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Thread: shop electrical question...

  1. #1
    markus shaffer Guest

    shop electrical question...

    I'll not bore everyone with the crazy details of my shop, but simply put, my space is shared. We have this giant air conditioner that was made by Chrysler in the late 50's or early 60's.. It's very large and about the size of a small car. Runs on 3 phase 208 power.. A few years back, we turned on the thing for the first time in the spring and we blew one of the main fuses coming into the box in the basement.. 100 amp fuse. We had an electritian out and he said that we were drawing too much amperage on one of the phases with everything else that is wired up in the space.. Today we blew another fuse and this time I replaced it. No big deal, but my question is, how do I determine how much amperage is being drawn from one of the phases and furthermore, how do you spread it out so that there is even draw across all the phases? Keep in mind, this is New York City we're talking about here.. Nothing is done to code, by law we have to use armored BX cable and electritians cost a small fortune.. If it's a matter of just replacing a fuse every few years, no problem. I can take care of that, but if possible, I'd like to fix it right.

    -Markus

  2. #2
    Markus. Let me start by saying I am not an electrician, but do all my own wiring, including 3-phase. I'm not sure how to help you out here.
    But one word of advice before you get into splitting circuits up on 3-phase.
    3-phase has a wild leg. A wild leg meaning that one of the hot lines is always more than 110 volt.
    2 lines will be 110, the wild leg will be 130-140 volt, somewhere around there.
    If this wild leg gets connected to something that cannot handle the extra voltage, it will fry it, say a control switch for your a/c or a switch on your 220 table saw.
    I am outa here.
    Steve


  3. #3
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    Markus. 3 phase power is simply that 3 seperate phases(A,B&C with respect to neutral). Each phase is 90 degrees off of the other phase. The phase angle is established by the physical location of the windings and taps in a transformer/ motor generator. An SCR( silicon rectifier diode) rectifier is used to balance the phase angle and current in each leg. In your shop all they have done is tap off a single phase and supply the rest of your loads.
    If I had to guess I would say that you are not the only person with this problem. somewhere between the pole and your building is a "box" inside this box should be the voltage regulators and SCR's to control your 3 phase power. This is not something you can adjust.
    If there truly is a current imbalance, that cannot be compensated for in the controller, the other option would be to redistibute the loads across the phases, I'm not sure if this can be done in accordance with city codes.I can do it at work, but not sure if it can be done outside the fence
    I used to lose a single fuse, of the 3 phase in my rod control system at work and drop a fuel rod into the reactor, What a pita!
    I will take a little umbrage here tho', replacing 100amp fuses in a 3 phase system is a big deal. The neutral side of three phase may be diode suppressed from ground due to the additional inductive loads. This means that you may have stray voltage on one side of the fuse block and a single phase on the other side of the fuse block. Take care when replacing that fuse.
    Oh yeah. the simplist way I know of to find out the current draw is with a clamp on ammeter, safest way also. Good luck
    Last edited by Mike Cutler; 07-23-2004 at 4:41 PM.

  4. #4
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    Hi Markus

    Buy a new air conditioner.

    Joe
    Last edited by Joe Mioux; 07-23-2004 at 7:17 PM.

  5. #5
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    Exclamation what the?

    Mike you sound like you really know your stuff but when you casually say that you lost CONTROL of a rod control and then DROPPED a fuel rod down in to the REACTOR that really scares us non-nuclear industries kind of people. We had a near catastrophe near here at the Browns Ferry Nukeplant in Al, when a "technician" couldn't find a flashlight and used a CANDLE instead and set the main control panel for the reactor on fire, according to their own reports it was an ***puckering event! Please, I know that it was all routine and not a problem but I just don't want to hear that kind of thing and Nuclear Power all in the same sentence. Oh and Markus, from what little I know about 'lectricity you will be better off bitting the bullet and hiring a pro to straighten it all out for you. Expensive up front, but cheaper in the long run when dealing with your life

  6. #6
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    Quote Originally Posted by Mike Cutler
    Each phase is 90 degrees off of the other phase.
    Mike, they are 120 degrees out of phase: 3*120 degrees = 360 degrees.
    Wood: a fickle medium....

    Did you know SMC is user supported? Please help.

  7. #7
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    Chris. Thanks for the correction, I'm not sure why I wrote 90 deg. The SCR firing angle isn't even 90, not sure what I was thinking there.
    Jack. I apologize if I sounded to casual or cavalier. It was not my intent at all. The design of the system requires, that in the event of power failure the control rods will insert to shutdown the reactor in a controlled manner . This is the safe, conservative condition. Once again, I apologize for being insensititve. I am extensively familiar with the incident at Browns Ferry, having spent hours in training sessions going over a timeline chronology minute by minute of that event. That incident caused every nuclear power station in the country to have to redesign their instrumentation and control schemes to provide for safe shutdown in the event of a similar incident. These are the Appendix R, Reg Guide 1.97 requirements for safe shutdown. All nuclear utilities are required by federal law to meet this criteria or shutdown the plant until they meet these requirements
    Markus. After all of this, Joe may have the best solution to buy a new AC unit

  8. #8
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    Thumbs up Nukes

    Hi MIKE, well I was really just kind of poking fun at you I certainly didn't think you were being insensitive to my concerns. I for one think that we will eventually have to bite the bullet on N power in this country and use it more. But I think that there is still a lot of work to be done about the waste issue and safety concerns from all aspects, especially after 9-11. I think that the Europeans and Japanese are using smaller more modern designs than what we use here aren't they? I almost went to work for TVA in the Nuclear division but landed a job with the Post Office first. As to the Browns Ferry plant not long after the "incident" a huge cooling facility there made of wood burned to the ground reducing the cooling capacity of the plant, never did hear the last on that problem. So hopefully people such as yourself will keep on making them safer and cheaper to run so we will all benefit.
    Last edited by Jack Wood; 07-24-2004 at 6:55 PM.

  9. #9
    markus shaffer Guest
    Mike,

    Most of what you said was well past my knowledge of electricity. But my understanding of what you're saying is that the problem may be from the supply lines from the street rather than anything that is going on inside here. I would love to just get a new air conditioner, but it's not an option.. Perhaps I ought to elaborate more on my problem as we just blew another fuse..

    The air conditioner did once blow a fuse on it's own which is why I was blaming it yesterday. However, today, I was running my planer (3 phase 240 volts) and and cyclone (1 phase 240 volt) at the same time. I imagine between these three machines and all the other crap that's on here, lights, computers, fan, etc.. that we're just drawing too much amperage. However, all this stuff has been run together in the past with no problem, so I'm curious as to why it's happening all of the sudden. The cyclone is fairly new, so perhaps that is the culpret. But like I said, it's not as if all of this hasn't been run together before. There have even been a few times where even more was running at this place which is why I'm concerned that we've blown two fuses in two days.

    A couple issues which I'm curious about though.. The fuses that have blown are in a safety switch box in the basement of the building.. I have to shut off all the power to open the box and replace the fuse. If the problem is that Con Ed is tapping off single phase and redistributing it as 3 phase, would that mean that the same fuse would blow each time? Today's fuse was a different one than yesterday's. The fuse that went yesterday was obviously really old so perhaps it was just it's time.. The other two (one of which that burned out today) seem newer. Also, shouldn't some breakers trip before the main fuses burn out?

    I'll be speaking with an electrician on Monday, so hopefully it'll all get sorted out then.. But if you have any other suggestions, I'd love to hear them..

  10. #10
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    Markus,
    I must say I am not an electrian either but have worked with it in my prevous vocation for many years. It got to point where we did our own ro told the electrians what to do.
    Anyway 100AMPS is not much. I have that at my house and 200AMP at the shop.
    There are 2 reasons a fuse will blow - over-current or heat.
    Heat is caused by loose connections, either the wire lug or the fuse holders.
    An infrared thermometer pointed at these connections can detect this fast.
    The other way is to turn the power of in the middle of a normal day and touch these connection to see if they are getting warm.
    Over-current - you need an ampmeter to check each leg of your 3 phase. Note each.
    Now you do need to know if you have a true 3 phase or the stinger leg type like Steve Clardy mentioned. Many old places have the stinger leg type. The voltage on this 3rd leg can be as high as 190volts. If you have the standard 3 phase( I think it is "star")
    With the cover off your breaker panel you can see which leg feeds which breaker. Each breaker can be unplugged and moved to another slot. Try to re-distrubute the load to even out the drawer on each leg.
    Roger

    The WoodCrafter

  11. #11
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    Markus. It wouldn't seem improbable that a different fuse could blow each time. I don't know the actual power distribution for your building, but I'm willing to bet it goes something like this. "A" phase to neutral is supplying the 120 60 cycle, single phase vac loads. "A" phase and "B" phase are supply 220 vac. 3 phase is "A","B" and "C". If at some time after initial installation it was convenient to reference a 120 off of a different phase it may have been done, this could lead to imbalances across the phases, if not compensated for. The larger loads that you are referring to are primarily inductive. This means that on startup there is a large inrush, or current surge. As demand or load increases the inductive field increases and current demand goes up. Another large load is the ballasts in your lighting. I've seen faults in ballast bring an electrical system to it knees. Regardless of all this tho. the controller should be controlling the current balance across the phases unless a fault exists somewhere in the system.
    Another potential problem is a voltage sag, or loss from the service, as voltage decreases and load increases current goes up.
    I'm relieved to hear that the fuses are located after a manual disconnect, sometimes older or renovated buildings are not up to current code and the fuses are "live"
    The bottom line though is this. If you are continually blowing fuses or popping breakers, something is wrong.
    As to the question of why the fuse is blowing and the breaker is not opening. there could be couple of reasons; breakers have a time response criteria to prevent inadverdant, or premature opening of a breaker, a "typical"breaker is looking for a continuous over current condition to exist( of course this is measured in milliseconds) fuses on the other hand are designed to meet a mutitude of criteria, a sudden large inrush of current may cause failure, current increase over time may cause failure, and some fuses have the engineered attributes of both. Fuses and breakers have temperature ratings also. as the temperature increases the device begins to operate closer to the point of failure, thus mitigating margin. Contrary to popular belief, having breakers open and fuses blow indiscriminately may not be "safer". And of course the reason for fuse failure and not breaker opening could be attributed to a breaker fault. Breakers have a cycle time life, and breakers can also just plain not work.
    You are definitely beyond the point of " I'll just learn as I go " here. It's time to bring in the liscence and maybe ConEd. It is an unrealisic expectation to require a building tenant or owner to be replacing 100 amp fuses. Take care and be safe

  12. #12
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    Quote Originally Posted by Steve Clardy
    Markus. Let me start by saying I am not an electrician, but do all my own wiring, including 3-phase. I'm not sure how to help you out here.
    But one word of advice before you get into splitting circuits up on 3-phase.
    3-phase has a wild leg. A wild leg meaning that one of the hot lines is always more than 110 volt.
    2 lines will be 110, the wild leg will be 130-140 volt, somewhere around there.
    If this wild leg gets connected to something that cannot handle the extra voltage, it will fry it, say a control switch for your a/c or a switch on your 220 table saw.
    I am outa here.
    Steve
    Steve,

    FYI, there are 2 types of 3-phase power - 120Y208 and 120D240 (at least where the base voltage is 120v). Those are abbreviations for "120 - Y - 208" and "120 - Delta - 240" 3-phase.

    A Delta 3-phase configuration has a wild leg, a Y configuration doesn't.

    In a "Y" configuration, all 3 legs have the same potential to ground. IOW, in a 120Y208 system, all 3 phases are 120v. The angles between the phases are 120 degrees.

    In a "Delta" configuration, 2 of the legs are the base voltage and the 3rd leg is a wild leg. The voltage of that wild leg is the base voltage * the square root of 3; for a 120v system that means the wild leg is 208v. The phase angles are a bit different here. The 2 base legs are 180 degrees out of phase and is our typical residential 120/240 single phase. The generated leg is 90 degrees out of phase to the 2 base legs.

    Marcus - have you replaced your fuses with slow-blo, main fuses? I don't know if there is a difference at that size, but in the smaller, glass screw-in size, there are "normal" and "slo-blo" fuses.

    Forgot - a clamp-on ammeter is the easiest way to measure current.
    Last edited by Rob Russell; 07-26-2004 at 7:59 AM. Reason: Forgot ammeter

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