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Thread: AF Safety concerns: 150kVA pad mount xfmr boosting 208 => 480 in Data Center

  1. #11
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    If the equipment covers are on, in of itself NFPA 70E would consider there to be no arc flash danger.

    Although not directly related, I would re-evaluate the Occupancy groups. A transformer does not make a S-2 occupancy, and UPS batteries do not make a H-3 occupancy.

    If that occupancy labeling were taken to the next level, then office space with garbage cans would take on an H-3 category.
    Ron

  2. #12
    Join Date
    Jul 2008
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    SF CA USA
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    But really in the worst case scenario
    What's the threshold boundary for a
    480 V 150 kVA transformer anyway
    Even with all the dead fronts
    the doors open

    18 inches ??

    in arc flash training there were
    different circles and radius
    This is No 12 KV
    Must just be at most arms length 36"
    "A problem clearly stated is a problem half solved."

  3. #13
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    Jul 2008
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    boost buck

    while some were snickering I had missed that not every xfmr can boost as well as buck
    as per sQd
    It seems your customer is reverse feeding/backfeeding the transformer, and we don't recommend that. Backfeeding causes very high excitation inrush, making coordination to breakers or fuses difficult without exceeding the limits set by the National Electrical Code.

    For a 208v - 480wye application use EX150T212H.
    he said that would generate lots of heat and noise and trouble - not recommended
    "A problem clearly stated is a problem half solved."

  4. #14
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    Rutland, VT, USA
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    Quote Originally Posted by wyreman View Post
    What is the actual Arc Flash danger here?
    In the normal operating state, there is no arc flash danger. An arc danger would exist when work is being done that could cause an flash. Examples are removing covers from energized equipment, testing with a multimeter, etc.

    I guess I am a little confused what the concern is on having a transformer there, humming is normal for a transformer as well as some heat. Looking the pictures, I would be more concerned with the housekeeping in an electrical room

  5. #15
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    This is a common xfmr
    rated for a 480 primary and 208 secondary.
    Back feeding it to 150kVA will cause
    Extra heat, humming and coordination trouble


    The arc flash question would be
    if the insulation is sufficient
    on the 208v rated assembly
    "A problem clearly stated is a problem half solved."

  6. #16
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    Jul 2008
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    anybody in the market for a slightly used 150kVA xfmr ?

    anyway, that was put in by someone else and I'm glad we spotted it, and are getting rid of it...
    anybody in the market for a slightly used 150kVA buck xfmr that was boosting?

    got to move it out of the network room because it generates too much heat.
    I am thinking about taking the opportunity to get rid of some of the 480 load as well -
    redistribute some of that load at 208
    It may help with the sequence of work


    Also have to get a mitsubishi a/c unit in the IDF room - the big issue is the condensate
    ... but first thing is to move that xfmr.
    "A problem clearly stated is a problem half solved."

  7. #17
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    It was a step down transformer that was stepping up. No buck or boost involved.

    mobile

  8. #18
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    we used to boost these up and down
    up to 480v for a triplex run and then
    down to 208 for the point of use on portable bldg.

    step down xfmrs on both sides - no problem for years
    we even had spare 4160's

    what is your opinion on the insulation on the low volt side here being a 480 secondary?
    probably work fine for years...

    just not what the manufacturer recommends tho
    "A problem clearly stated is a problem half solved."

  9. #19
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    Springfield, MA, USA
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    As far as conductor insulation is concerned, it pretty much doesn't matter if you use the transformer for step up or step down. The 208V side is being used at 208V, and the 480V side is being used for 480V.

    The problems come in with other features of the design.

    1) Grounding. In most situations you want to use a grounded electrical system. The 'standard' approach for 3 phase systems is to ground the neutral of a wye secondary. Being used in reverse, your transformer doesn't have a wye neutral, so grounding is less obvious...and might require more expensive switchgear or cause problems with equipment.

    2) Primary side 'taps'. Transformers are usually designed with 'taps' on the primary coils, to permit adjustment for different supply voltages. The transformer might be nominally 480V HV, but have taps for 456V, 468V, 480V, 492V and 504V. This lets you get proper magnetic saturation and proper output voltage over a range of input voltages. When the transformer is reversed, these 'taps' end up on the secondary side. If your supply voltage is high, then you might over-saturate the transformer core.

    3) Inrush current. Transformers are designed so that the primary has more 'leakage inductance' which acts to limit the surge of current just as the transformer starts up. When you swap primary and secondary you don't get this design feature, and will see higher proportional inrush current, which might do things like trip breakers when you first energize the transformer. For smaller transformers this often is not a problem, and specialized switchgear can 'pre-charge' a transformer to get around this sort of problem...but it is easier to simply have a proper step up transformer.

    All of the above can frequently be tolerated or accommodated with proper design; if I was in a rush I would not hesitate to use a 480V delta to 208V wye transformer 'in reverse' to get 480V from 208V. But if I had time to get the correct transformer that would be my very strong preference.

    To expand on GoldDIgger's terminology issue: buck and boost are 'terms of the art' that specifically describe transformer connections used to make small changes in voltage where the transformer secondary is placed in series with the supply voltage. See http://www.emerson.com/resource/blob...boost-data.pdf for example

    In 'normal English' you are boosting 208V to 480V. But in 'transformer English' you do not have a 'boost' configuration. Instead you have an isolating transformer connected to step up voltage

    -Jon

  10. #20
    Join Date
    Jul 2008
    Location
    SF CA USA
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    609
    for the grounding... how should the backfed xfmr be grounded?
    I havent opened it yet, but there is a rod driven and a small EGC I'll follow

    also, the here is a video of the sound of the backfed load on the main gear
    Name:  IMG_2334.jpg
Views: 20
Size:  131.1 KB
    "A problem clearly stated is a problem half solved."

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