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  1. #1
    If your breaker is sized properly, it seems a fused disconnect is not a good idea. If only one of the fuses blows during a storm or something, the unit may single phase. A breaker will trip all 3 legs. What good are they?
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  2. #2
    Join Date
    Jun 2005
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    I agree Dice. Some PE's reqiure them.I do alot of school jobs and larger Commercial jobs and for some reason they will call for (example) 100 amp feed to a machine and the max fuse would be 80 on the machine and fuse the disconnect. Not sure exactly why they would do that. Maybe added protection? Whats even worse is when you encounter this and dont have the right fuse and have to run and find one.. I just take some 7/8 copper and stick them in there lol.. J/K. Another reason is that fuses have different trip and temp caracteristics then a breaker and maybe the trip point (some see fit) would be better at the machine then at the panel and if the breaker fails, it wont take out the building service and important computers, I have seen this more then once.. Just another added safty point. JMO.. P.S.. If one of your fuses blow, there is a 33% chance that it would take out your control circut and leave the machine disabled, if the wrong fuse blows and the machine single phases, that draw is usually enough to take out another fuse and then, thats it. I see your point though.

    [Edited by ultratec on 06-15-2005 at 07:50 PM]

  3. #3
    Join Date
    May 2004
    Location
    Los Angeles, CA
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    Dear Ultratec,

    I would agree with most of what are saying regarding the fused service discount being added protection for the unit, while adding another level of safety to our building’s main circuit breakers. Your 33% chance of knocking out the control circuit if only one leg(3 phase only) blew is really a 66% one though.

    EXAMPLE

    L1, L2, and L3 feed the unit

    L1 and L2 power the control circuits

    Blowing either L1, or L2 will remove at least one leg of the control circuit, in other words, two out of three, or 66% chance of removing the control circuit if only one leg drops out.

    I would also have to agree with you that under almost all, but not every, circumstance, one of the two other remaining legs will blow as well.

    Respectfully Submitted,
    John J. Dalton


  4. #4
    Join Date
    May 2004
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    Dear Ultratec,

    I’ve also seen partially melted building breakers that still have their circuits through them, but I’ve never seen a fuse that failed to blow under their designed fail conditions, they are, by design, construction, and nature, a failsafe open safety device.

    In our area (Los Angeles, CA) they are required by most municipal and state electrical codes.

    Respectfully Submitted,
    John J. Dalton

  5. #5
    But on older units, by the time the other fuse blows, so has a motor or the compressor, like I said, what good are they, I don't see any added protection, I see more problems with them.. I dunno.......anyone else see anything good about em?
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  6. #6
    I guess in rare situations a breaker may fail Like JD said, still, many more times one fuse blows and a unit is worse off as a result.
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  7. #7
    Join Date
    Mar 2005
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    burlington county n.j.
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    any fuses in outdoor switches are a pain in the a**.
    had a carrier rooftop on a dialysis center with a 3ph blower motor that smoked twice due to utility supply dropping leg out at pole.both times tripping duct smoke alarm and bringing out the friendly area fire dept. wired extra relay into control transformer primary so if any leg went out unit shut down. no problems since then. compressors seem to handle the dropped leg with internal overload, but any 3 ph blowers should have circuit to drop them out.

  8. #8
    Join Date
    Jun 2005
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    Yes John, what was I thinking, I feel like a Jack^%$^$. 66 is correct lol. Dice, from what I have seen, a machine with fuses is much likly not to have damage from a lost leg, ask me why... I dont know. Even w/ time delay fuses. (John?) I think they trip faster then a breaker. All of my service contracts are commercial, If the machines are older and dont have solid state controls to look at incoming power, I always try and sell phase monitors to protect against this situation of single phasing and over or under voltage situations. Motors (compressors) usually will trip on overload or internal line break (hopefully) before there toast from a single phase condition. Semi's have external motor protectors to protect them as well. In a perfect world, this would all work lol.

  9. #9
    Join Date
    Jun 2005
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    New York
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    Originally posted by t527ed
    any fuses in outdoor switches are a pain in the a**.
    had a carrier rooftop on a dialysis center with a 3ph blower motor that smoked twice due to utility supply dropping leg out at pole.both times tripping duct smoke alarm and bringing out the friendly area fire dept. wired extra relay into control transformer primary so if any leg went out unit shut down. no problems since then. compressors seem to handle the dropped leg with internal overload, but any 3 ph blowers should have circuit to drop them out.
    Phase(voltage) Monitor, problem solved.

  10. #10
    Join Date
    May 2004
    Location
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    Dear Diceman,

    I would agree with Ultratec that these fused service disconnects do add another level of safety between the main building’s distribution panel or motor control center and the unit itself, although I have also seen these same fused service disconnects kill motors and compressor as stated here on this thread.

    After all is said and done though, I would have to put them in the same category as car or truck safety belts, sometimes wearing them will kill you, but the vast majority of the time, they will save your life…or at least the life of the motors and compressors in your units.

    Just my own personal take on the subject guys.

    Respectfully Submitted,
    John J. Dalton

    PS: Don't feel bad....I make a Jack#&^@ out of myself on a wekkly basis my friend.

  11. #11
    Join Date
    May 2004
    Location
    Los Angeles, CA
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    1,560
    Dear Ultratec,

    Regarding your previous post:

    “Phase(voltage) Monitor, problem solved.”

    I would agree 100%.

    Respectfully Submitted,
    John J. Dalton


  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Under a dead short a fuse blows quicker then a breaker trips.

    All fuses are a pita when you don't have the one you need.
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  13. #13
    Join Date
    Dec 2003
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    1,739
    you can put a much lower sized fuse (slo blow) in a motor circuit than you can a CB. So that gives you much better protection. Fuses also have a 100,000 amp to 200,000 amp fault current rating. Most breakers have 10k to 20k. What difference does that make? If you have a ground fault there is a possibility of very large amounts of amperage over a very short period of time. If you have ever seen a meltdown of a commercial service you will know what I mean. You go into the mechanical room and there is nothing but carbon and melted copper and steel. On commercial sites you will notice that there may be some series rated panels and breakers with 20k or more fault current because of this. I can't remember ever seeing a motor fail because of only one fuse going out, that doesn't mean it never happens. If you are working on larger equipment then phase protection is in order which will eliminate any single phasing. On smaller equipment (single phase) if one fuse goes then the circuit is inoperative. If you don';t have the right size fuse, and the breaker is protecting the unit, you can put in a piece of copper pipe, as a temp till you get one. then it becomes temporarliy a non-fused disconnect. This seems to be a potato po tat o discussion among people. Electricians always will go with fused, hvac guys tend to go with non-fused. I have never had a problem with fused disconnects on any system. imho
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