Grounding Vs. Bonding
The Big Picture
- What is “grounding”? What is “bonding”? What’s the difference?
Grounding and bonding is probably the most discussed issue here, aside from 210.52’s design requirements. The following is based on a solidly grounded conventional wiring system. For simplicity's sake, I will not discuss corner grounding, impedance grounding, or ungrounded systems in this post.
The terms are defined in Article 100 and 250.2 of the NEC. Section 250.4 provides the performance requirements of Article 250. Grounding is a connection to earth, and bonding is the connection of items to each other.
Bonding is crucial inside a structure, because without it, if something goes wrong and an ungrounded conductor comes in contact with a piece of metal that someone can touch, that someone will receive a shock and potentially be electrocuted due to the uncleared fault. A quick and dirty definition for bonding is connecting electrical devices together in the attempt to trip a breaker, if an ungrounded conductor touches surface metal associated with the system.
What does the earth have to do with this? Nothing.
Then why is it called an “Equipment Grounding Conductor (EGC)” in the NEC if it’s primary purpose is to “bond” things together? Simple answer: tradition. It’s always been called that, and the terms in the NEC have served to confuse people for a long time. Proposals have been made to change the term, and progress has been made, but the EGC continues to hold it’s misnomer.
Electricity does not seek the path of least resistance to the earth. It seeks all available paths back to its source, in proportion to their resistance. The reason that a person gets shocked when touching an ungrounded conductor and the earth is because the neutral of the system is repeatedly connected to earth in a grounded electrical system. The earth becomes part of a return path to the transformer – it’s part of one route back to the source; the earth is not the destination for the electricity.
Driving a ground rod to ‘ground’ any electrical equipment does not provide the low-resistance path required to trip breakers. Driving a ground rod, or using a Ufer, or a metal water pipe is not a substitute for an EGC. A ground rod with 25 ohms to earth will allow almost five amps to escape the system into the earth when directly energized from a 120V source. Five amps will never trip a 15A or 20A breaker, and in the meantime everything bonded to this ground rod will be energized to 120V.
Thanks to both Chris and Bob for their help on this very important topic!
If any of the items discussed here does not make sense to you, or if you disagree, please start a thread publicly or send a private message to me to discuss it privately, if you desire. It is critical that this issue be understood to ensure the safety of everyone involved.
Edit to clarify solidly grounded systems discussed only.