A hot conductor faults to a metal enclosure. What is the main job of the bonding (equipment grounding) conductor at that moment?

B. To give the fault current a low-impedance path back to the source, so enough current flows to trip the protective device fast

A. To carry the fault current into the earth, where it disappears

C. To keep the neutral current balanced

Why is the neutral bonded to ground at one point (the service or the source of a separately derived system) and not again downstream?

B. Because a second bond puts normal neutral current onto the bonding conductors, conduit and enclosures

C. Because the code book likes round numbers

D. It makes no difference where the bonds are

What makes a transformer secondary a "separately derived system"?

B. Its secondary has no direct electrical connection to the supply conductors, including the supply neutral, so it needs its own system bond and grounding

What is the difference between grounding and bonding?

B. Grounding connects to earth (the electrode system); bonding joins metal parts together and back to the source so they sit at the same potential and form a fault path

A. They are two words for the same thing

C. Bonding is only for water pipes

D. Grounding is only for lightning

You find the neutral and ground bars bonded together in a subpanel downstream of the service. What does that tell you?

B. There is likely an extra neutral-to-ground bond; the neutral current now has parallel paths on the bonding system. It needs a look against the drawings and the code book

A. Nothing — that is normal in every panel

C. The subpanel is a separately derived system

Troisième année Lesson 10 of 13 · about 15 min

Grounding and bonding as a system

What you’ll learn

The two separate jobs: grounding to earth, and bonding metal back to the source.
The fault return path, traced end to end, and why the earth is a poor conductor for it.
Why the neutral is bonded to ground at one point only.
What a separately derived system is and why a transformer secondary needs its own bond.

Before this: Hot, neutral, ground · Bonding vs grounding

Grounding and bonding are two different jobs that meet at one point. Grounding ties the electrical system to the earth. Bonding ties every piece of exposed metal together and back to the source, so a fault has a low-impedance road home and the protective device opens fast. In first and second year you learned the wires; in third year you look at the whole system — service, feeders, transformers and all.

Two jobs

System grounding connects one point of the supply — usually the neutral of a wye — to the grounding electrode system (rods, plates, a building’s steel or a water pipe, as your book allows). That holds the system at a steady voltage to earth, gives lightning and switching surges somewhere to go, and makes a fault to earth detectable.

Bonding joins all the metal that does not normally carry current — boxes, raceways, enclosures, motor frames, cable armour — into one connected system that runs back to the service. With everything bonded there is no dangerous voltage between two things a person could touch at once, and a hot conductor touching any of it creates a short circuit that trips the breaker instead of an energised enclosure that waits for someone.

The fault return path

Fault current goes back to the source, not into the ground: out on the hot, through the fault into the frame, back on the bonding conductor, across the main bonding jumper (MBJ) to the neutral, and home. The electrode does its own job; it is not on this road.

Current only flows in a loop, and the loop for a fault ends at the transformer that made the voltage. So the question is never “is it grounded?” but “how good is the path back to the source?” The lower the impedance of that loop, the higher the fault current and the faster the protective device opens.

Worked: the bonding path versus the earth

A 120 V circuit. The hot conductor out to the fault is 0.2 Ω; the bonding conductor back is 0.2 Ω. Loop = 0.4 Ω (ignoring the transformer).
Fault current = 120 ÷ 0.4 = 300 A — far above a branch breaker’s rating, so it trips in its fast region.
Now remove the bonding conductor and let the fault find its way home through a ground rod at the motor and the earth, say 25 Ω in total.
Fault current = 120 ÷ 25 = 4.8 A. The breaker will never notice. The motor frame sits near 120 V to anything grounded nearby — until someone touches it.

One bond, at one place

The neutral and the bonding system are joined at one point: the main bonding jumper at the service (or the system bonding jumper at the source of a separately derived system). Downstream of that point they are kept apart — the neutral bar in a subpanel is isolated from the can, and the bonding conductors land on their own bar. If they are joined again downstream, normal neutral current finds parallel paths over conduit, enclosures and bonding conductors. That current makes shocks, heats joints, hums in audio and data, and trips ground-fault protection for no visible reason.

Separately derived systems

A transformer with separate primary and secondary windings — a 600 V or 480 V delta primary feeding a 208Y/120 or 480Y/277 secondary, say — creates a brand-new system. Its secondary neutral has no connection at all to the building’s service neutral. So it needs its own neutral-to-ground bond at the transformer or its first disconnect, its own connection to the grounding electrode system, and bonding conductors that run back to that point. A generator whose neutral is switched by the transfer switch is treated the same way. The exact method and sizes are in your code book; the idea is simply that every system gets exactly one bond, at its own source.

Sur le chantier

Tips

Trace the fault loop in your head for any new installation: source, hot, fault, bonding, bond point, neutral, source. Every break in it is a problem.
In a subpanel, look for the green bonding screw or strap before you land the neutral — it usually comes out.
Use proper bonding fittings where a raceway is the bonding path; paint and loose locknuts are high impedance.
At a transformer, find where the system bond is supposed to be on the drawings, and confirm there is one — and only one.
Clamp a meter around a bonding conductor on a live system: steady current there often means an extra neutral bond somewhere.

Common mistakes

Believing a ground rod will trip a breaker.
Bonding neutral to ground again in a subpanel.
Forgetting the system bond on a transformer secondary, leaving a floating system.
Using the neutral as the bonding path downstream of the service.
Relying on a flexible or painted raceway joint as the only bonding path.

Check yourself

0 right of 0 answered 7 questions · pick one answer each

1.A hot conductor faults to a metal enclosure. What is the main job of the bonding (equipment grounding) conductor at that moment?
2.A 120 V fault returns only through the earth via a ground rod with 25 Ω resistance. Roughly what current flows?
3.Why is the neutral bonded to ground at one point (the service or the source of a separately derived system) and not again downstream?
4.What makes a transformer secondary a "separately derived system"?
5.What is the difference between grounding and bonding?
6.A fault loop has 0.2 Ω of hot conductor and 0.2 Ω of bonding conductor on a 120 V circuit. Ignoring the source impedance, what fault current flows?
7.You find the neutral and ground bars bonded together in a subpanel downstream of the service. What does that tell you?

Answers

B. To give the fault current a low-impedance path back to the source, so enough current flows to trip the protective device fast — Fault current wants to get back to the source (the transformer), not "into the ground". A solid bonding path back to the service, and through the main bonding jumper to the neutral, makes the fault loop low-impedance, so a big current flows and the breaker or fuse clears quickly.
A. 4.8 A — I = V ÷ R = 120 ÷ 25 = 4.8 A. That will not trip a typical branch breaker, so the enclosure stays energised. The earth is a poor return path — which is exactly why the bonding conductor, not the rod, is what clears faults.
B. Because a second bond puts normal neutral current onto the bonding conductors, conduit and enclosures — Downstream of the one bond, the neutral and the bonding path are separate. Bond them again in a subpanel and normal load current splits between the neutral and every metal path in parallel — conduit, enclosures, bonding conductors — which causes shocks, noise, heating and nuisance ground-fault trips.
B. Its secondary has no direct electrical connection to the supply conductors, including the supply neutral, so it needs its own system bond and grounding — An isolating transformer (such as a delta-wye) makes a brand-new system on its secondary. Its neutral point has no connection to the upstream neutral, so it gets its own neutral-to-ground bond and grounding electrode connection at the transformer or its first disconnect, per your code book.
B. Grounding connects to earth (the electrode system); bonding joins metal parts together and back to the source so they sit at the same potential and form a fault path — Grounding ties the system to the earth — stabilising voltage to ground and giving lightning and surges somewhere to go. Bonding ties all the non-current-carrying metal together and back to the source, so there is no dangerous voltage between parts someone can touch and a fault has a low-impedance path home.
B. 300 A — Total loop impedance = 0.2 + 0.2 = 0.4 Ω, so I = 120 ÷ 0.4 = 300 A — many times a branch breaker's rating, so it trips in its fast region. Compare it with the 4.8 A the earth path gave.
B. There is likely an extra neutral-to-ground bond; the neutral current now has parallel paths on the bonding system. It needs a look against the drawings and the code book — In a subpanel fed from the service the neutral bar is normally isolated from the enclosure and the bonding bar. A bonding screw or strap left in place creates a second bond. Report it and confirm against the drawings — it is a correction, not a guess.

Questions fréquentes

Does a ground rod make a circuit safe?

Not on its own. A rod has far too much resistance to the earth to carry enough fault current to trip a breaker — a few amps on 120 V, typically. The rod and the rest of the grounding electrode system do other jobs: they stabilise the system voltage to earth and give lightning and surges a path. Fault clearing is done by the bonding conductors back to the source.

Why is it called "equipment grounding conductor" in some books and "bonding conductor" in others?

Different codes use different words for the same wire. The Canadian Electrical Code calls it a bonding conductor; the NEC calls it an equipment grounding conductor. Either way it is the conductor that ties exposed metal back to the source so a fault clears. Use your own book's term on the exam.

What is an ungrounded (or high-resistance grounded) system?

Some industrial systems deliberately have no solid bond between the system and ground, or bond it through a resistor. A first ground fault then causes little current and the plant keeps running, while a detector raises an alarm to find it before a second fault on another phase causes a line-to-line short. They need their own monitoring and procedures, and the rules for them are in your code book.

Voir aussi

Bonding vs grounding — the short definition
Hot, neutral, ground — the three wires by function
Overcurrent protection and coordination
Three-phase transformer connections — where separately derived systems come from
Les bases des transformateurs

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