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BigWheel

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BigWheel last won the day on June 9 2017

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About BigWheel

  • Rank
    P.E.

Previous Fields

  • Engineering Field
    Architectural Engineering
  • License
    PE
  • Calculator
    TI
  • Discipline
    Electrical

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  • Gender
    Male
  • Location
    Alabama
  • Interests
    Guitar, Music Recording, FPGA development, Microcontrollers

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  1. Senate Bill 316 has been signed by the Governor, making it Act 2018-550. This Act is in regards to the Alabama Board of Licensure for Professional Engineers and Surveyors. It encompasses a number of changes, but the most important change made was with respect to the sequence of events that need to be satisfied before one could become licensed as a Professional Engineer in the State of Alabama. Under the previous Act (signed 1975), one had to accomplish the following: 1. Graduate from an "approved" four-year engineering college or school program, 2. Pass the Fundamentals of Engineering Exam (FE Exam), 3. Accumulate 48 months of progressive work experience (72 months of experience is required if graduating from a "non-approved" engineering college or school), and then 4. Pass the Principals and Practices of Engineering exam (PE Exam). Under the new Act (signed 2018), you no longer have to accumulate the required working experience before you are allowed to sit for the PE Exam. You can now take the PE Exam whenever you wish (after completing steps 1 and 2 above, of course). Note, however, that you will not be eligible for licensure until you accumulate the requisite 48 months (or 72 months) of experience. A summary of these changes and the marked-up Senate Bill can be found at the Alabama bels website: http://www.bels.alabama.gov/Index.aspx I don't know when these changes will take effect, but hopefully it will be in time for some of you young'uns to take the September exam!
  2. There are two types of grounds in an electrical system. You have the Equipment Grounding Conductor (EGC) and Grounding Electrode Conductor (GEC). NEC Article 100 Definitions (Italicized text is my additional information): Equipment Grounding Conductors (EGC) - The conductive path(s) that provides a ground-fault current path and connects normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. This bonds the parts of the equipment that you do not expect to carry current under normal conditions (i.e., exposed metallic parts). This conductor is typically a bare copper conductor or has a green jacket or green jacket with a yellow stripe. This conductor is responsible for ensuring that all of the non-current-carrying parts of the equipment are of equal potential. This conductor does not carry current unless there is a fault condition. This is the "BONDING" conductor. Grounding Electrode Conductor (GEC) - A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system. This conductor is physically tied to earth (typically through a ground rod, but metallic plumbing direct buried in earth can be used, too). EGC are ultimately tied back to this point to provide a low-impedance path to ground in the event of a ground fault within the equipment. This is the "GROUNDING" conductor. Three Phase Delta loads typically have only three phase conductors and a GEC that is somewhere near the three phase input terminal block. All EGCs within the equipment are tied back to this point as well. Three Phase Wye loads typically have three phase conductors, one neutral, and one GEC. The Neutral is typically tied to the GEC and all EGCs are tied to this GEC. If a phase-to-phase fault occurs in the equipment, the neutral, which normally does not carry current under normal conditions, will suddenly carry current in the event of the fault; therefore, the Neutral is tied to the GEC and provides a path to ground that will not energize the exposed conductive surfaces of the equipment. The picture you drew and provided above is called a "high leg delta" or "stinger." That type of connection is reserved for when you want to be able to tap a 240V single phase from a 480 three-phase service. Where I work, we do not permit high leg delta connections. On the rare installations where they exist, but have not been demolished yet, they are specially marked to alert the electricians of the potential danger. I can't think of an installation where you would use a high leg delta on a purely three-phase load...in other words, I would not expect to see a three phase chiller pump connected to the source you drew above. As a point of reference; some electrical systems have all six conductors! One example is a UPS-backed Power Distribution Unit, where the input feed from normal power would be a three phase wye source, complete with three Phase conductors, a Neutral conductor, a EGC conductor connecting the source panel and the UPS-backed PDU together, and a GEC tying the entire UPS-backed PDU and feeding panelboard to earth ground (Note that the panelboard is tied to earth through its own GEC as well). In effect, the source panel feeding the UPS-backed PDU and the UPS-backed PDU itself are both bonded together and solidly grounded. All three phase systems will have a ground in one form or another (or both) - I think what you're asking is will all three phase systems have a grounded neutral. The answer is no. Purely three phase deltas do not have a grounded neutral, but high leg deltas and wye services do.
  3. I'm not trying to give anything away - I'm just trying to say that Engineering Economy questions on the PE are kind of "softball" questions compared to the FE exam where they were wanting to make sure you understood the underlying principles of "Engineering Economics." I don't know how to speak to your specific experience, as you're an examinee who is trying to figure out where to spend your valuable study time, but I would say if you think you could pass an FE-type style of Engineering Economics question, then I think you could pass a PE-style type of Engineering Economic question. The FE is looking to make sure you understand the underlying principles while the PE is looking to ensure that you understand how the underlying principles can be used to make something beneficial to society... Does this make sense?
  4. HA! I thought the FE Engineering Economics was more comprehensive, while the the PE was much simpler. If you did well on the FE Engineering Economics problems, you'll have no trouble with the PE.
  5. It might help to think of "leading" vs. "lagging" in terms of current and voltage instead of positive and negative. Power factor is generally used in calculations as an absolute value, so the sign of the PF is irrelevant. A leading power factor means the current is "leading" the voltage (i.e., current crosses the zero-axis first) while a lagging power factor means that the current is "lagging" behind the voltage (i.e., current crosses the zero axis second).
  6. Yep, that's true. The three phases serve as return paths for each other, but I think dwelling on this too much will just create more confusion than necessary for people trying to prepare for the exam when all they want to know is why a single phase VD calculation counts the conductor length twice while the 3-phase VD calculation only counts it once. There are two types of 3-phase configurations: delta and wye. In a delta configuration, there is no neutral, so that was easy - just one conductor length is considered. In a wye configuration, there is a neutral. The return current on the neutral in a grounded-wye three-phase system is a vector sum of the three (3) phase currents. In a balanced, grounded-wye configuration, the vector sum of the currents will result in 0 amps in the neutral, so no voltage drop in the neutral; therefore, only one conductor length is considered. In an unbalanced, grounded-wye configuration, the current in the neutral depends on what the unbalanced conditions are. It can be very close to zero, meaning we're almost (but not quite) balanced (typical), or it can be as high as one of the phase currents alone (atypical, and is just one of the reasons why the NEC requires that the neutral conductor in a grounded-wye configuration is the same gauge/size as the phase conductors). But the total three-phase voltage drop is a vector sum of the voltage drop across each phase and the neutral. The sqrt3 simply falls out of the 3-phase formulae derivations. When performing 3-phase voltage drop calculations in practice, a balanced three-phase load is usually assumed because you're working with something like a motor, pump, transformer, VFD (intrinsically balanced), or you're hanging a new three-phase panelboard, and if you're doing that (hanging a new panelboard), a good engineer recognizes that it's good practice to arrange the circuits such that the anticipated loads are balanced across all three phases of the new panelboard.
  7. In a single-phase system, current travels to the load on the line conductor and returns on the neutral conductor, so this "series" circuit voltage is dropped across both the line and neutral conductors. In a three-phase system, you have two conditions: balanced and unbalanced. In a balanced three-phase system, there is no current in the neutral conductor (each phase's current cancels the other two phase's currents). Some examples of balanced 3-phase loads include 3-phase motors, pumps, and VFDs. Conversely, with an unbalanced 3-phase system, current is produced in the neutral; however an unbalanced 3-phase system is rarely (if ever) calculated considering the line and neutral impedances because the effort to perform the calculations isn't worth the results. I'm happy to talk to this and prove what I just claimed, but I'll spare the community until pressed to do so, but I will conclude with this: a three-phase system is not a series system; it's a parallel system, so the math is considerably more complicated than a single phase analysis, and the kicker is that you can do the math and calculate the true 3-phase voltage drop of an unbalanced 3-phase system, but the results of these calculations will leave you wondering why you even bothered. Finally, it helps to understand why "Voltage Drop" is such a hot topic. Is it because it's a code requirement? It's not a code requirement, but rather a recommendation. It is absolutely true that too much voltage drop will cause the load to behave erratically, and can even be a very dangerous (and even deadly) situation for craftspersons working at the load; nevertheless, it's not a "CODE REQUIREMENT." The NEC recommends no more than a 3% voltage drop on the branch circuit and no more than 2% on the feeder. Voltage drop calculations are essential to the difference between a true engineered design and a classroom assignment.
  8. I can tell English is a second language for you, but the mathematics language is universal. You post good work on here, @rg1...I have no doubts about your passing. @cos90 is also mathematically strong. Both of you guys will do well. Just don't make the mistake of overthinking things. The actual PE exam questions have been vetted through-and-through before they appear in the actual exam, so if you miss the question, well, you missed it fair-and-square. In other words, you should not have to "figure out what they mean;" it will be obvious to the casual observer what they're asking for. You will see questions that are obviously "test questions;" these are questions that they are "testing" for viability, but won't be actually included in the final scoring. I recognized one right away because the grammar, spelling, and setup was so utterly confusing that I couldn't figure out what it was EXACTLY they were looking for. Honestly, the questions are worded so carefully that there will be no room for interpretation. You either know the answer or you don't. This is why I know you both will pass. Both of you find mistakes in your reference material and are able to deduce why it's wrong, what should have been asked based on what the "right" answer is, and can rationalize both why the question and answer conflict. I would wish you both good luck, but neither of you need it. Instead, I'll wish you both a foregone conclusion of "CONGRATULATIONS!"
  9. Agreed. I predict both of you will pass your PE Exams, and will have plenty of time to play with this later.
  10. You're right. I missed the connection. I interpreted the question as what would be the peak voltage of the given phase voltage at Vab (perhaps that was the intended question), but the question simply asks what what is the peak voltage at Vab, so your answer is right. I think you meant 400 * sqrt2 = 565V, though. Thanks.
  11. Here is a brief explanation: https://electronics.stackexchange.com/questions/12453/if-a-standard-three-phase-400v-ac-connection-is-rectified-what-dc-voltage-comes
  12. Where you have your voltmeter in your sketch will read sqrt3 * 125V, or 216.5V...that is not where Vab is measured from. Vab is measured from the outputs of the rectifier, as shown in my attachment.
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