Grounding, Ground Fault Protection, and Power Quality
Grounding, Ground Fault Protection, and Power Quality covers electrode resistance, touch and step voltage, GFP coordination, harmonics, and voltage quality. The exam tests whether you can separate safety grounding from power-quality symptoms.
Core rules
- Ground resistance is measured by fall-of-potential or clamp-on; isolate the electrode where required.
- Lower resistance is not always the only goal; the grid must limit touch and step voltage and provide a fault path.
- Ground-fault protection uses residual or zero-sequence current; coordination must avoid nuisance trips while clearing faults.
- Power quality: THD = √(sum of harmonic squares)/fundamental × 100%. IEEE 519-style limits apply at the point of common coupling.
- Voltage sag is a short-duration rms reduction; harmonics are steady-state distortion.
Worked example
A grounding electrode measures 12 Ω; the design target is 5 Ω. Add parallel rods or a ground ring, then retest. Two identical 12 Ω rods spaced far apart give about 6 Ω; four give about 3 Ω. Spacing must exceed the rod length for independence. For power quality, a 100 A fundamental with 30 A of harmonic current has THD = 30/100 × 100 = 30%.
Traps
Measuring ground resistance with the electrode still bonded to the system; assuming low resistance equals safety; confusing bonding with grounding; using a clamp-on meter on a single unbonded conductor; ignoring harmonic phase angles.