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Bizarre partial power failure exprienced...

Discussion in 'Electrical Engineering' started by Zorin the Lynx, Feb 26, 2004.

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  1. So at around 9PM, here in Miami, the power blinked for about three
    seconds. No biggie, I have a UPS. Then I got some a report that a power
    blink ocurred at work, but only certain circuits were affected. I
    confirmed that both failures happened at the same time.

    Is it possible for one phase feeding a substation to open up, resulting
    in a power failure for only a certain fraction of the single-phase loads
    fed from that substation? And yes, I'm certain that the loads that
    remained powered at work are not on any sort of emergency generator or
    battery backup system.

    Either that or it was just a coincidence that both power failures
    happened at once, and that there was a malfunction on the power grid at
    work.

    Any thoughts? My curiosity is getting the better of me...

    -Z
     
  2. In the US electric utililities generally provide no protection from
    "single-phasing" and state so in the terms of service in the tariff. It is
    the customers responsibility to protect their facility from a loss of one
    phase. Most customers don't seem to realize this even though it is right
    there in the service document.

    Charles Perry P.E.
     
  3. Beachcomber

    Beachcomber Guest

    Why is it that the Electrical Codes do not normally require this
    protection then? I know the standard answer is that the electrical
    codes are concerned with safety and not convenience. But, this would
    seem to be a safety issue...

    Case in point. I was once vice-president of a condo association that
    sustained $24,000 in damages when all the elevator motors burned out
    due to a loss of one of the three phases when there was a problem at
    the fire pump. (The utility fuse at the pole had blown for one phase
    only).

    These were critical life-safety elevators serving elderly people in
    wheelchairs. Everyone assumed that the building developer and his
    electrical contractor had done their job properly and, of course, by
    bare minimum standards, they claim that they did. It seemed to be an
    afterthought that a few inexpensive devices costing ($100 - $150)
    could have fully protected us, yet they were not installed because
    they were not required. It seems to me that if fuses or circuit
    breakers are required to protect your wiring, an protection from
    single phasing should be required to protect your motors.

    Beachcomber
     
  4. Don't ask me. I once did a power quality investigation at a brand new, very
    expensive, cash processing center. They had a gold plated electrical
    system...it was all way over built. They had 4 times the onsite generation
    that they needed and had 2.5 times the UPS capacity. But.....they tried to
    save $15k and left of the loss of phase protection. It cost them about 3
    times that is repairs to elevator equipment. The person responsible for
    cutting out the protection (a VP) was "released" I believe.

    Charles Perry P.E.
     
  5.  
  6. Chris Oates

    Chris Oates Guest

    Because of Starling migration we used to suffer with line
    line shorting due to their weight suddenly landing on the lines
    at 4 o'clock every day during the winter - we installed phase loss
    detection which coped quite well except when the problem
    wa reduced voltage as that caused chatter in the contactors
    which burnt them out so we had to go to voltage sensing
    with dropout & manual reset - luckily the Starlings have now moved on
     
  7. Wow, that was pretty strange. Did you hear anything about it in the news
    or from friends/coworkers the next morning? Like people complaining that
    equipment was damaged? Or was it very localized and no one cared?

    I had something similar happen once here; when hurricane Andrew hit us
    back in 1992, the power flickered (little blinks) for the first 20
    minutes or so of the storm, then suddenly all the lights got very dim,
    much as you described, and the refrigerator sounded like it was
    lurching... This condition lasted for about 10 seconds or so and then it
    failed completely and didn't come back for six days... I didn't have
    enough time to get a voltmeter; from the sound of it you did, so it must
    have persisted longer (or you had the voltmeter very handy)

    So apparently now I'm enlightened; it was probably a phase going out
    somewhere.

    -Z
     
  8.  
  9. daestrom

    daestrom Guest

    One idea that I haven't seen mentioned yet in this thread is a single-phase
    fault on a third feeder.

    Imagine you and your work are fed from a three-phase substation. And
    consider further another load, unrelated to you also fed from that
    substation.

    Now, the third load develops a large phase-ground or single phase-phase
    fault. If it is severe, it can cause the substation secondary voltage to be
    pulled down near zero. So you and work see a loss of one phase. Then the
    fault is cleared by any number of protection schemes and the 'lost' phase
    voltage restores to the remaining loads (you and work).

    With a severe fault, this can happen in the blink of an eye. Medium faults
    may take a second or two to be cleared depending on how they are protected
    (fuse, inverse-time-delay relay, mho-relay, etc...). Or if primary
    protection failed and a backup trip had to take place to clear an outgoing
    line from the substation.

    Just speculation.

    daestrom
     
  10. Beachcomber

    Beachcomber Guest

    Did the managment of the utility company not have any common sense?

    Assuming they were in control and could shut things down completely,
    did they not realize that running perhaps hundreds or thousands of
    their customers refrigerators at 60 volts for several hours was going
    to create liability issues?

    Beachcomber
     
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