Overcurrent Protection and Grounding/Bonding
What you must understand
Know the types of overcurrent protective devices (fuses, circuit breakers), their ratings, interrupting capacity, and the standard ampere ratings. Understand the difference between overload and short-circuit/ground-fault protection, and how motor circuits use separate overload and branch-circuit protection. Study the grounding and bonding system: service grounding electrode system, equipment grounding conductors, bonding jumpers, and the main bonding jumper. Know when GFCI and AFCI protection is required and how the grounded conductor is handled (never bonded downstream of the service disconnect).
Traps
A frequent trap is confusing the grounding electrode conductor with the equipment grounding conductor. Another is bonding the neutral at a subpanel, which creates parallel return paths. Questions on fuse-versus-breaker selection often hinge on the standard rating table and the next-higher-standard rule for certain loads. Motor questions test that overload protection protects the motor from overload while branch-circuit protection handles short circuits. Watch for interrupting rating questions where the available fault current exceeds the device rating.
Rule of thumb and worked example
Rule of thumb: the service disconnect is the only place where the grounded conductor is bonded to the equipment grounding system. Example: a 100-ampere service with a 4 AWG copper grounding electrode conductor must terminate in the grounding electrode system without splices except by irreversible means. If a subpanel is fed from that service, its neutral bar must be isolated from the enclosure and the equipment grounding conductor must be separate.