Electric Power Distribution Handbook, T. A. Short

8. Faults

Questions

  1. A fault at location B is measured as 1 kA. For this event, how much current is measured at location A?

    circuit

    1. 7 kA
    2. 3 kA
    3. 1 kA
    4. 0.5 kA

  2. If you are one mile from a 12.5-kV substation, which answer is likely closest to the available L-G fault current?

    1. 7.0 kA
    2. 3.5 kA
    3. 1.5 kA

  3. On a 12-kV circuit, what’s a typical line-to-ground fault current 4 miles from the substation?

    1. 200 A
    2. 500 A
    3. 1500 A
    4. 2500 A

  4. On a 25-kV circuit, what’s a typical line-to-ground fault current 4 miles from the substation?

    1. 200 A
    2. 500 A
    3. 1500 A
    4. 2500 A

  5. Which of the following helps reduce high-impedance faults? Mark all that apply.

    1. Faster relaying
    2. Fuse taps
    3. Use covered wire
    4. Stronger conductors

  6. On a 25-kV circuit that’s 4 miles from the substation, which has the lowest current?

    1. Line-to-ground fault
    2. Line-to-line fault
    3. Line-to-line-to-ground fault
    4. Three-phase fault

  7. For a line-to-ground fault on the secondary side of a delta to wye transformer, what is the primary line current?

    1. About 58% on two phases
    2. 115% on one phase
    3. 100% on one phase and the neutral/earth return

  8. On an overhead system, which is more likely?

    1. 80% of faults are permanent
    2. 60% of faults are temporary

  9. A digital relay measures a fault current of 8 kA. Where is the fault likely to be?

    1. Close to the station
    2. About 3 miles out
    3. More than 5 miles out
    4. Need more information

  10. Arcs tend to travel?

    1. Towards the source
    2. Away from the source

  11. Which of these are likely to cause temporary faults (rather than permanent).

    1. Trees
    2. Animals
    3. Both

  12. If an arc has a voltage of 1000 V, about how long is that arc? Assume open air.

    1. 2 in
    2. 1 ft
    3. 3 ft
    4. 10 ft

  13. For a circuit with an X/R ratio of 10, when is the asymmetry worst?

    1. When the fault happens when the voltage is near zero.
    2. When the fault happens when the voltage is near peak.
    3. When the fault happens when the voltage is about 50% and increasing.
    4. When the fault happens when the voltage is about 50% and decreasing.

  14. True/false: for an open fuse feeding a three-phase customer, backfeed voltages can appear on the open phase, but only with a delta to wye connection.

    1. True
    2. False

  15. True/false: to protect customer equipment from damage from primary faults into the secondary, smaller, fast transformer fuses are better.

    1. True
    2. False

  16. Faster relaying helps with which of the following? Mark all that apply.

    1. Conductor slapping
    2. Tree faults
    3. Evolving faults
    4. Insulator damage

  17. When using station-class arresters on distribution circuits to protect customers against overbuilt to underbuilt faults, which of the following are true? Mark all that apply.

    1. Station arresters are used because they have a higher energy rating, so they can handle energies involved.
    2. Station arresters have a lower clamping voltage than other arresters on the circuit.
    3. Isolators are not used, because we want the arresters to stay in the circuit.

  18. Which is likely to be the most energetic failure? (Assume the durations are the same.)

    1. Fault in a transformer
    2. Fault in a cable
    3. Fault in a splice

  19. Fault location using substation data tends to be more accurate for which of the following. Mark all that apply.

    1. Faults past 3 miles because load currents are less
    2. Faults closer to the substation
    3. Tree faults (relative to other fault types)
    4. Underground systems

  20. For a line-to-ground fault on the secondary side of a delta – grounded-wye transformer that is 10 per unit (relative to the full-load rating of the transformer), what is the fault current on the primary side?

    1. 0 per unit
    2. 5.8 per unit
    3. 10 per unit

Problems

  1. Using the impedances in tables 2.7 and 2.8, calculate the following bolted fault currents:

    • Line to ground
    • Line to line
    • Three phase

    Circuit parameters include:

    • Fault location = 2 miles from the substation
    • Voltage = 12.5 kV
    • Substation transformer: 20 MVA OA rating, 9% impedance, X/R = 10
    • Peak feeder load current = 400 A
    • Phase conductors = 336-kcmil AAC
    • Neutral conductor = 2/0 AAC

  2. Calculate the line-to-ground fault currents for problem #1 using a 1-ohm arc resistance.

  3. Calculate the line-to-ground fault currents for problem #1 using a 1 kV arc voltage.

  4. Calculate the line-to-ground fault currents for problem #1 using a 40-ohm arc resistance.

  5. For 1” x 3/8” busbar, calculate the length of copper bus melted for a 20-kA fault for 1 sec.

Projects

  1. Fault app

    Build a spreadsheet app, web/JavaScript app, or R/Matlab app for plotting fault currents based on user inputs.

  2. Youtube downed conductors

    Look on youtube or other internet video sites, and find and document three cases of downed wires, and identify the cause as best you can as well as any options that might have prevented it.



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To the extent possible under law, Tom Short has waived all copyright and related or neighboring rights to these study questions. This work is published from the United States. Please use this material however you want.