Stray Voltage and Bonding Investigation

Purpose

Somebody felt a tingle off a metal thing, and the number you are about to measure depends entirely on three choices nobody writes down: what you referenced it to, what impedance your meter presented, and whether any load was across the two points. Change any one of those and the same site reads 12 V or half a volt. That is why stray voltage calls get argued about for months, and why this procedure fixes all three before the first reading.

The finding then lands on one of two owners. Voltage arriving on the grounded service conductor from a multi-grounded distribution system is the utility's, and it exists at some level everywhere. Voltage created by the customer's own load returning through a path it should not be on is yours, and a neutral bonded to ground downstream of the service is the version that turns a pool bonding grid into a return conductor.

Scope

Covers a reported shock, tingle or measurable voltage on metal parts in or around a dwelling or small commercial occupancy, including pools, spas, wet bars, laundry and outbuildings, through to a premises-side defect or a utility escalation.

Does not cover routine verification of grounding electrode and bonding installation, which the Grounding and Bonding Verification SOP owns, or the general voltage complaint screen, which the Power Quality Complaint Investigation SOP owns. Does not cover agricultural stray voltage mitigation design or utility distribution engineering.

Roles and handoffs

Procedure

1. Close the water and open the equipment circuits before any diagnosis at all. Where anyone has felt a shock in or near a pool, spa, pond or wet area, nobody enters or re-enters the water, everyone already in it is directed to leave without being touched, and any rescue is made from the deck with a non-conductive reach pole rather than by entering. Acceptance: water cleared, the facility physically closed, and every pool, spa, pump, heater and lighting circuit opened at the panel and tagged, all before a meter comes out. Wrong looks like measuring first and closing the pool afterward. Stop rule: a customer unwilling to close the facility gets the refusal in writing, the office notified the same day, and the tech does not perform in-water or waterline measurements. Hazard: a person in an energized field in water can be incapacitated and unable to swim, so power comes off first and nobody enters the water to help.

2. Map where the voltage is felt and what it follows. Record which metal parts, whether the person was wet, whether they were barefoot on earth or on a deck, and what was running. Acceptance: a written list of the parts involved and at least two instances tied to a coincident load. Wrong looks like recording "shocks at the pool" with no part named. Stop rule: a report of shock from a receptacle, an appliance cord or a switch is a fault on that equipment rather than a bonding question, and it leaves this procedure. Hazard: none at this step, it is an interview conducted away from the affected metal.

3. Take the reading twice, high impedance then low impedance, against a stated reference. Measure from the metal part to a stated reference, a driven remote earth reference for a true stray-voltage reading or a named local ground where you say so, first on a normal digital meter and then on the meter's low-impedance setting. Acceptance: both readings recorded with the reference named on the same line. Wrong looks like reporting one number with no reference and no impedance stated, which is not a measurement anybody can repeat. Stop rule: a voltage that collapses to near zero on the low-impedance setting is capacitively coupled with no source current behind it, and the investigation stops there rather than chasing it. Hazard: you are making contact measurements on possibly energized metal while standing on wet ground, so stand on a dry insulating mat and wear rubber insulating gloves tested to ASTM D120 in the class your shop assigns for the system voltage.

4. Load the measurement with a defined shunt and state the resistance you used. Place a known resistance across the two points and re-read, then divide the loaded voltage by the shunt resistance to get the current the path can deliver. Acceptance: three numbers on one line, the loaded voltage, the shunt resistance and the resulting current. Wrong looks like an open-circuit voltage quoted alone, which any high-impedance source produces and no load sustains. Stop rule: the 500 ohm shunt used in animal-contact stray-voltage work is used here only as a defined, repeatable load so two readings can be compared; it is not a human body model, and what current is hazardous to a person is a question for the standard your jurisdiction applies. Hazard: the same wet contact exposure as step 3, same mat and gloves, and the shunt is placed with an insulated tool rather than held.

5. Run the split test by opening the main, which is what names the owner. Open the service disconnect and repeat steps 3 and 4 at the same points against the same reference. Acceptance: the voltage collapsing with the premises dead, making the source premises load current, or persisting substantially, making it voltage arriving on the grounded service conductor from the distribution system. Wrong looks like skipping this and arguing about it later. Stop rule: a reading that persists with the whole premises de-energized ends the premises investigation and goes to step 7. Hazard: opening the main drops a sump, a freezer, an alarm and possibly medical equipment, so what depends on power is arranged before the handle moves.

6. Walk the premises fault paths with everything proved dead. Open the feeders, prove 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 check the main bonding jumper, the separation of grounded and equipment grounding conductors at every panel downstream of the service, equipment grounding continuity, and any bond orphaned by a metal water line replaced with plastic. Acceptance: bonding jumper present and torqued at the service, the two conductor types separated everywhere downstream, and continuity confirmed on every equipment grounding path. Wrong looks like a neutral bonded to the equipment grounding bar in a subpanel, which sends normal return current through the grounding conductors and any bonding grid tied to them. Stop rule: a downstream bond ends the search, is corrected first, and the facility stays closed until step 8 clears it. Hazard: a neutral bonded downstream carries load current, so it is never lifted with the feeder energized; the feeder breaker is opened and every conductor in that panel proved dead first.

7. Escalate a persisting reading to the utility with all three numbers. Give them the high-impedance reading, the low-impedance reading, the shunted voltage with its resistance and computed current, the reference point, the time, and the fact that it persisted with the service disconnect open. Acceptance: a ticket number, a response class and a name in the job record. Wrong looks like reporting a single open-circuit voltage, the number their own engineer discounts first. Stop rule: no commitment inside their stated window and the office escalates the same day, with any pool or spa closed meanwhile. Hazard: none at this step, it is a phone call.

8. Repair, restore, and re-measure the same three ways at the same points. Repeat steps 3 and 4 at exactly the points and reference used before. Acceptance: high-impedance, low-impedance and shunted readings recorded against the original set, with the computed current stated. Wrong looks like re-reading at a different metal part or against a different reference, which compares nothing. Stop rule: a reading that has not moved means the finding was wrong and step 6 resumes rather than the facility reopening. Hazard: this is where energy goes back with water present, so before the pool reopens confirm continuity between every bonded metal part and the equipotential bonding, noting that the NEC specifies the conductor and the connections rather than a resistance value, so the check is continuity and connection integrity rather than a published ohm figure, and confirm every ground-fault device serving pool equipment trips and resets on its test button.

The record this produces

One investigation record per complaint: the step 1 closure time and who was told; the parts and coincident loads from step 2; the step 3 pair with the reference named; the step 4 loaded voltage, shunt resistance and computed current; the step 5 split-test pair taken with the main open; the condition of each premises path checked; the utility ticket where one was raised; and the full step 8 re-measurement set.

The office reads the owner call to know whether it is quoting work or chasing a utility. The utility reads the split test, because a reading that persisted with the customer's main open is the number that moves their ticket. The customer reads the before and after at the same point, which is what reopens a pool on evidence rather than on reassurance.

Worked pass: 1998 house, tingle from the pool ladder when the pool pump runs

Step 1: the pool is cleared and closed at 10:12 a.m., and the pump, heater and pool light circuits are opened and tagged before any meter is used.

Step 2: the ladder and the skimmer lid, always with the pump running, twice in the last week, both times barefoot on a wet deck.

Step 3: from the ladder to a driven remote earth reference, a high-impedance meter reads 4.2 V with the pump running, and the low-impedance setting reads 3.9 V. The reading does not collapse, so there is real source current behind it and the investigation continues.

Step 4: with a 500 ohm shunt across the same two points the voltage falls to 3.1 V, and 3.1 divided by 500 is 0.0062 A, or 6.2 mA. All three numbers recorded on one line with the reference.

Step 5: the service disconnect is opened after arranging for the sump, and the same measurement at the same points against the same reference reads 0.3 V high impedance and 0.3 V low impedance. The reading collapsed from 4.2 V to 0.3 V with the premises dead, so this is the customer's own load current and not distribution neutral-to-earth voltage. No utility call.

Step 6 fails and takes its stop rule. Feeders opened and every conductor proved dead on a known live source before and after. At the pool equipment subpanel the grounded conductor is bonded to the equipment grounding bar, so pump return current has been dividing between the feeder neutral and the equipment grounding conductors, and the pool bonding grid is tied to those. The bond is removed and an isolated neutral bar fitted, with the feeder still open throughout. The facility stays closed.

Step 7 does not run, because step 5 put this inside the premises.

Step 8: power restored, pump running, same ladder, same remote earth reference. High impedance 0.4 V, low impedance 0.4 V, and with the same 500 ohm shunt 0.35 V, so 0.35 divided by 500 is 0.0007 A, or 0.7 mA, against the 6.2 mA measured before. Continuity is confirmed between the ladder, the skimmer lid, the pump housing and the equipotential bonding, checked as continuity and connection integrity rather than against an ohm figure, and both ground-fault devices serving pool equipment trip and reset on their test buttons. The pool reopens on that record.

References

• NEC Article 250 for main bonding jumper and grounded-versus-equipment-grounding conductor separation checked at step 6, and Article 680 including 680.26 for pool equipotential bonding and the required ground-fault protection verified at step 8, in the edition your authority having jurisdiction has adopted
• ASTM D120 for the rubber insulating gloves used at steps 3 and 4, with the class selected for the system voltage by your shop's electrical safety program
• 29 CFR 1910.333(b)(2) for work practices, with NFPA 70E-2021, 120.5 for the live-dead-live sequence used at step 6
• See related: Grounding and Bonding Verification SOP, Power Quality Complaint Investigation SOP, Open Neutral Diagnosis on a Service SOP