Grounding and ground resistance testing on the PE Power exam

Grounding questions on the PE Power exam draw on two references. The NCEES handbook holds soil resistivity surveys and electrode tests under Ground Resistance Testing, and step voltage, touch voltage, grid resistance and Schwarz's formula under Grounding. NEC Article 250 holds the building side: which electrodes count, and how to size the grounding electrode conductor, the equipment grounding conductor and the bonding jumpers.

What you actually need to own

• Four-pin soil resistivity. With four equally spaced pins, ρ = 2πaR, where a is the spacing between adjacent pins. That short form holds only when the pins are driven no deeper than a tenth of the spacing. Deeper pins need the general form, which carries the burial length. An unequally spaced array has its own relation, which reduces to a short form when the outer spacing is much larger than the inner pair's separation. Take a and b as the handbook figure dimensions them.
• Fall of potential. The handbook prints this layout as the variation of depth, or driven rod, method. R = V/I, read with the potential probe at 62% of the distance from the electrode under test to the current probe, measured from the electrode. The current probe has to be far enough away that the resistance curve has a flat middle.
• Rod resistance. R = ρ/(2πl) · [ln(8l/d) − 1] for a rod of length l and diameter d. Resistance is proportional to resistivity, so one rod reads higher in dry soil than in wet. A test rod gives ρ, which then sizes a proposed rod.
• Grid resistance. For a grid buried shallower than 0.25 m in uniform soil, the minimum is (ρ/4)·√(π/A) and the upper limit adds ρ/L, with A the area the grid occupies and L the total buried conductor length. A grid combined with rods uses Schwarz's formula, which includes the mutual resistance between the two groups.
• Touch, step and potential rise. The permissible touch and step voltages differ in the factor on the surface term. Ground potential rise is the current entering the grid times the grid resistance to remote earth.
• System grounding. With a neutral grounding resistor that dominates the zero-sequence path, ground-fault current is about V_LN/R_n. Zero-sequence current cannot flow in the lines on the delta side of a transformer. It circulates inside the delta winding. On an ungrounded system, a ground fault on one phase raises the other two to line-to-line voltage above ground.
• Three NEC tables, three entry arguments. Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service-entrance conductor. Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device ahead of it. Table 250.102(C)(1) sizes main and system bonding jumpers from the supply conductors, with parallel sets added together.

Where people lose points

• Using ρ = 2πaR with deep pins. Check the pin length against the spacing first.
• Measuring the 62% from the current probe, or taking a voltage reading from another position.
• Using the resistivity of a soil layer the rod never reaches.
• Combining a grid and rods as parallel resistors when a mutual resistance is given.
• Swapping the touch and step relations, or derating for a surface layer the person is not standing on.
• Adding a 3 or √3 to ground potential rise. When the problem gives the current entering the grid, use it as given. A factor of 3 belongs only when it gives the zero-sequence current instead.
• Missing the two adjustments. 250.66(A) limits the conductor that runs only to a rod, pipe or plate electrode, and building steel is none of those. 250.122(B) increases the equipment grounding conductor in proportion when the ungrounded conductors are upsized for any reason other than ampacity correction or adjustment. Voltage drop is the usual case.
• Trusting a clamp-on reading on a bonded rod. The tester reads the loop its jaws encircle.

A note on the codes

The exam supplies its codes, including the 2020 NEC and the 2017 NESC, and scores code answers against those editions. See the PE Power handbook and code books. In Article 250, 250.4 states what system grounding, equipment grounding and bonding are for, 250.52 lists the electrodes, and 250.53 covers their installation. Under 250.53(A)(2), a single rod, pipe or plate electrode needs a supplemental electrode unless its resistance to earth is 25 ohms or less. A metal underground water pipe is covered separately in 250.53(D)(2).

How to study it

Work one site end to end with your own numbers: resistivity from a four-pin reading, one rod, a grid's lower and upper limits, then potential rise for an assumed fault current. For Article 250, size all three conductors for one service. Zero-sequence paths are covered under symmetrical components, and conductor upsizing under voltage drop and conductor sizing.

Part of the General Applications area of the PE Power exam. → Practice it on CastorPrep