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Thread: Fuses vs Circuit breakers in switch gear

  1. #21
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    Quote Originally Posted by mbrooke View Post
    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?
    Bolted pressure switch uses an operating mech (Springs, etc..) , QMQB is just a handle attached to the contact arm.

  2. #22
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    fuses may not be cheaper
    13.8 kv 400 A

    disconnect switch, fuse rack, another dss if on a bus
    the fuses alone might be 3-4k each

    cb
    as mentioned basically a vaccum contactor with shunt trip coil
    relays and ct's

    one cycle of fuses likely wipes out any cost advantage
    might be close even in initial cost

  3. #23
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    Quote Originally Posted by Ingenieur View Post
    no it isn't
    fuses do not have the same characteristics as relaying
    Thats obvious- and something you must take into account when doing coordination studies.

    IF the fault is in the cable
    a UG cable fault is far less likely than an OH one for obviously reasons
    If there is no overhead portion. An underground to overhead setup without a recloser would require auto-reclosing the cable- that is unless the cable is first in line and you have MHO impedance elements which will drive the relay to lockout should it pickup for the cable zone- at least the the first 90% of the cable anyway.

    a relay will respond the same
    better flexibility, accuracy, repeatability
    Thats true- but is such speed and accuracy really needed all the time?

    not so good for 3 ph motors
    or the lines carrying the extra load to supply them
    Very valid- which is why you want to have single phasing protection at the MCC / 480 volt portion of the systems. How many POCOs feed large motor customers with fuse protected risers and transformers?

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

    Substation breaker is always a breaker- but down stream there is lots of room to chose.
    I'm in over my head...

  4. #24
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    Quote Originally Posted by zog View Post
    Bolted pressure switch uses an operating mech (Springs, etc..) , QMQB is just a handle attached to the contact arm.
    This mech is manual, right? Or does some relay kick it?
    I'm in over my head...

  5. #25
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    Quote Originally Posted by mbrooke View Post
    Thats obvious- and something you must take into account when doing coordination studies.

    If there is no overhead portion. An underground to overhead setup without a recloser would require auto-reclosing the cable- that is unless the cable is first in line and you have MHO impedance elements which will drive the relay to lockout should it pickup for the cable zone- at least the the first 90% of the cable anyway.

    Thats true- but is such speed and accuracy really needed all the time?

    Very valid- which is why you want to have single phasing protection at the MCC / 480 volt portion of the systems. How many POCOs feed large motor customers with fuse protected risers and transformers?

    Substation breaker is always a breaker- but down stream there is lots of room to chose.
    obvious to some
    you are stating the obvious
    and you will still not match a relay with a fuse

    ????

    yes
    unless you can tolerate damage

    but up stream is a cb
    and for large industrial users they get a util sub...w/cb

    there are numerous reasons it is not done
    you are asking open ended questions that need specifics to be answered
    and those have been answered in dozens of texts and decades of practice
    do you design T&D grids?

    hi Z gnd fault
    a fuse will sit there all day
    a cb with ct's configured for 0 seq will trip and clear
    the most common fault, as 90% of all util dist faults

  6. #26
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    Quote Originally Posted by mbrooke View Post
    This mech is manual, right? Or does some relay kick it?
    manual
    like a cam over mechanism
    crank it to a break point then snaps over with spring assist, very fast

  7. #27
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    Quote Originally Posted by Ingenieur View Post
    obvious to some
    you are stating the obvious
    and you will still not match a relay with a fuse
    Yes- only because you stated the obvious and I was basically agreeing.

    ????
    You have never used MHO elements at MV? It can be done.

    yes
    unless you can tolerate damage
    In most cases you can.

    but up stream is a cb
    and for large industrial users they get a util sub...w/cb
    For smaller guys it varies- as does by POCO.

    there are numerous reasons it is not done
    you are asking open ended questions that need specifics to be answered
    and those have been answered in dozens of texts and decades of practice
    do you design T&D grids?

    And numerous reasons that it is done- otherwise major manufacturers (like S&C for example) would not be offering enter lineups based on fused protection.

    But my question is more around over all 40-60 year costs when fuses are compared with breakers. This is my #1 intrigue.

    hi Z gnd fault
    a fuse will sit there all day
    a cb with ct's configured for 0 seq will trip and clear
    the most common fault, as 90% of all util dist faults

    This is a very valid point and I would like to know more regarding how often High Z faults occur on XLPE/EPR cables- but answer me this: even though high impedance faults are common on over head networks, why are taps almost always fused with cutouts in many POCOs?
    I'm in over my head...

  8. #28
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    Quote Originally Posted by Ingenieur View Post
    manual
    like a cam over mechanism
    crank it to a break point then snaps over with spring assist, very fast
    Makes sense- fast breaker to quickly break the load (increased interrupting rating) I am assuming.
    I'm in over my head...

  9. #29
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    Quote Originally Posted by drktmplr12 View Post
    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.
    Generally fuses are faster than breakers. A short time or momentary dip in voltage, when a breaker operates, would not happen when a fuse operates for a fault.

  10. #30
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    Quote Originally Posted by Sahib View Post
    Generally fuses are faster than breakers. A short time or momentary dip in voltage, when a breaker operates, would not happen when a fuse operates for a fault.
    There are published current vs trip time curves you can look up for a lot of breakers and fuses.

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