or past) the 50% acceptable value. The higher current seems to have more of an impact with "cleaning" the contacts using higher current to "blast away" any gunk or grease. I've been able to change a few pieces of equipment from a fail to a pass by doing this. I would say, however, that I generally use a 10A ductor for most applications, and have never used a 100A ductor for runs of bus.

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    Quote Originally Posted by nolanholt View Post
    From what I've picked up from tribal knowledge, I've been taught that a 100A ductor is generally used more for medium voltage gear or low voltage switches/motor starters/breakers that are rated for 100A+ if contact resistance is at the edge of (or past) the 50% acceptable value. The higher current seems to have more of an impact with "cleaning" the contacts using higher current to "blast away" any gunk or grease. I've been able to change a few pieces of equipment from a fail to a pass by doing this. I would say, however, that I generally use a 10A ductor for most applications, and have never used a 100A ductor for runs of bus.

    This has been the case in my experience as well. Our company only has a couple of 100A ductors and they are typically used for MV or HV equipment. Most techs carry a 10A on their truck at all times.

    100A ductors are very robust and the test connections are usually bulky clamps, which can make them difficult to work with. In this particular application, a 100A ducter may be beneficial for a long bus duct run but I've had success testing LV bus duct risers in large commercial buildings with 10A.

    With all of that said, I've heard instances of breaker manufacturers not doing warranty work for 10A contact resistance tests, they want to see 100A. It's really all depends on the specific application but in general go with 10A and bump up the current only if readings are questionable.

    As others have stated, be careful not to alter the device under test with excessive currents. Also be mindful of any components in the test circuit that can't handle such levels, such as when testing PT fuses.

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    Quote Originally Posted by SecondGen View Post
    This has been the case in my experience as well. Our company only has a couple of 100A ductors and they are typically used for MV or HV equipment. Most techs carry a 10A on their truck at all times.

    100A ductors are very robust and the test connections are usually bulky clamps, which can make them difficult to work with. In this particular application, a 100A ducter may be beneficial for a long bus duct run but I've had success testing LV bus duct risers in large commercial buildings with 10A.

    With all of that said, I've heard instances of breaker manufacturers not doing warranty work for 10A contact resistance tests, they want to see 100A. It's really all depends on the specific application but in general go with 10A and bump up the current only if readings are questionable.

    As others have stated, be careful not to alter the device under test with excessive currents. Also be mindful of any components in the test circuit that can't handle such levels, such as when testing PT fuses.
    This is especially true with SF6. I've also found that you need adequate burn in time for consistent and accurate results. I've never found a direct answer as to why, but I believe it's due to initial electron absorption from some SF6. I've seen 20-40 percent drops in the minute value vs. the spot value.

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