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Thread: Fuses vs Circuit breakers in switch gear

  1. #11
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    Quote Originally Posted by mbrooke View Post
    Noted and makes sense.


    By any chance have you ever worked with QMQB bolted pressure switches?
    QMQB's are not bolted pressure switches, they are just fused switches.

  2. #12
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    Quote Originally Posted by zog View Post
    True, but say you spend $20k on a MV breaker, that's it for 20 years or so until you have to refurb it and/or replace VI's, etc..
    Which costs how much more (to refurbish)? My understanding is that a fuse and switch does not need a refurbish after 20 years. There is also the cost of maintaining the relays and batteries with breakers.


    MV fuses can be a few grand each, plus as mentioned you will need a switch to house them in ($10k). A phase to phase fault will blow 2 of those fuses and that costs a few grand everytime it happens. Google 400E fuse and look at the prices, not cheap.


    What about "K" links? I'll have to check, but I think you can get S&C gear that takes standard fuse tubes found in overhead cutouts.




    Quote Originally Posted by Ingenieur View Post
    many faults on the utility side are self clearing
    trees, wind, birds, critters, etc.
    every time you have one of these and blow the fuse you are down for hours, while with a CB you can reclose when cleared

    a cb with relaying can be coordinated much better than any fuse set-up

    Thats true for overhead wire- but not for underground cables. You don't want to reclose on those.
    What is esoteric knowledge today will be common knowledge tomorrow.

  3. #13
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    Quote Originally Posted by zog View Post
    QMQB's are not bolted pressure switches, they are just fused switches.
    Ok- my mistake them.

    Not to derail the thread- but just a quick question: whats the difference between a bolted pressure switch and a fused switch?
    What is esoteric knowledge today will be common knowledge tomorrow.

  4. #14
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    Quote Originally Posted by mbrooke View Post
    Thats true for overhead wire- but not for underground cables. You don't want to reclose on those.
    ??? no one said you would never reclose on UG, although they often do
    the fault may be reflected from a customer and on premisis protection may clear it so reclosing on the util side may be desirable
    many more OH than UG lines anyways
    lost revenue, labor and material cost, possible damage due to clearing times, ineffective/unpredictable/imprecise coordination
    will a fuse react the same at -10 F vs 90 F?
    a cb opens all 3 phases, a fuse on a gnd fault may lead to single phasing
    dangerous: replace a fuse and closing it on a load vs a remote operated enclosed cb

    if it was a good idea everybody would be using fuses



  5. #15
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    Quote Originally Posted by Ingenieur View Post
    ??? no one said you would never reclose on UG, although they often do
    the fault may be reflected from a customer and on premisis protection may clear it so reclosing on the util side may be desirable

    That would be a miss-cordination issue. And whats to say that they downstream OCPD will be open when the breaker recloses back in?


    many more OH than UG lines anyways
    lost revenue, labor and material cost, possible damage due to clearing times, ineffective/unpredictable/imprecise coordination
    When you reclose an underground cable, you are increasing the incident energy at the failed point.

    will a fuse react the same at -10 F vs 90 F?

    Of course not- thats why you take ambient temps and even preloading into account during fault simulation and coordination studies.

    a cb opens all 3 phases, a fuse on a gnd fault may lead to single phasing

    if it was a good idea everybody would be using fuses

    Which keeps the lights on for none effected parts of the system while protecting healthy cable portions from thermal damage.
    What is esoteric knowledge today will be common knowledge tomorrow.

  6. #16
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    that article makes me think plants need to do a better job checking batteries instead of applying fuses all over the place.

    your switchgear batteries are just as important, if not more, as doing vibration tests on large vertical pumps.

    wait, they don't check that either?

    the vibration switch was bypassed because it kept triggering?

    they calibrate by hitting it with the back of their screwdriver???

    that said, it's not unusual to see fused switches in lieu of "circuit breakers" when feeding from a MV switchgear lineup. i use quotes because at MV they are really glorified contactors. in many cases the downstream breakers will stop a fault before the fuse pops.

  7. #17
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    Quote Originally Posted by drktmplr12 View Post

    that said, it's not unusual to see fused switches in lieu of "circuit breakers" when feeding from a MV switchgear lineup. i use quotes because at MV they are really glorified contactors. in many cases the downstream breakers will stop a fault before the fuse pops.

    Is this because they breaker is faster on clearing a short circuit or because faults first leak "low" levels of current to ground picked up by differential/ground elements?
    What is esoteric knowledge today will be common knowledge tomorrow.

  8. #18
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    Quote Originally Posted by mbrooke View Post
    Is this because they breaker is faster on clearing a short circuit or because faults first leak "low" levels of current to ground picked up by differential/ground elements?
    trip ratings being equal.. I can't say if a fuse or breaker is faster without know the fault current and the exact fuse and breaker we are comparing.

  9. #19
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    Quote Originally Posted by drktmplr12 View Post
    trip ratings being equal.. I can't say if a fuse or breaker is faster without know the fault current and the exact fuse and breaker we are comparing.
    Including the ground element?
    What is esoteric knowledge today will be common knowledge tomorrow.

  10. #20
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    Quote Originally Posted by mbrooke View Post
    That would be a miss-cordination issue. And whats to say that they downstream OCPD will be open when the breaker recloses back in?

    When you reclose an underground cable, you are increasing the incident energy at the failed point.

    Of course not- thats why you take ambient temps and even preloading into account during fault simulation and coordination studies.

    Which keeps the lights on for none effected parts of the system while protecting healthy cable portions from thermal damage.
    no it isn't
    fuses do not have the same characteristics as relaying

    IF the fault is in the cable
    a UG cable fault is far less likely than an OH one for obviously reasons

    a relay will respond the same
    better flexibility, accuracy, repeatability

    not so good for 3 ph motors
    or the lines carrying the extra load to supply them

    that is why no one does it on util T&D
    it would be a very unstable, unreliable system with only fusing



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