Per the NESC, do all ground rods on multigrounded systems have to be below 25 ohms?
On an ungrounded system, what voltage will the phase-to-ground voltage rise to for a ground fault?
Can you be killed by hand-to-hand contact across 20 V?
Which soil has the lowest resistivity?
For an 8-ft rod in dry sand, what’s the most likely resistance to ground?
What’s the leading cause of electrical fatalities on distribution circuits?
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?
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?
You measure 10 V on a streetlight, and the total harmonic distortion is 2%, which is more likely?
What’s the leading cause of electrical fatalities on distribution circuits?
Given two cases of contact voltage, one energized at 20 V and the other energized at 5 V, which is the most appropriate response?
True/false: manhole events from a primary splice failure may be worse in a smaller manhole.
Why are lockout-tagout failures more of a concern on underground distribution?
True/false: vented manhole covers relieve pressures from explosions.
If the available fault current is twice as high, what happens to arc flash incident energy? Assume a 0.2-sec clearing time.
Comparing a 25-kV system to a 12.5-kV system. Which is true (assuming the same fault current).
Which scenario has the worst arc flash hazard?
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.
Calculate the arc flash incident energy using the IEEE 1584 equations and assumptions for the following conditions:
Estimate the maximum duration allowed for arc flash at 12.5-kV with 8 cal/cm^2 clothing for the following conditions:
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:
For the tree resistance diagram in Fig. 14.26, find the body current for these scenarios:
For a current of 100 mA, what’s the duration required for the threshold of defibrillation?
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:
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:
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:
Arc flash
Implement an IEEE 1584 calculator in R, Octave, Matlab, JavaScript, a spreadsheet, or some other tool.
Tree contacts
Build an impedance model of a tree and include various contact points to estimate body currents and risks.
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