022: Grounding vs. Bonding: Safety Implications
Discussion leader
duties for this session:
Review the definitions of
grounding and bonding in the
NEC, Article 100. Review any
company policies on grounding. Be prepared to discuss
where the word “bonding”
should be used in place of
“grounding.”
What this Safety Talk
covers:
How grounding and bonding
differ, and how to avoid confusing the two.
Discussion notes:
Background
The misuse of the word “grounding” in place of “bonding” dates back to the early
days of electrical applications, when electrical theory was poorly understood. The
unfortunate choice of wording has stuck with us, and the resulting confusion is
dangerous.
Some grounding and bonding facts
■ When you bond one metallic object to another, the extremely low resistance
path between them eliminates a difference in potential, for all practical purposes. You can verify this by drawing the circuit and applying Ohm’s Law.
■ When you connect one metallic object to another only through an earth connection, you have a high-resistance path between them. The resistance value
through earth varies wildly, even in the same location. Differences in potential
when using earth as a bonding jumper can be large and varied.
■ The NEC provides clear definitions of bonding and grounding in Article 100.
But the NEC then uses “grounding” where it actually means “bonding.”
OSHA has followed this same pattern. From the context, you can see where
bonding is the intention. Don’t drive a ground rod in lieu of bonding. Doing so
will leave a difference of potential.
■ Just because something is “grounded” doesn’t mean it’s safe to touch. It can
still be at a high difference of potential from other nearby objects, including
you.
If you can’t decide whether to ground or to bond
■ If the goal is to create an equipotential plane (eliminate difference in potential), then you bond (not ground).
■ Draw the circuit and apply Ohm’s Law. You want to provide a low-resistance
path for unwanted electricity.
■ Draw the circuit and apply Kirchoff’s Law of Parallel Circuits. Putting a highresistance path in parallel with a circuit doesn’t cause all electricity to flow
only in the high-resistance path. In fact, it does very little to protect you.
■ Determine where the electricity needs to get back to. If the source is anything
but lightning, electricity is trying to get back to the supply. Earth is a high
resistance path, and bonding is a low-resistance path.
■ Determine if you are trying to protect the premises from lightning or trying to
protect people from electric shock. If shock, then bonding is your choice.
Review and discussion
1. Which method provides a low-resistance path: grounding or bonding?
2. If you need to eliminate differences in potential, which method do you
use and why?
3. If the NEC or any other standard calls for grounding, is this necessarily what is meant?
4. How can you apply basic electrical theory—Ohm’s Law and Kirchoff’s Law—to help you clear up the confusing misuse of grounding where bonding is meant?
5. Give some examples of where bonding should be used even though the terminology in the requirement is
“grounding.”
6. Where is electricity always trying to get back to?
7. Explain why this statement is false: “If something is grounded, it’s safe to touch.”
8. You are working on a new installation. You notice separate ground rods driven for each of the utilities,
including the gas main, water, communications lines, and power. None of these are connected with bonding
jumpers. Why is this installation unsafe?
9. During a retro-fit, you notice many IG (isolated ground) receptacles. These are installed such that the green
wire runs to a ground rod that is not bonded to the main system in any way other than through earth. Why is
this installation unsafe? [Note to presenter: This installation seeks to suspend Kirchoff’s Law and Ohm’s Law. It violates the
grounding and bonding theory presented in IEEE 142 (the Green Book) and violates NEC 250.4, among other references.]
10. You are installing new control panels on a large robotic welder that runs on medium voltage. You notice
ground rods driven into the cement. There’s one at the main drive motor, one at the welding power supply,
and another on the main frame. Upon further inspection, you find no bonding jumpers from any of these
places and no bonding jumpers to the main grounding conductor. If there’s power to this unit, is there also a
major shock hazard? Why or why not?
Participant’s Signature and Date
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