Electric Power Distribution Handbook, T. A. Short

14. Grounding and Safety

Questions

  1. Per the NESC, do all ground rods on multigrounded systems have to be below 25 ohms?

    1. Yes
    2. No

  2. On an ungrounded system, what voltage will the phase-to-ground voltage rise to for a ground fault?

    1. 125%
    2. 150%
    3. 173%

  3. Can you be killed by hand-to-hand contact across 20 V?

    1. Yes
    2. Probably
    3. Highly unlikely

  4. Which soil has the lowest resistivity?

    1. Sand
    2. Clay
    3. Gravel

  5. For an 8-ft rod in dry sand, what’s the most likely resistance to ground?

    1. 5 ohms
    2. 50 ohms
    3. 500 ohms

  6. What’s the leading cause of electrical fatalities on distribution circuits?

    1. Missing ground
    2. Equipment failure
    3. Human error

  7. For the top part of a tree touching a 12.5/7.2-kV line, is my life at danger with one hand touching the tree?

    1. Yes
    2. Probably
    3. Highly unlikely

  8. If I’m up in a tree cutting a large branch, and it falls into a 12.5/7.2-kV line, is my life in danger?

    1. Yes
    2. Probably
    3. Highly unlikely

  9. You measure 10 V on a streetlight, and the total harmonic distortion is 2%, which is more likely?

    1. NEV
    2. Contact voltage

  10. What’s the leading cause of electrical fatalities on distribution circuits?

    1. Missing grounds
    2. Equipment failure
    3. Human error

  11. Given two cases of contact voltage, one energized at 20 V and the other energized at 5 V, which is the most appropriate response?

    1. Both need to be repaired soon.
    2. The 20-V case is urgent; the other can be addressed later.

  12. True/false: manhole events from a primary splice failure may be worse in a smaller manhole.

    1. True
    2. False

  13. Why are lockout-tagout failures more of a concern on underground distribution?

    1. No work-site grounds
    2. Higher fault currents
    3. Confined space

  14. True/false: vented manhole covers relieve pressures from explosions.

    1. True
    2. False

  15. If the available fault current is twice as high, what happens to arc flash incident energy? Assume a 0.2-sec clearing time.

    1. Stays the same
    2. About doubles
    3. Almost quadruples (I2t)

  16. Comparing a 25-kV system to a 12.5-kV system. Which is true (assuming the same fault current).

    1. 25-kV energy is about 2 times the 12.5-kV energy
    2. 25-kV energy is about equal to the 12.5-kV energy
    3. None of the above

  17. Which scenario has the worst arc flash hazard?

    1. 480-V transformer secondary compartment with 20 kA available fault current
    2. 480-V self-contained meter with 10 kA available fault current

Problems

  1. Calculate the resistance to ground for a ground rod with a diameter of 1/2 in and a length of 10 ft where the soil resistivity is 100 ohm-m.

  2. Calculate the arc flash incident energy using the IEEE 1584 equations and assumptions for the following conditions:

    • 12.5-kV switchgear
    • 0.5-sec duration
    • 6-kA fault current
    • 24-in worker distance

  3. Estimate the maximum duration allowed for arc flash at 12.5-kV with 8 cal/cm^2 clothing for the following conditions:

    • ARCPRO, 2-in arc, 15-in working distance
    • ARCPRO, 15-in arc, 15-in working distance
    • IEEE 1584, 24-in working distance
    • Padmounted switch equation, 24-in working distance

  4. Calculate the earth fault factor for faults in each of the unfaulted phases for both a L-G fault and a L-L-G fault at a location 6 miles from the substation for the following two cases:

    • 12.5 kV, 5-MVA, 6% station transformer, 3/0 ACSR with a #2 neutral
    • 12.5 kV, 5-MVA, 6% station transformer, 3/0 ACSR with a 3/0 neutral

  5. For the tree resistance diagram in Fig. 14.26, find the body current for these scenarios:

    • Case 1:
      • Hand-to-foot body resistance = 1000 ohms
      • Foot-to-earth resistance = 2000 ohms
      • 25/14.4-kV single-phase contact at 10.8 m
      • Hand contact at 2 m
      • Bark resistance = 50 kohms
    • Case 2:
      • Hand-to-foot body resistance = 1000 ohms
      • Foot-to-earth resistance = 2000 ohms
      • 25/14.4-kV single-phase contact at 10.8 m
      • Hand contact at 2 m
      • Bark resistance = 0 ohms
    • Case 3:
      • Hand-to-foot body resistance = 1000 ohms
      • Foot-to-earth resistance = 2000 ohms
      • 25/14.4-kV single-phase contact at 7 m
      • Hand contact at 2 m
      • Bark resistance = 0 ohms

  6. For a current of 100 mA, what’s the duration required for the threshold of defibrillation?

Projects

  1. Ground-fault overvoltages

    Build a model in OpenDSS (or an equivalent tool) that models a three-phase mainline with each phase and the neutral modeled independently. Apply grounds periodically. Apply a fault near the substation. Plot current flows on the neutral and the grounds. Compare with idealized equations.

    Try these variations:

    • Vary the fault location.
    • Break the neutral connection.
    • Vary the neutral conductor size.

  2. NEV

    Similar to the previous problem. Build a model in OpenDSS (or an equivalent tool) that models a three-phase mainline with each each phase and the neutral modeled independently. Apply grounds periodically. Apply unbalanced loads. Plot neutral voltages along the circuit.

    Try these variations:

    • Break the neutral connection.
    • Vary the neutral conductor size.
    • Vary the magnitudes of individual grounds.
    • Apply a L-G fault.
    • Add a capacitor bank, and try to create a resonance. See how high the voltage can be with reasonable assumptions.

  3. Underground personnel protection

    Build a model in OpenDSS (or an equivalent tool) that models each cable phase and neutral. Break the cable at a splicing location. Install bracket grounds upstream and downstream. Apply faults at various locations and evaluate all touch potentials at the work site.

    Try these variations:

    • Vary distances of bracket grounds.
    • Instead of bracket grounds, isolate the phases at bracket locations.
    • Jumper the neutrals together at the work site.
    • Jumper the neutrals to a ground mat at the work site.

  4. Arc flash

    Implement an IEEE 1584 calculator in R, Octave, Matlab, JavaScript, a spreadsheet, or some other tool.

  5. Tree contacts

    Build an impedance model of a tree and include various contact points to estimate body currents and risks.



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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.