Grounding, Bonding, and Overcurrent Protection
Core concepts
Distinguish system grounding (intentionally connecting a circuit conductor to earth) from equipment grounding (bonding non-current-carrying metal parts to provide a low-impedance fault path) and from bonding (electrically joining metal parts to ensure they remain at the same potential). Know the grounded conductor versus the grounding conductor, the role of the main bonding jumper, grounding electrode systems and electrodes, and the requirement that equipment grounding conductors be sized relative to the overcurrent device. Understand overcurrent device types, interrupting rating, series ratings, coordination, and the rules for tap conductors and motor circuits.
Traps in questions
- Using the grounded conductor as an equipment grounding conductor on the load side of the service disconnect.
- Sizing an equipment grounding conductor from load current instead of from the overcurrent device rating.
- Ignoring the interrupting rating; a device must be able to clear the available fault current.
- Confusing ground-fault protection of equipment with personnel ground-fault circuit-interrupter protection.
- Overlooking that bonding jumpers around water meters, gas piping, and structural steel are bonding, not grounding.
Worked example
A 200 A feeder requires an equipment grounding conductor. Look up the permitted size in the conductor table keyed to the overcurrent device rating rather than to the phase conductor ampacity. Rule of thumb: if the phase conductors are increased for voltage drop, the equipment grounding conductor generally must be increased proportionally.
Practical link to inspection
Grounding and bonding defects are the most consequential safety findings because they defeat the fault path that makes overcurrent protection work.