Ground Fault Location on a Branch Circuit

Purpose

A branch circuit that trips the instant the breaker is reset has a fault to ground somewhere in a few hundred feet of cable, and the only two ways to find it are to open every box in the house or to bound it by measurement. Each measurement halves the length still in question, so a circuit with sixteen candidate boxes is down to two in four tests.

It also prevents the wrong instrument giving a clean answer: a multimeter's resistance range drives a few volts across the insulation, and a staple bruise or a damp box reads open at a few volts and conducts at line voltage. An insulation resistance test at 500 V DC finds what the multimeter misses, which is why "I ohmed it out and it was fine" and "it trips every time" are both true about the same circuit.

Scope

Covers locating a fault to ground on a de-energized 120 V or 240 V branch circuit in a dwelling or small commercial occupancy, from the confirmed trip through to a bounded segment, its repair, and the re-test that releases the circuit.

Does not cover a dead circuit with no fault, which the Dead Outlet Investigation SOP owns, or nuisance operation of a ground-fault device with sound wiring, which belongs to the GFCI and AFCI Nuisance Trip SOP. Does not cover feeder or service-level faults, motor winding testing, or fault current calculation.

Roles and handoffs

Procedure

1. Confirm the fault is in the wiring and not in what is plugged into it. Unplug or disconnect every load on the circuit, including hardwired equipment, then reset the breaker. Acceptance: the breaker holds with nothing connected, which puts the fault in a load, or it trips again with nothing connected, which puts it in the wiring. Wrong looks like resetting with a lamp still plugged in and drawing the wrong conclusion. Stop rule: a breaker that holds with everything disconnected ends this procedure, and the fault is chased in the loads, one at a time. Hazard: a breaker being reset onto a fault releases energy at the breaker, so stand to the hinge side rather than square in front of the deadfront, keep the panel door between you and the enclosure, and reset once only.

2. Isolate the circuit completely, both conductors, and prove it dead. Open the breaker, then prove the circuit dead on a known live source before and after per NFPA 70E-2021, 120.5, with work practices at 29 CFR 1910.333(b)(2), and lift the circuit conductor off the breaker and its neutral off the bar so the wiring is floating free of the panel. Acceptance: no voltage on the ungrounded conductor, no voltage on the neutral, and both conductors physically off their terminals and capped. Wrong looks like relying on the breaker alone, which leaves the neutral bonded to ground at the panel and makes every reading you are about to take meaningless. Stop rule: a neutral that still shows voltage to ground with the breaker open means the neutral is shared, and the Shared Neutral and Multiwire Branch Circuit Identification SOP runs first. Hazard: the panel bus is live while you work at the breaker, so use insulated tools, work one-handed, and keep the free hand out of the enclosure.

3. Disconnect every device and electronic load before any test voltage is applied. Remove or disconnect dimmers, smoke alarms, arc-fault and ground-fault devices, LED drivers and surge protective devices. Acceptance: a written count of devices disconnected matching the count on the circuit. Wrong looks like leaving a dimmer in place, which damages the dimmer and pulls the reading down so you chase the dimmer's own internal path. Stop rule: a device you cannot reach means that segment gets tested separately rather than with the device inline. Hazard: opening boxes in an attic or crawl disturbs the insulation packed around them, and loose-fill in a pre-1980 building you cannot positively identify may be vermiculite, which can contain asbestos, so it is not swept, blown or pushed aside; route around it or have it sampled first.

4. Take the three insulation resistance readings at 500 V DC and record all three. With the circuit floating, measure ungrounded conductor to equipment grounding conductor, neutral to equipment grounding conductor, and ungrounded conductor to neutral. Acceptance: the long-standing field minimum for 600 V class building wiring is 1 megohm, and sound dry branch wiring normally reads in the hundreds of megohms, so anything between is a developing fault rather than a pass. Wrong looks like recording only the failing reading, when the pattern is the diagnosis: low hot-to-ground with a clean neutral is insulation damage on the ungrounded conductor, while low neutral-to-ground with a clean hot trips a ground-fault or arc-fault device and never trips a standard breaker. Stop rule: all three above the floor means the fault is intermittent or moisture-dependent and goes to the Intermittent Fault Investigation and Monitoring SOP. Hazard: the instrument puts 500 V DC on those conductors, so announce the test, confirm nobody is at an open box on that circuit, keep fingers behind the probe guards, and let the tester discharge the run before touching either conductor, because a long cable holds a charge after the test ends. It is not applied at all where any conductor of the run is beyond your announcement, where the run shares a cable or raceway with fire alarm, communications or control conductors, or where the far end lands in another tenancy; isolate shorter and test inside your control.

5. Halve the run and re-test each half rather than walking it end to end. Open the circuit at a splice near the middle, cap both sides, repeat the step 4 readings on each half. Acceptance: one half above the 1 megohm floor and the other at or below it, bounding the fault to that half. Wrong looks like both halves reading clean, which means the fault was in the splice you just opened. Stop rule: both halves failing means two faults or a fault in the shared grounding path, so test each half against a separate known ground before continuing. Hazard: you are in boxes on a dead circuit sitting next to live ones, so prove each box dead on its own rather than assuming, and re-cap the far side before the next test so nobody meets an energized cap.

6. Repeat the halving until the fault sits between two adjacent accessible points, then look at it. Acceptance: a bounded segment named by its two end boxes, failing on itself and passing on everything either side. Wrong looks like stopping at a bounded half and starting to open walls. Stop rule: a bounded segment with no accessible cable between the two boxes gets written up and the access quoted before any drywall is cut. Hazard: pulling or flexing cable in a joist bay dislodges debris and whatever it passes through, so eye protection stays on and nobody works directly beneath the pull in a crawl space. Where the run passes through loose-fill of unknown identity in a pre-1980 building, it is not pulled or flexed until that material is identified or sampled; eye protection does not address a fiber route.

7. Repair or replace the bounded segment and record what was actually wrong. Where the cause is a staple bruise, a nail, a rodent bite or a wet box, repair in an accessible listed enclosure or pull a new length. Acceptance: the segment above the 1 megohm floor, in practice well into the hundreds of megohms, on all three step 4 measurements. Wrong looks like taping over damaged insulation inside a wall cavity. Stop rule: cable damaged in more than one place on the same run is replaced end to end, because the cause was mechanical and the rest of the run saw the same event. Hazard: cutting and stripping throws fragments, so eye protection stays on, and charred insulation is handled as a burned-cable condition rather than trimmed and reused.

8. Restore the circuit and prove the protection you removed still works. Re-land the neutral and the conductor, reinstall every device removed at step 3, close the breaker, load the circuit. Acceptance: the breaker holds under a load at a stated current, every receptacle tests correct for polarity and grounding, every ground-fault or arc-fault device reinstalled trips and resets on its own test button, and every smoke and carbon monoxide alarm removed at step 3 is refitted, powered, sounding on its own test button, and where the set is interconnected proved to sound the others from one unit; a set that will not interconnect is not released. Wrong looks like releasing on the breaker simply holding, when the arc-fault device removed at step 3 was never put back or came back on the wrong neutral. Stop rule: a protective device that will not trip on its test button is replaced before release, not noted. Hazard: this is the step that puts energy back with the customer in the room, so clear the space in front of the panel, stand to the hinge side, and close the breaker with the flat of the hand.

The record this produces

One fault record per circuit: the step 1 result with everything disconnected; the step 3 device count; the step 4 trio in megohms with the test voltage stated; every halving point with its pair of readings; the bounded segment named by both end boxes; the physical cause; the post-repair trio; and the step 8 restoration checks including each protective device's test-button result.

The office reads the bounded segment to price access, the customer reads the before and after readings because they cannot see inside a wall, and the next tech reads the cause, since a rodent bite and a construction nail predict different futures for the rest of that house.

Worked pass: 1998 two-story, basement receptacle circuit tripping on reset

Step 1: every load unplugged, including the freezer and a hardwired sump controller. The breaker still trips on reset. The fault is in the wiring.

Step 2: breaker open, circuit proved dead on a known live source before and after, conductor lifted off the breaker and neutral lifted off the bar, both capped. Neutral reads no voltage to ground with the breaker open, so the neutral is not shared.

Step 3: eleven devices on the circuit, of which two are dimmers and one is a hardwired smoke alarm. All three disconnected, along with eight receptacles left in place but with their conductors freed. Step 4 fails. At 500 V DC, ungrounded conductor to ground reads 0.04 megohm, neutral to ground reads 210 megohm, and conductor to neutral reads 190 megohm. Against the 1 megohm floor, 0.04 megohm is well under, and the pattern points at insulation damage on the ungrounded conductor with the neutral sound.

Step 5: the run is split at the sixth box, roughly midway. The panel-side half reads 180 megohm hot to ground and passes. The far half reads 0.04 megohm and fails, so the fault is bounded to the far half.

Step 6: the far half is split again at the ninth box. Boxes 6 to 9 read 240 megohm and pass; boxes 9 to 11 read 0.04 megohm. One more split at box 10 bounds it between boxes 10 and 11, a single 14 ft length running through a rim joist above a laundry sink.

Step 7: the cable is wet and the jacket is split where a shelf bracket screw went through it. The full 14 ft length is replaced rather than patched. The new segment reads 320 megohm hot to ground, 340 megohm neutral to ground and 310 megohm conductor to neutral, all far above the 1 megohm floor.

Step 8: neutral re-landed, conductor re-landed, all eleven devices reinstalled including both dimmers and the smoke alarm. Breaker closed and the circuit loaded to 9.6 A for ten minutes with no trip. Every receptacle tests correct for polarity and grounding, and the smoke alarm sounds on its test button. Circuit released.

References

• NEC 110.7, in the edition your authority having jurisdiction has adopted, for the requirement that completed wiring be free of short circuits and unintended connections to ground
• NFPA 70B, in the edition your shop's maintenance program has adopted, for insulation resistance testing practice and the 1 megohm field minimum applied to 600 V class wiring at step 4, alongside your test instrument manufacturer's own instructions
• 29 CFR 1910.333(b)(2) for work practices, with NFPA 70E-2021, 120.5 for the live-dead-live sequence used at step 2
• See related: Dead Outlet Investigation SOP, Shared Neutral and Multiwire Branch Circuit Identification SOP, Intermittent Fault Investigation and Monitoring SOP