Grounding and Bonding Verification

Purpose

Prove that a service's grounding and bonding is doing the two separate jobs it exists to do: carrying fault current back to the source hard enough to open an overcurrent device quickly, and holding every touchable metal part at the same potential as everything else a person can touch at the same time. Earth resistance does neither of those. It is the fault return path through the equipment grounding conductor and the main bonding jumper that clears faults, and that path is invisible from the deadfront.

Without this procedure a shop signs off systems where a fault that should trip in a fraction of a cycle instead sits on a cabinet as a shock hazard, or where a bootleg neutral-to-ground bond in a subpanel puts normal load current on every ground wire in the building. Both pass a visual inspection. Both are found in under an hour with a clamp meter and a plan.

Scope

Covers verification of grounding and bonding on an existing or newly installed single service at a residential or light-commercial site: the electrode system, the grounding electrode conductor and its terminations, the main bonding jumper, feeder and branch equipment grounding conductors, piping bonds, and the intersystem bonding termination.

Does not cover sizing an electrode system on a new service, which the service-panel-upgrade SOP owns; separately derived systems fed from a transformer or standby generator, where the system bonding jumper location is the whole question and which belongs with the generator transfer-switch SOP; or pool and spa equipotential bonding, which is a bonded grid rather than a return path.

Roles and handoffs

Procedure

1. Decide energized or de-energized before you touch a cover, and gear for the harder answer. Steps 5 and 7 need load on the system, so most of this procedure runs with the service live. Removing a deadfront on energized equipment is energized work: it needs a documented justification and an energized electrical work permit under NFPA 70E-2021, Article 130, in the edition your employer's electrical safety program adopts, plus arc-rated PPE selected for that equipment. Acceptance: permit in hand or the panel is de-energized, locked and tagged under 29 CFR 1910.333(b)(2) and proved dead live-dead-live per NFPA 70E-2021, 120.5. Wrong looks like a tech reaching for cover screws with a shirt on and no permit. Stop rule: nobody opens anything until one of those two states exists.

2. Locate and identify every electrode present, and write down the ones that are absent. Walk the service: rod or rods, metal underground water pipe in earth contact for 10 ft or more, concrete-encased electrode, ground ring, building steel. Acceptance is a written list where each entry says what it is, where it lands and how it is connected. Wrong looks like "ground rod, present." Stop rule: a qualifying metal underground water pipe that is not bonded is a defect logged now, not discovered at step 6 and blamed on a rod. Hazard here is the crawlspace, not the electricity: check for standing water before a knee goes down, and back out and call the utility if the meter pit is flooded.

3. Inspect the grounding electrode conductor end to end for continuity, splices and termination quality. It must be continuous or spliced only by irreversible compression connectors listed for the purpose or by exothermic weld, per NEC Article 250 in the edition your authority having jurisdiction has adopted. Acceptance: no split-bolt or wire-nut splices, clamps listed for direct burial where buried, no paint or corrosion under the lug, terminations at the torque marked on the equipment. Wrong looks like a taped splice in a crawlspace. Stop rule: a spliced GEC gets written up and quoted, not re-torqued and passed.

4. Verify the main bonding jumper at the service disconnect, and that it is the only one. The grounded conductor and the equipment grounding system connect at exactly one place. Acceptance: the bonding screw or strap is installed, seated and torqued, and the neutral bus is bonded to the enclosure there and nowhere downstream. Wrong looks like a green bonding screw still in the panel's parts bag while the installer assumed a factory strap. Stop rule: a service with no main bonding jumper has no low-impedance fault return path at all, so de-energize under 29 CFR 1910.333(b)(2), install it, and re-verify before anything else here means anything.

5. Clamp each feeder equipment grounding conductor with the downstream panel under load. This is the test that finds a neutral-to-ground bond downstream of the service. Put a clamp ammeter with 0.01 A resolution around each subpanel's feeder EGC, then switch a known 120 V load in that subpanel on and off. Acceptance: the reading sits at the meter's noise floor and does not move with the load. Wrong looks like a reading that tracks the load, meaning neutral current is dividing between the feeder neutral and the EGC. Stop rule: do not close out; find the bond, usually a neutral bar still bonded to the can, and remove it. Hazard clause: jaws on one conductor only and your free hand out of the enclosure, because a clamp meter tells you nothing about the bus behind that conductor.

6. Measure electrode resistance only where a number is actually required. A single rod, pipe or plate electrode is excused from a supplemental electrode only if its resistance to earth is 25 ohms or less, per NEC Article 250; that 25 ohms is permission to skip a second rod, not a performance target for the system. Acceptance: a documented fall-of-potential result under 25 ohms following IEEE Std 81, or a second electrode installed at least 6 ft from the first, after which no test is required. Wrong looks like a clamp-on ground resistance reading on an isolated electrode, where the instrument has no parallel return path and the number is meaningless. Stop rule: a plateau that will not settle means the probe run is too short, not the electrode too good.

7. Prove the branch-circuit fault return path, not just its presence. At the farthest outlet on each of three representative circuits, confirm the equipment grounding conductor is continuous back to the ground bus and that hot, neutral and ground each land on their own terminal. Acceptance: continuity confirmed and no neutral-to-ground voltage beyond the drop the loaded circuit explains. Wrong looks like a three-prong receptacle on a two-wire circuit with a jumper from neutral to ground, which reads correct on a plug-in tester and energizes the appliance case on a neutral fault. Stop rule: a bootleg ground is a red-tag item; label the outlet, tell the occupant the third pin is connected to nothing, and quote the correction.

8. Verify the piping bonds and the intersystem bonding termination, then restore and route. Interior metal water piping likely to become energized is bonded, gas piping is bonded through the equipment grounding conductor of the circuit that may energize it, and an accessible intersystem bonding termination exists for the communications grounds. Acceptance: each bond on clean metal with a listed clamp, every lug you opened back at marked torque with a calibrated tool, and every failure above carrying an owner and a date. Wrong looks like a cable ground clamped to its own separate rod. Stop rule: the job does not close until each failure reads corrected, quoted, or refused in writing.

When the site does not match the assumption

The electrode is under a finished slab and nobody will open it: record it present-but-inaccessible with a photo of where the conductor disappears, verify everything upstream, and state which acceptance items you could not reach. Do not write a resistance number you did not measure.

The customer refuses the remediation: get it in writing on the field report, in their words, with the defect named, and leave a copy. A verbal "they said no" is not a record.

The subpanel is a listed main-breaker panel with a factory bond and no removable strap: it is being used in a way its listing does not support. Quote the correct panel rather than improvising an isolation. Detached structures and mobile homes have their own bonding rules and go to the lead rather than through this step 4.

The record this produces

One verification record per service, filed against the customer: date and technician; service size and panel make; the step 2 electrode list with each entry marked present, absent or inaccessible; GEC size, material and splice condition; main bonding jumper present with the torque used; a per-subpanel line giving the clamped EGC current with the test load on and off; the electrode resistance result with the method named, or the note that a second rod went in instead; the three circuits sampled in step 7; piping and intersystem findings; and a defect list with an owner and a date.

The office reads it to build the quote. The next technician reads it before pulling a deadfront, because it says whether the ground bus is carrying current. A buyer's inspector reads it years later, which is why absent and inaccessible have to stay distinct.

Worked pass: 1962 ranch, service upgrade final

Service is 200 A, single panel, one 8 ft rod, copper water service. Step 1: work runs energized under a permit with arc-rated clothing, because steps 5 and 7 need load.

Step 2 list: one rod at the east foundation corner; copper underground water pipe entering the crawlspace, bonded at the meter; no concrete-encased electrode. Step 3: GEC is 6 AWG copper, continuous, clamp listed for direct burial, lug clean. Step 4: bonding screw present and torqued.

Step 5 fails. With a space heater and lighting on the garage subpanel, the feeder EGC reads 3.1 A and falls to 0.02 A when the load switches off. The feeder has a 6 AWG copper neutral and a 10 AWG copper EGC. From NEC Chapter 9, Table 8, DC resistance for uncoated stranded copper is 0.491 ohms per 1,000 ft at 6 AWG and 1.24 at 10 AWG, so a parallel neutral path through the EGC carries 0.491 divided by (0.491 plus 1.24), or 28.4 percent of the imbalance. The measured 3.1 A implies an imbalance near 10.9 A, and the clamp on the subpanel neutral reads 11 A. Two independent readings agreeing is what turns a suspicion into a finding. Stop rule taken: the neutral bar in the garage subpanel is still bonded to the can. It comes out on this visit, panel de-energized and proved dead first, and the EGC reads 0.02 A afterward with the same load on.

Step 6: fall-of-potential on the rod. The rod is 8 ft, so the current probe goes out 80 ft and the potential probe is read at 52, 62 and 72 percent of that span, at 41.6, 49.6 and 57.6 ft. Readings are 36, 38 and 39 ohms. That is a plateau, so the result stands at 38 ohms, above the 25-ohm exception, so a second rod goes in 8 ft from the first, clear of the 6 ft minimum. No retest is performed, and none is required after supplementing.

Step 7: three circuits sampled, all continuous, no bootleg grounds. Step 8: water piping bonded; no intersystem bonding termination, with the cable company's ground clamped to the interior water pipe. Logged and quoted.

The record closes with two corrected items, one quoted item, and the 38-ohm result written with its method so nobody repeats the test.

References

• NEC Article 250, in the edition your authority having jurisdiction has adopted, for electrode systems, the main bonding jumper, and equipment grounding conductor requirements
• IEEE Std 81, for the fall-of-potential method and probe spacing
• 29 CFR 1910.333(b)(2), the general-industry lockout and verification practice for electrical work, and NFPA 70E-2021, 120.5, for the live-dead-live proving sequence
• NEC Chapter 9, Table 8, for the conductor resistances used in the worked pass
• See related: Service Panel Upgrade SOP (electrode system design), Generator Transfer Switch Install SOP (separately derived system bonding)