Open Neutral Diagnosis on a Service

Purpose

An open or high-resistance neutral on a 120/240 V single-phase service keeps destroying the customer's equipment while you stand there setting up a meter. With the neutral not carrying, the two 120 V halves stop being independent supplies and become a series divider across the full 240 V, so the lightly loaded leg climbs and the heavily loaded leg collapses, and the split moves every time something switches on.

This procedure confirms the condition with a test a quiet house cannot fake, puts the fault on one side of the meter so one party owns it, and gets the load off. What it prevents is the visit that reads 121 V and 119 V at an idle panel, calls the service healthy, and leaves the customer to lose a second round of appliances.

Scope

Covers confirmation, localization and disposition of a suspected open or high-resistance neutral on a 120/240 V single-phase service at a dwelling or small commercial occupancy, through to a utility escalation or a named premises-side defect.

Does not cover the screen that decides whether a general voltage complaint is a neutral problem at all, which the Power Quality Complaint Investigation SOP owns and runs first. Does not cover service entrance replacement, meter relocation, or any work on the line side of the meter.

Roles and handoffs

Procedure

1. Take the intake and decide whether this is an emergency dispatch. Ask which rooms go bright while others go dim, whether more than one appliance has failed recently, and whether lights change when a large load starts. Acceptance: two symptoms pointing opposite directions at the same moment, written down, which no single-circuit fault produces. Wrong looks like logging it as a routine flicker call. Stop rule: two directions at once, or any burning smell, goes out today, and the customer is told by phone to unplug electronics and leave the range and dryer off. Hazard: none at this step, it is a decision made at a desk.

2. Read all three service voltages as found, before you change anything. At the service disconnect record L1 to neutral, L2 to neutral and L1 to L2, plus what is running. Acceptance: both line-to-neutral readings inside the ANSI C84.1 Range A service band of 114 V to 126 V on a 120 V nominal base, within a few volts of each other, and summing to within a couple of volts of the line-to-line reading. Wrong looks like one leg high and one low with the pair still summing to line-to-line, the divider signature. Stop rule: readings inside the band do NOT clear the service, because a house at a small imbalance shows almost nothing, so go to step 3 rather than closing the call. Hazard: an energized reading, so wear the arc-rated clothing and face protection your program assigns for that panel per NFPA 70E-2021, 130.5 and 130.7, use a meter and leads rated CAT III 600 V or better, and seat both leads before you look at the display.

3. Drop the house to a known baseline and re-read. Open every branch breaker, leave the main closed, read the same three voltages. Acceptance: both legs within about 2 V of each other, baseline written down. Wrong looks like expecting the fault to show here; an unloaded divider is balanced by definition, so a clean reading proves nothing and is only the reference step 4 is measured against. Stop rule: legs still far apart with every branch breaker open means the imbalance is created ahead of the panel, so skip to step 5. Hazard: you are operating breakers on a panel that may sit at elevated voltage, so stand to the hinge side rather than square in front of the deadfront and operate with the flat of the hand.

4. Run the load-shift test, the confirming measurement. Close one branch breaker feeding a known resistive load you are willing to lose, a portable heater or a work light and never the customer's electronics, and watch both line-to-neutral readings as it comes on. Acceptance on a healthy neutral: both legs move under about 2 V and stay inside 114 V to 126 V. Wrong, and diagnostic, is the loaded leg falling and the unloaded leg rising by tens of volts within a second, the two still summing to line-to-line. Stop rule: any leg above 132 V ends the test, that breaker comes off immediately, and you go to step 5 without repeating it on the other leg. Hazard: you are feeding a load at an unknown voltage, so keep it resistive and portable, hands off it while it runs, and never leave it where the customer can switch it back on.

5. Get the service off, with the occupants briefed first. Ask what depends on power (medical equipment, a sump in a wet basement, refrigerated medication, an alarm), arrange for it, then open the main and tag it. Acceptance: main open, tag applied, customer told the service is not coming back until the neutral is repaired and re-proved. Wrong looks like leaving it energized so they have some power, which leaves the divider swinging every time the furnace cycles. Stop rule: a customer who refuses gets the refusal, the readings and the overvoltage risk in writing, the office notified the same day, and a list of loads to keep off. Hazard: the load side is dead now but the service conductors, meter and main lugs stay energized and unfused, so nothing ahead of the main gets touched at any point in this procedure.

6. Inspect the four premises-side candidates with the panel proved dead. Prove the load side 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), then examine the neutral landing at the main lug, the neutral bar and its bonding to the enclosure, the main bonding jumper, and the service entrance neutral at its entry. Acceptance: all four tight to the manufacturer's marked torque value and free of discoloration, corrosion and heat damage. Wrong looks like white powdery deposit on an aluminum neutral, a lug that moves by hand, or a jumper missing. Stop rule: any one failing makes this a premises defect that stays with your shop, and step 7 does not happen. Hazard: the line side of the main and the meter terminals stay live with the main open, so keep tools and the back of your hand out of that half of the enclosure.

7. Escalate to the utility with numbers, not a description. Where all four premises candidates are sound, the fault is upstream by elimination. Give them the as-found trio, the load-shift pair with the leg it was taken on, the time, the meter number, and the words "open neutral, equipment being damaged." Acceptance: a ticket number, a stated response class and a name in the job record. Wrong looks like a voicemail about flickering lights, which gets routed as a routine outage. Stop rule: no commitment inside the utility's own emergency window and the office escalates the same day. Hazard: none at this step, it is a phone call from the truck.

8. Re-prove the service before it carries load again. After any repair, yours or theirs, close the main with branch breakers still open, re-read the three voltages, then re-run the step 4 load-shift test on both legs in turn. Acceptance: both legs inside 114 V to 126 V at rest and both moving under about 2 V under the shifted load, recorded in the same units and at the same point as the as-found readings. Wrong looks like taking the utility's word that it is fixed. Stop rule: a leg still moving more than about 5 V means a high-resistance neutral rather than a fully open one, the main goes back off and the escalation reopens. Hazard: re-energizing is where an unfound fault announces itself, so stand to the hinge side, confirm the main bonding jumper is reinstalled and torqued before the main closes, and verify continuity from the neutral bar to the equipment grounding bar so the protective path you disturbed is proved working.

9. Write the damage conversation down rather than having it twice. Tell the customer what you measured and what it exposes equipment to. Acceptance: a written report carrying the readings, the times and the side of the meter, handed over. Wrong looks like an opinion about who owes them for a television. Stop rule: whether the utility is liable for a specific failed appliance goes to the customer's own insurer or counsel rather than being answered by you. Hazard: none at this step, it is paperwork.

The record this produces

One service record per event: the step 2 as-found trio with the coincident load; the step 3 unloaded baseline; the step 4 load-shift pair with the leg identified and the sum check written out; the step 5 shutdown time; the condition and torque state of all four step 6 candidates; the ticket number, class and contact; and the step 8 re-prove trio and load-shift pair.

The office reads the ticket number and response class to know when to chase. The utility reads steps 2 and 4, and a ticket arriving with two voltage pairs and a time is worked differently from one saying the lights are funny. The customer's insurer reads the dated readings, the part of this that survives a year.

Worked pass: 1968 ranch, overhead service, two failed appliances in a month

Step 1: bedroom lights go bright when the well pump starts while the kitchen goes dim, and a microwave and a television have failed in five weeks. Two directions at once, so it goes out the same day.

Step 2, as found at 2 p.m. with the house nearly idle: L1 to neutral 121 V, L2 to neutral 120 V, L1 to L2 241 V. Both inside 114 V to 126 V, 1 V apart, and 121 plus 120 is 241, matching the line-to-line reading. Every step 2 criterion passes. This is the trap the step warns about, and closing here is the mistake.

Step 3, every branch breaker open: 120 V and 121 V, 241 V line to line. Baseline recorded.

Step 4 fails and takes its stop rule. A 1500 W portable heater is landed on a branch fed from L2. Within a second L2 falls to 78 V and L1 rises to 163 V, and 163 plus 78 is 241, still equal to the line-to-line reading. That is 163 minus 120, or 43 V over nominal, which is 36 percent high and past the 132 V ceiling the step names, so the heater breaker comes straight back off and no test runs on the other leg. The five weeks of dead appliances are explained: everything on the lightly loaded leg sat above nominal every time the pump or the dryer loaded the other side.

Step 5: a sump is running in a wet spring basement, so a cord is run from a neighbouring service before the main opens. Main open at 2:24 p.m., tagged, occupants briefed.

Step 6: panel proved dead on a known live source before and after. Main lug neutral tight at the marked value, neutral bar clean, bonding jumper present and torqued, service entrance neutral sound at its entry. All four pass, so this is not a premises defect.

Step 7: utility called at 2:51 p.m. with both voltage pairs, the time and the meter number, and the phrase "open neutral, equipment being damaged." Ticket issued in the emergency class. They find a corroded splice at the weatherhead on the service drop neutral.

Step 8: main closed with branch breakers open, 120 V and 121 V, 241 V line to line. The load-shift on L2 moves it 1.4 V and moves L1 0.9 V, and the same test on L1 moves both under a volt, so both clear the 2 V acceptance. Bonding jumper confirmed torqued before the main closed, and continuity from the neutral bar to the equipment grounding bar reads under an ohm. Service released.

References

• ANSI C84.1, in the edition your utility references, for the Range A service band of 114 V to 126 V on a 120 V nominal base used at steps 2, 3 and 8
• NEC Article 230 and Article 250, in the edition your authority having jurisdiction has adopted, for the service conductor and main bonding jumper requirements checked at step 6
• 29 CFR 1910.333(b)(2) for work practices, with NFPA 70E-2021, 120.5 for live-dead-live and 130.5 and 130.7 for the arc-flash risk assessment and PPE selection
• See related: Power Quality Complaint Investigation SOP, which owns the screen that routes a complaint here, and Grounding and Bonding Verification SOP