Cables, Grounding Systems, and Ground-Fault Protection
Cables, Grounding Systems, and Ground-Fault Protection
This domain tests cable insulation and fault-location tests, grounding electrode and grid measurements, soil resistivity, and GFCI/GFP operation.
Core rules
- Cable insulation tests include IR, DC or VLF withstand, tan delta, and partial discharge; use the test voltage and duration specified for the cable type and age.
- Time-domain reflectometry locates opens and splices; a thumper locates high-resistance faults.
- Grounding: use fall-of-potential for electrode resistance, clamp-on for continuous ground paths, and the Wenner four-pin method for soil resistivity (rho = 2 pi a R).
- Ground-fault protection: verify GFP sensor polarity, pickup, and time delay; test GFCI with a listed tester and at the device.
- Ground grid continuity and step/touch potential are separate from electrode resistance.
Worked example
A fall-of-potential test uses a 100 ft current stake and a potential stake at 62 ft (62% of the current-stake distance). The meter reads 4.8 ohm. With a 5 ohm target, the electrode passes, but the reading must be corrected for stake spacing and soil conditions. The reasoning: 62% spacing places the potential stake in the flat portion of the voltage curve, reducing measurement error.
Traps
- Using a DC hi-pot on aged XLPE without manufacturer guidance.
- Measuring ground resistance with the electrode still bonded to the grid.
- Confusing soil resistivity with electrode resistance.
- Testing a GFCI only at the receptacle and not at the breaker.