GND-1 — Grounding Versus Bonding and the Fault-Current Path

Separate the conductor names by function

Grounding is connecting to earth or to a conductive body that extends the earth connection. Bonding joins conductive parts to establish electrical continuity and conductivity. They work together, but they solve different problems. Grounding a system to earth helps stabilize system voltage and limit voltages imposed by lightning, line surges, or accidental contact with higher-voltage conductors. Bonding creates the conductive path needed when an ungrounded conductor contacts metal.

A grounded conductor is intentionally grounded. In a common 120/240 V single-phase system, the neutral is both the grounded conductor and the conductor connected to the winding midpoint. It normally carries unbalanced load current. Do not call every grounded conductor a neutral; a neutral is tied to the system's neutral point, while some grounded systems have no neutral point brought out.

An equipment grounding conductor (EGC) connects normally non-current-carrying metal parts and forms part of the ground-fault current path back toward the source. It may be a wire or a qualifying metal raceway or other recognized method. A grounding electrode conductor (GEC) connects the grounded system conductor or equipment to the grounding electrode or grounding electrode system. A bonding jumper ensures required conductivity between metal parts that might otherwise have an unreliable connection. Paint, enamel, scale, or other nonconductive coatings must be removed or addressed by fittings designed to make the connection as required by 250.12.

The word "ground" in casual speech can hide these different jobs. A grounding electrode conductor to a rod is not an equipment grounding conductor for a branch circuit. A neutral is not a routine equipment bonding conductor downstream. An EGC normally carries no load current, but it must be ready to carry high fault current. Identify both endpoints and the intended current before naming a conductor.

Trace the effective ground-fault current path

Section 250.4(A)(5) requires electrical equipment, wiring, and conductive material likely to become energized to create a low-impedance circuit that facilitates operation of the overcurrent device or ground detector. The path must safely carry the maximum likely ground-fault current from any fault point back to the electrical supply source. The last destination is the source winding, not merely "ground." The section expressly states that earth shall not be considered an effective ground-fault current path.

For a service-supplied branch circuit, trace a typical bolted enclosure fault as a loop:

1. the source winding drives current through the ungrounded conductor;
2. the conductor contacts the metal enclosure;
3. the EGC, bonded raceway, and bonding connections carry current toward the service;
4. the main bonding jumper connects that equipment-grounding path to the grounded service conductor; and
5. the grounded service conductor returns current to the transformer winding.

The high current causes the branch breaker or fuse to open. The grounding electrode system is connected at the service and controls voltage to earth, but soil is not the intended clearing conductor. Calling the path "back to ground" invites a dangerous mistake: installing a rod at a machine does not replace the EGC back to the source.

A simplified Ohm's-law comparison shows why. Assume 120 V and ignore source and conductor impedance except for the stated return path. A bonded metallic path of 0.10 ohm permits 120 V ÷ 0.10 ohm = 1,200 A, likely driving a 20 A breaker into prompt fault operation. A 25 ohm earth path permits only 120 V ÷ 25 ohms = 4.8 A, far below the breaker's rating. Actual current and trip time require the complete impedance and the device curve, but earth resistance plainly cannot be treated as a reliable breaker-clearing path.

Apply the performance requirements

Sections 250.4(A)(1) and (A)(2) address connections to earth that limit imposed voltage and voltage to ground. Sections 250.4(A)(3) and (A)(4) require normally non-current-carrying equipment and other likely-to-be-energized conductive material to be connected together and to the source to establish the effective path. Section 250.4(A)(5) then states the path's impedance, current-carrying, and protective-operation objectives. A low measured electrode resistance does not excuse broken bonding, and excellent bonding does not eliminate the grounding electrode requirements.

The effective path must be permanent and continuous enough for the installation. Loose locknuts, concentric knockouts not bonded by an approved method where required, flex fittings outside their permitted grounding conditions, corrosion, and paint under lugs can add impedance or open the route. A continuity beep alone does not prove that a path can carry available fault current safely. Verify conductor or raceway qualification, sizing, fittings, terminations, and bonding around impaired connections.

Prevent objectionable current

Section 250.6 addresses objectionable current on grounding conductors or equipment. On the load side of the service bonding point, connecting neutral to an enclosure or EGC creates a parallel normal-current route. Neutral load current then divides between the grounded conductor and metal paths such as EGCs, raceways, building steel, cable armor, and connected piping. Those parts can remain energized with normal current even when no fault exists.

In a simplified example, a 40 A neutral load has two equal 0.05 ohm return paths: the intended neutral and an accidental bonded-metal path. Ideal division sends 20 A on each path. Real current divides inversely with impedance, so it may not split equally, but any normal current on the unintended equipment path is the warning. The remedy is not to remove required bonding or the EGC. Find the unintended grounded-conductor connection and restore the required separation.

Do not confuse normal neutral current with ground-fault current or leakage. During a fault, the EGC is supposed to carry current long enough to operate protection. During normal operation, it should not be a parallel neutral. For every exam diagram, trace normal load current and fault current separately; identify the source, every conductive segment, the bonding point, and the device expected to open.