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Should we be checking DLRO on Motor Starter fuses or not?

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    Should we be checking DLRO on Motor Starter fuses or not?

    We are performing maintenance on Low Voltage Motor Starters with Fused Protection. I was always trained to ductor fuses. My guys are telling me we don't ductor fuses ever. I haven't been in the field for several years and can't find supporting documentation from NETA. Should we be checking contact resistance on fuses or not? If the answer is no, why not? Thanks

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    Quote Originally Posted by bbryant View Post
    We are performing maintenance on Low Voltage Motor Starters with Fused Protection. I was always trained to ductor fuses. My guys are telling me we don't ductor fuses ever. I haven't been in the field for several years and can't find supporting documentation from NETA. Should we be checking contact resistance on fuses or not? If the answer is no, why not? Thanks
    There is nothing in MTS-2019 for fuses on low voltage starters. It does specify for MV starter fuses a tolerance of 15% on power fuses. With that said, it's not a bad idea to test them if time allows as it's a simple, quick test.

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    Quote Originally Posted by bbryant View Post
    We are performing maintenance on Low Voltage Motor Starters with Fused Protection. I was always trained to ductor fuses. My guys are telling me we don't ductor fuses ever. I haven't been in the field for several years and can't find supporting documentation from NETA. Should we be checking contact resistance on fuses or not? If the answer is no, why not? Thanks
    I only tell techs to not test fuses on PT's because you are using a 10A ductor and most PT fuses are 1A or less. Always verify the fuse size before testing, or you might be buying new fuses.
    As for any other piece of equipment, you can verify the fuse is still good and if they are experiencing damage.

    I had a customer that was running single phase on a MV contactor starter due to a blown fuse, but when we went to replace it, the engineer told me it wasn't possible to be single phase on the motor cause he would know about it. Went out the the operator and found out they had replaced 2 sets of bearings (over 2 year period), and were currently replacing the motor due to overheating conditions but never realized it was just a blown fuse.

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    Quote Originally Posted by Kalbi_Rob View Post
    I only tell techs to not test fuses on PT's because you are using a 10A ductor and most PT fuses are 1A or less. Always verify the fuse size before testing, or you might be buying new fuses.
    As for any other piece of equipment, you can verify the fuse is still good and if they are experiencing damage.

    I had a customer that was running single phase on a MV contactor starter due to a blown fuse, but when we went to replace it, the engineer told me it wasn't possible to be single phase on the motor cause he would know about it. Went out the the operator and found out they had replaced 2 sets of bearings (over 2 year period), and were currently replacing the motor due to overheating conditions but never realized it was just a blown fuse.
    Very good point and a mistake I've seen rookies fall victim too. If you are going to ductor small fuses be mindful of your test current. Start on a very low setting, it's a delicate balance between getting an accurate measurement and not blowing the fuse.

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    Quote Originally Posted by SecondGen View Post
    Very good point and a mistake I've seen rookies fall victim too. If you are going to ductor small fuses be mindful of your test current. Start on a very low setting, it's a delicate balance between getting an accurate measurement and not blowing the fuse.
    On LV Motor Starter a Multi-meter is good. A meter should be fine for any LV Fuses.
    On MV Motor Starter - always check Main Power Fuses (15%) and Fuse Clips with DLRO, and use a meter for PT fuses, because not all DLRO will have a 1A setting.

    Sometimes is hard to get a good surface contact at the fuses clips - What I see is Techs doing an overall fuse check from the incoming of the motor switch to the line of the contactor - an overall.

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    As others have said I always check the fuse resistance but I always check the rating of the fuses to be sure I'm well under the amp rating of the fuse with my test current.

    If you cant get under the rated current with test current then I use a Multi-meter.

    It is always important to ensure all the fuses are with 15% of the lowest as they can become stressed and have increased resistances that could lead to a failure/nuisance tripping

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