EGC Resistance Reads Low But Fault Current Path Fails: Decision Tree

Why this matters

A low ohmmeter reading on an equipment grounding conductor (EGC) feels like proof the fault path is good, but ohms tell you almost nothing about whether the circuit will actually clear a ground fault. A multimeter pushes microamps through the EGC; a real line-to-ground fault pushes hundreds of amps, and a path that looks like 0.4 ohms on the bench can balloon to several ohms or open entirely under load because of a loose lug, a painted conduit thread, a bonding bushing left finger-tight, or a star-washer biting only paint. When the impedance of the effective ground-fault current path (NEC 250.4(A)(5)) is too high, the overcurrent device never sees enough current to trip, leaving metal energized at 120 V while your meter swears the ground is fine. This is the failure mode behind shocks from "grounded" equipment, and it is the single most misread test in the trade.

Symptom presentation

You measure EGC-to-neutral or EGC-to-ground continuity at a receptacle or piece of equipment and read a clean low value, often under 1 ohm. Yet a deliberate or accidental hot-to-frame fault either does not trip the breaker, trips slowly, or you read elevated touch voltage on the enclosure. Receptacle testers may show "correct" because they only verify low-current continuity. The tell is that the metal stays live or warm, GFCI/AFCI behavior is erratic, or a megohm/loop test contradicts the simple ohmmeter.

Quick checks

• Confirm power is off and the circuit is locked out before touching conductors. Verify with a meter, not the breaker label.
• Re-read EGC continuity but wiggle every termination, junction, and conduit coupling while watching the meter. A reading that jumps means a marginal joint.
• Inspect every bonding point in the suspected path: panel ground bar lug torque, bonding jumper on metal raceway, bonding bushing on concentric/eccentric knockouts (NEC 250.92), and any star washer seated on bare metal rather than paint.
• Check that conduit is actually approved as the EGC for this run, or that a separate wire EGC is present and unbroken (NEC 250.118).
• Look for a missing or undersized main bonding jumper at the service (NEC 250.24, 250.28) and confirm the neutral-ground bond exists at exactly one point.

Isolation tree

Start at the load and work toward the service. First, decide is the EGC even electrically the path that would carry fault current. Open the device, separate the wire EGC from the box, and read continuity wire-only versus box/conduit-only. If wire reads low but box reads high, the metallic path is the problem; if box reads low but the device frame reads high, the device-to-box bond (mounting screw, yoke clip) is the problem.

Next, branch on where impedance appears. Use a four-wire or clamp-style ground loop tester if available, since it forces a known current and reads true loop impedance, not bench microamp ohms. If the loop value is high while the handheld ohmmeter is low, you have a connection that conducts at low current but chokes at fault current, the classic loose-lug or paint-under-washer signature.

Then branch on raceway versus wire EGC. For metal-conduit-as-EGC systems, megger or loop-test each section: a bonding bushing left off a concentric knockout, a reducing washer carrying the bond, or a flex connector without a bonding jumper (NEC 250.102) will read low cold and open hot. For wire-EGC systems, the failure is almost always a backed-out lug, a green screw on paint, or an EGC spliced with a wirenut that never seated.

Finally branch on the service bond. If the path is good all the way to the panel but fault current still cannot return, verify the main bonding jumper and the grounded-conductor-to-enclosure bond. A subpanel with neutral and ground improperly bonded, or with a missing equipment bonding jumper between sections, breaks the return even when every downstream EGC reads zero.

Confirming diagnosis

Confirm with a forced-current method, never with a handheld ohmmeter alone. A ground loop impedance tester or an analog earth-ground tester applies enough current to expose marginal joints; a path good for fault clearing typically shows loop impedance low enough that available fault current exceeds several multiples of the breaker rating. As a controlled bench confirmation on a de-energized, isolated circuit, some technicians apply a known load between hot and EGC through a current-limited source and watch whether the breaker trips in its expected time; if it does not, the path impedance is too high regardless of the cold ohm reading. Re-torque the suspect lug to manufacturer spec and retest; if the loop value drops dramatically after torquing, you have proven the loose connection was the fault-path failure.

Remediation

Repair the specific high-impedance joint, then re-verify the whole path. Re-terminate loose lugs to the manufacturer torque value with a calibrated driver. Install bonding bushings and bonding jumpers on every concentric/eccentric knockout and every flex/EMT-to-box transition that the bond depends on (NEC 250.92, 250.102). Replace star washers seated on paint with a fastener landed on clean bare metal, or use a listed grounding screw in a tapped hole. Where conduit was assumed to be the EGC but joints are suspect, pull a properly sized wire EGC per NEC 250.122 sized to the overcurrent device. Confirm exactly one neutral-ground bond at the service and that subpanels keep neutral and ground separated. After any repair, re-run the loop/impedance test and confirm the value will drive the breaker into its instantaneous or short-time trip region.

References

• NFPA 70 (NEC) 250.4(A)(5), Effective Ground-Fault Current Path performance requirement
• NFPA 70 (NEC) 250.92 and 250.97, Bonding of service and over-250V enclosures and raceways
• NFPA 70 (NEC) 250.102, Equipment bonding jumpers, and 250.118, Types of equipment grounding conductors
• NFPA 70 (NEC) 250.122, Sizing equipment grounding conductors to the overcurrent device
• NFPA 70E, Standard for Electrical Safety in the Workplace, energized-work and verification practices
• NECA 331, Standard for Building and Service Entrance Grounding and Bonding