When the neutral wire in a branch circuit opens, usually the power goes off and nothing else happens in terms of fire.
When the Service Lateral Neutral wire opens on a 120/240 or 120/208 volt system, the 240 and 208 voltages remain but the 120 volt supply can automatically vary wildly e.g., from 40 to 200 volts as devices, such as refrigerators, cycle on and off. This condition does not exceed the voltage rating of the structure wiring (often 600v), nor create excessive structure wiring current (because the branch circuit breaker will trip first). However over-voltages will cause excess currents, and eventually fires, in the continuously connected loads, e.g., clocks, electronics, light ballasts, appliances, computers, chargers, alarms, etc. The voltage swings occur as a function of the ratio of connected loads on each of the 120 volt legs. The redundant neutral-to-utility-transformer ground connection cannot be relied upon to maintain the proper voltage ratio. As a forensic electrical engineer I have verified more than a handful of open-neutral caused fires in 25 years and 600 cases.
Why is automatic protection important? When an open neutral occurs it is necessary to disconnect the load from the utility immediately, yet many electricians, and certainly most occupants, do not know the unique indicators. In one of the online "ask an expert" sites an owner complained that her lights were blinking on and off, sometimes on but dim, and there was the smell of smoke. The seemingly well credentialed master electrician suggested that she go to her local hardware store, buy a meter, and begin creating a record of the voltages in her house over time. The correct answer was turn off your main breaker immediately and call your electric utility provider (they will respond quickly and often the open wire is in their service drop, or will suggest calling an electrician).
The NEC requires over-current protection for all installations, yet this important and not uncommon condition, that readily causes fires, is not accounted for! How did this happen?
The most practical solution is to install a main breaker at the service entrance that senses the imbalance in service voltage, and trips in a manner similar to a GFCI. A patent search reveals that all of the circuit breaker manufacturers hold existing patents that would cover this design. The retrofit is easy in most cases as the new "ONCI" (Open Neutral Circuit Interrupter) will install like a familiar GFCI circuit breaker. There is no money to be made in inventing the ONCI but lots to be gained through a national retrofit program, both economically and safety-wise.
The ONCI can safely incorporate a time delay of several seconds and a tolerance of a few volts of imbalance, minimizing nuisance trips, as the over-voltage fires develop over some time. The ONCI could contain a mechanical indicator indicating "circuit trouble, call an electrician," (a different trip condition from the usual over-current or short). Design wise, if the owner resets the ONCI main breaker, it will simply trip out again on the next voltage imbalance. Otherwise the facility remains safe. Should the broken neutral connection occur during a wind storm when the homeowner is otherwise away, the ONCI performs its function safely with no further user attention. (Loss of electric heaters in houses exposed to freezing conditions could expose the building to water damage however 240 volt heaters are not subject to the open neutral hazard and might be connected before the main breaker, requiring a similar exception as with fire pumps.) Otherwise ONCIs appear to be an elegant fail-safe and user-proof design.
Utility companies are beginning to demand evidence of "surge protectors," (which are actually spike protectors), before they will pay for user devices damaged from their open neutral wire. However these protectors do not protect against long term over voltages and usually don't activate below 330 volts. If they did respond to a spike that contained more energy than their surge rating (in Joules) they would burn out and cease to provide protection (compare to the proposed ONCI). This requirement before payment is electrically absurd as has been demonstrated for several claims. The surge protector has no relevance in protection from open neutral conditions.
Why has the NEC not addressed this obvious source of electrically caused fires?
How do open-neutral fire rates compare to arc fault fires, and other electrical causes?
Why aren't these ONCI main breakers not readily available from the circuit breaker manufacturers?
Why haven't the insurance or utility companies demanded ONCIs and offered discounts for their installation and use?
How can this oversight be incorporated into the NEC?
Ralph Crawford, P.E.
Forensic Electrical Engineer