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Single Phase, 230V

Discussion in 'General Discussion' started by mountaineer, Jun 15, 2010.

  1. mountaineer Sparks Level

    I am working on an arc flash study that includes several 240V single phase pump stations for a municipality. They are 2hp, 230v, single phase grinder pump stations, one for each homeowner's lot and the control panel is in the homeowner's basement.

    Not sure if these fall under the <240v category. Is this a situation where the tables are better used?
    mountaineer, Jun 15, 2010
    #1

  2. Flash Junior Level

    Most testing shows self extinguishing arcs at other than high arcing currents at 240 volt three phase. In single phase systems where you get voltage zero, it is highly unlikely that an arc can sustain itself. I do not believe there is any valid testing, therefore, anyway to calculate low voltage single phase arc flash energies.

    Chet Davis from Easy Power has delivered an interesting paper on 208V arc flash energies. He uses a max arc time or cut-off of .5 seconds and mathematically shows this approach gives a safety factor of five. If you use that same logic, the self-extinguishing nature of single phase (at 230 volts) circuits would show negligible incident energy.

    To take it one step further, I do not see how you ever show energies greater then 1.2 cal for an overcurrent device that small. It was never the intent to take studies down to circuits that small, no matter how hard some engineers try to convince their clients otherwise.
    Flash, Jul 20, 2010
    #2

  3. viper57 Sparks Level

    Dwellings

    A couple of thoughts... it appears you are dealing with dwellings. Research the NFPA 70E to see the requirements for dwellings (exempt). Most electric services to single family dwellings (and many commercial facilities) are <125kVA <= 240V so there is no arc flash hazard. Chet's article has shed a lot of light on the topic!
    viper57, Jul 30, 2010
    #3

  4. brainfiller Administrator

    This issue resurfaces from time to time and continues to have people scratching their heads. I am heading up the task group in IEEE 1584 that is reviewing the lower limit. There is some concern that using the 125 kVA transformer size (actually I believe the next standard size is 112.5kVA) as a lower limit might not be low enough.

    In the lab we found under certain conditions you can sustain an arc flash down towards 5,000 Amps of short circuit current at 120/208V. Nothing official yet but it does indicate the 125 kVA might not be a low enough cut off.

    I would be VERY CAREFUL about using the term “No Arc Flash Hazard Exists” for these cases. Even if the arc flash does not sustain, there is still an arc flash, it is just not a large one, more like a big “pop” but if someone is standing near, there can still be a good spray of sparks that will get your attention and be a threat to your eyes. << from experience :eek: had an "oops" in my sister's house a few years ago, loud "pop" lots of sparks, nothing major but it was an arc flash (and eye threat) non the less.
    brainfiller, Jul 30, 2010
    #4

  5. Vincent B. Sparks Level

    Have you done some lab tests at 240 V center tap? Or aware of somebody else who did some? That's what normally goes into dwellings. The absence of the 2 other phases should lessen the amount of energy available to be released during an arc flash event, when the arc current is the same.
    Vincent B., Jul 30, 2010
    #5

  6. brainfiller Administrator

    No I have not looked at this. We ran a few single phase tests from the three phase set up and just got a "pop" out of it. Without more tests, I can't be sure but I think you might see a similar result with the 240V center tap. 240 V three phase with lower short circuit current can sustain given the right conditions.

    I'm sure it would also depend on the short circuit current. i.e. a suburban residential neighborhood typically has smaller transformers / low short circuit current. Neighborhoods in more dense urban settings could have a much higher short circuit current if the transformers are larger. That might influence the result.

    The real issue is arc sustainability. The lower short circuit current results in a low energy per time. Time becomes the issue and at the lower currents/lower voltages, the arc frequenly does not sustain very long so the total incident energy is low.

    I believe this is another "hole" that needs filled in the future.
    brainfiller, Jul 30, 2010
    #6

  7. stevenal Sparks Level

    230 is a nominal utilization voltage for 240 services. I would not call this < 240, since measured voltage will likely be higher. In my opinion the <240 rule is for 208 services.
    stevenal, Jul 30, 2010
    #7

  8. brainfiller Administrator

    Correct! The intent of <240V was not to include 239V and below but to include 208V systems.
    brainfiller, Jul 30, 2010
    #8

  9. viper57 Sparks Level

    <240 125kva

    I agree that this is the most controversial issue pertaining to arc flash studies. This is also a major cause for the arc flash study results' (for these voltage levels) loss of credibility. While that "pop" and molten BB's dancing on the floor gets our attention, I don't believe that is the crux of the arc flash hazard analysis. While this may technically be an arc flash, so is turning off a 120v light switch. So, where do we say that there is no arc flash hazard? Technically the answer should be never. In reality, we know that the "pop" is not the hazard that we need to address as compared to the high energy 3 phase systems. The studies that I have seen where a <240v arc did sustain, it didn't have the explosive energy associated with the classic arc flash.

    Hopefully, we will work toward a realistic solution that protects employees without being a cumbersome process. That is why I like Chet's interim solution... it blends safety with common sense.

    My only question is "Why were you working on the panelboard with it energized?"... hopefully it was for diagnostics.:D I too have ruined more screwdrivers than I like to admit! :eek:
    viper57, Aug 9, 2010
    #9

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