Electrical · Lesson 5 of 13Intermediate7 min read
Grounding and Bonding
Most of the time the ground wire does nothing at all. It’s there for the day something goes wrong, such as a frayed hot wire touching the metal case of a tool. This lesson explains how grounding and bonding turn that dangerous moment into a tripped breaker, and why your plugs are shaped the way they are.
Three wires, three jobs
Modern wiring uses a three-wire system. The hot wire supplies voltage and current to the appliance. The neutral is the return path that completes the circuit. The third wire, the ground, connects to the metal case of the appliance. It carries no current in normal use.
The neutral is connected to the earth at the service, which holds it near zero volts. Even so, the neutral carries current whenever the circuit is on, and a loose connection or a wiring mistake can put voltage on it. Electricians treat every wire as live until they test it.
The fault path
Picture a drill whose hot wire has rubbed through its insulation and is touching the metal housing. Now the housing is at 120 volts.
- Without a ground wire, nothing happens until you pick up the drill while touching something grounded, like a faucet or damp floor. Then the current’s path to ground runs through you.
- With the ground wire intact, the moment the hot wire touches the case, current rushes through the low-resistance ground wire back to the panel. That big current trips the breaker and shuts the power off. The tool stops working, which forces a repair.
OSHA puts it this way: a properly installed grounding conductor has a low resistance and greatly reduces the amount of current that passes through your body. Grounding is a backup protection, though. It doesn’t guarantee you won’t be shocked, which is one reason GFCIs exist (Lesson 6).
WhyWhy a big current helps
A breaker needs a lot of current to trip fast: at about 8 to 12 times its rating, the magnetic trip opens at once (Lesson 3). A solid, low-resistance ground path lets that big current flow, so the fault is cleared in a moment instead of leaving a live metal case. That’s why the path to ground must be permanent and continuous, and able to carry any fault current safely.
The grounding electrode
At the service, the neutral and the equipment ground wires are connected together, and both are connected to a grounding electrode through the grounding electrode conductor. A grounding electrode is a conductor buried in the earth. OSHA lists “made electrodes” such as metal rods, pipes and plates, and says a single rod, pipe or plate with more than 25 ohms of resistance to the earth must be backed up by a second electrode.
The earth connection ties the whole system to the ground around the house. It’s easy to think the earth is what trips the breaker, but the earth is a poor conductor compared with copper wire. Even a ground rod that just meets OSHA’s 25-ohm limit would pass only about 120 ÷ 25 ≈ 5 amps at 120 volts, not enough to trip a 15-amp breaker. When a metal case becomes live, the fault current returns mostly through the equipment ground wire to the panel, where the ground and neutral are tied together, and from there back toward the source. That low-resistance copper loop is what lets enough current flow to trip the breaker fast.
Bonding
Bonding is the permanent joining of metal parts to form a continuous path that can carry any current likely to be put on it. A bonding jumper is a wire that connects metal parts that must be connected together. Metal boxes, conduit, panels and other metal parts that could become energized are bonded to the grounding system. If a hot wire touches any of them, the fault current has a path back and the breaker trips, instead of leaving a live piece of metal for someone to touch.
Plugs and outlets
Plug shapes are part of the safety system:
- Three-prong plugs have a grounding prong that connects the appliance’s metal case to the equipment ground. A standard three-prong plug fits only one way.
- Polarized two-prong plugs have one blade wider than the other, so they fit an outlet only one way. That keeps the hot and neutral wires connected to the right parts of the appliance. The neutral should never be wired to the wrong terminal and reverse that polarity.
- Double-insulated tools and appliances have two layers of insulation and no exposed metal parts, so they don’t need a ground prong. They’re marked as double-insulated.
Never force a three-prong plug into a two-slot outlet, and don’t bend the blades of a plug; bent blades usually cause worse overheating later. In older homes with two-slot outlets and no ground wire, a GFCI can still protect against shock. OSHA’s workplace rules allow a GFCI-protected replacement outlet where no ground exists if it’s marked “No Equipment Ground.” Adding that is a job for an electrician.
| Part | What it does | Signs of trouble |
|---|---|---|
| Equipment grounding conductor | Connects metal cases and boxes to the panel’s ground bar. | Missing in two-slot outlets; broken ground prongs. |
| Neutral (grounded conductor) | Carries normal return current; grounded at the service. | Loose neutrals cause flickering or odd voltages. |
| Grounding electrode | A rod, pipe, plate or other conductor in the earth. | A cut or corroded wire to the rod. |
| Grounding electrode conductor | Connects the service’s neutral and grounds to the electrode. | Damage near the meter or panel: call an electrician. |
| Bonding jumper | Connects metal parts that must be connected together. | Missing or loose after other work was done. |
| Three-prong plug | Connects an appliance’s metal case to ground. | A missing or bent ground prong. |
| Polarized plug | Keeps hot and neutral on the right sides. | A plug that only fits when forced. |
| Double insulation | Two insulation layers instead of a ground. | Cracked housings or damaged cords. |
StopLeave grounding work to an electrician
Grounding and bonding are invisible when they work and dangerous when they don’t. Changing grounding connections, adding a ground wire, or replacing two-slot outlets is work for a licensed electrician. If you get even a slight shock or tingle from an appliance, a metal case or a faucet, don’t touch it again until an electrician has checked it.
Check yourself
Tap an answer to check it, or open “Show the answer.”
What does the equipment ground wire carry during normal use?
Show the answer
Nothing. The ground carries current only when something goes wrong. The neutral carries the normal return current.
A hot wire touches a grounded metal tool case. What should happen?
Show the answer
A large current flows through the ground wire and trips the breaker. The low-resistance ground path lets a big fault current flow, which trips the breaker fast.
Why is one blade of a polarized plug wider?
Show the answer
So it fits only one way and hot and neutral stay on the right sides. The asymmetric plug and outlet keep the hot and neutral connected to the right parts of the appliance.
According to OSHA, what happens if a single ground rod has more than 25 ohms of resistance to earth?
Show the answer
It must be backed up by a second electrode. OSHA’s construction rule says a single rod, pipe or plate over 25 ohms must be augmented by one more electrode.
Why don’t double-insulated tools need a ground prong?
Show the answer
Two layers of insulation and no exposed metal protect the user. Double insulation is an approved alternative to grounding, and such tools are marked to show it.
Go deeper
- OpenStax College Physics 2e, 23.8 Electrical Safety: Systems and Devices The three-wire system, GFCIs and isolation transformers.
- OSHA 3075: Controlling Electrical Hazards (2002, revised) OSHA’s guide to grounding, GFCIs and safe work practices.
- NIOSH: Electrical Safety, Safety and Health for Electrical Trades, Student Manual (Publication 2009-113) NIOSH’s student manual for the electrical trades.
- Being an Adult 101: Test and Reset a GFCI Outlet Test and reset a GFCI outlet.
Sources
- OpenStax College Physics 2e, 23.8 Electrical Safety: Systems and Devices
- OSHA 3075: Controlling Electrical Hazards (2002, revised)
- OSHA 29 CFR 1910.304: Wiring Design and Protection
- OSHA 29 CFR 1926.404: Wiring Design and Protection (Construction)
- OSHA 29 CFR 1910.399: Definitions (Electrical)
- U.S. Consumer Product Safety Commission staff (Lee, Trotta and King): New Technology for Preventing Residential Electrical Fires, Arc-Fault Circuit Interrupters
- U.S. Fire Administration: Appliance and Electrical Fire Safety
- U.S. Consumer Product Safety Commission: CPSC Guide to Home Wiring Hazards (Publication 518)
- NIOSH: Electrical Safety, Safety and Health for Electrical Trades, Student Manual (Publication 2009-113)
- U.S. Consumer Product Safety Commission: Home Electrical Safety Checklist (Publication 513)
- OpenStax College Physics 2e, 20.2 Ohm’s Law: Resistance and Simple Circuits
A free course from Teddy Rook Book. It explains how home electrical systems work in general and follows U.S. sources; your local electrical code, your inspector and a licensed electrician are the final word for your home.