Grounding, Bonding, and Electrical Safety
This domain tests grounding and bonding for power systems and low-voltage systems, plus safe work practices that affect installation and inspection.
Core rules
- Article 250 performance rules in 250.4 require a low-impedance fault path, bonding of metal parts that may become energized, and a grounding electrode system.
- Section 250.50 requires all present electrodes to be bonded into one system. Rod electrodes are at least 8 ft long and need a supplemental electrode if resistance exceeds 25 ohms (250.53(A)(2)). The grounding electrode conductor is sized per 250.66; the equipment grounding conductor per 250.122 and is upsized when phase conductors are upsized.
- Communications, CATV, and broadband systems have their own grounding rules in 800.100, 820.100, and 830.100. They should land on the intersystem bonding termination required by 250.94.
- Safety: test before touch, use lockout/tagout, and select PPE and approach boundaries for the voltage present. MIOSHA rules apply on the job.
Worked example
A communications cable enters a building and its shield must be grounded. The installer runs a copper conductor from the communications grounding busbar to the intersystem bonding termination at the service, sized per Table 800.100(B), instead of to a nearby water pipe. The water pipe qualifies only if it is part of the grounding electrode system and bonded per 250.104.
Traps
Grounding to an unbonded water pipe; using the equipment grounding conductor as a signal reference; assuming low-voltage systems need no grounding; treating 25 ohms as a hard failure instead of a trigger for a supplemental electrode; omitting the intersystem bonding termination.