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The Case of the Floating Neutral

Jul 16, 2014Bruce Crow ly, P.E.| Electrical Construction and Maintenance

http://ecmweb.com/accidents-investigations/case-floating-neutral

Crowly is a professional engineer with B.C. Engineering Services, Inc. in Malvern, Pa. He can be reachedatbcesi@comcast.net.

Fire breaks out in homes second floor bedroom after cable technician cuts coaxial cable

When the lights flicker for a split second, most people dont read too much into it. It happens just a fluke.But for one homeowner, who was experiencing ongoing problems for several weeks straight with lightsbrightening and dimming when he turned appliances on and off, there was obviously an underlyingproblem. In addition, the cable signal on the television was inconsistent, and the Internet service wasdropping on a regular basis.

While trying to identify the source of the issue on his own, the homeowner noticed a section of coaxialcable on the outside of the house had a melted jacket. At this point, he called the local electric utility andcable company to file a request for service.

The scene

A lineman for the electric utility, who arrived the next evening, examined the connections on the pole-

mounted transformer. Although it was dark, he noticed some deterioration on the service drop cablebetween the electric utility pole and the house. Because the home was located some distance from theroad, the electric utility was only responsible for the meter, transformer, and wire to the first pole on theproperty. In this instance, the electric utility did not own and was not responsible for the service dropcable. Therefore, the lineman advised the homeowner to get an electrician out there to check the integrityof the cable. Unfortunately, this recommendation would prove to be too little, too late.

The fire

A technician from the cable company arrived the following day, at which time the homeowner showed himthe melted jacket on the coaxial cable. After performing several tests, the technicians supervisor told himto go ahead and cut the coaxial cable. Once he did, some lights in the house dimmed, others brightened,and a fire broke out in the second floor master bedroom. Discovering the fire, the son, accompanied by his

mother and father, quickly vacated the house without injury. They called 911 immediately, and localfirefighters were dispatched to extinguish the fire. The fire left behind damage to the second floor bedroomsas well as smoke and water damage throughout the house. Although the monetary amount of damage tothe structure and its contents is unknown, the home was ultimately repaired after the incident.

The lawsuit

I was retained as a forensic expert by the homeowners insurance company to ascertain the cause of thefire. Although the exact failure that caused the fire was not determined, the electric utility and cablecompany were named as defendants in the lawsuit due to evidence that indicated the actions of theiremployees contributed to the cause of the fire.

Investigation and analysis

Fire investigators are typically retained by the homeowners insurance company to determine the origin and

cause of a fire. If it is decided that electricity or failure of electrical devices/appliances are potential causesof the fire, then an electrical engineer is also retained. In this case, there were problems with the electricalsystem (i.e., lights dimming and brightening) and the cable system (i.e., television picture interference andInternet failure), so I was retained at the same time as the fire investigator.]

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Photo 1. View of t he service drop cable showing t he severed neutral conductor.

Although it was determined that the fire originated in the master bedroom at the head of the bed, the exactcause was not determined. The remains of a duplex receptacle, electric blanket, and radio in the area oforigin were examined. However, extensive damage did not indicate a failure that could have caused thefire.

Examination indicated that the service drop cable to the residence had a severed neutral, as shown onPhoto 1. The failure of the neutral conductor appeared to be caused by deterioration due to age andpossible contact with tree limbs. Examination of the coaxial cable indicated melted insulation on a bend

(Photo 2), indicative of heat caused by current flowing through the shield.

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Photo 2. View of the coaxial cable showing the melted plastic jacket.

Documents produced during the lawsuit included depositions from the lineman and cable TV technician.The lineman testified that he was dispatched to the house for a flickering light problem. He was aware thatsuch circumstances could be caused by a severed neutral conductor and if he had seen any breaks inthe wire he would have de-energized the service cable until repairs were made. This action would haveprevented the fire. However, he arrived after dark and although he saw deterioration of the cable hedid not see any breaks in the neutral.

The cable technician testified that when he measured the voltage on the coax shield, he got readings of

20V to 35V, which was not normal. He called his supervisor to discuss the situation and was told to removethe ground connection. After removing this connection, the voltage measurement increased to 200V on theshield. Realizing this created an even bigger problem, he reconnected the ground and again called hissupervisor. After another discussion, the technician was directed to cut the coax (although the reason forthis action was not provided).

Theoretically, electrical current flows through all bonding and grounding connections to return to the electricutility transformer. The lower the resistance path, the more current will flow. The neutral conductor in theservice drop cable provides the lowest resistive path to the transformer. When the neutral wire in theservice drop cable was severed, electrical current sought other paths to return to the transformer. Thosepaths included the ground, through ground rods and a water pipe, and the cable shield that was bonded tothe neutral on the electric utility pole where the transformer was located. Current flowing through the shieldcreated heat that caused melting of the jacket. Losing the neutral also eliminated the 0V reference so thatvoltage on one line conductor increased while voltage on the other line conductor decreased. This resultedin the dimming and brightening of lights.

Lessons learned

The case was settled prior to trial, the terms of which were confidential. However, several lessons can belearned from the sequence of events in this case.

The lineman was aware of the dimming light problem and that it could indicate a problem with the neutral,but he did not investigate further since the service drop cable was owned by the homeowner. Although hesaw deterioration of the cable, he testified that if he had visually identified a severed neutral conductor hewould have de-energized the transformer until cable repairs were made, which would have prevented thefire.

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The cable technician testified he was aware that lights dimming and a melted jacket on the coax wereindications of electrical problems. Furthermore, measurements indicating voltage on the cable shield alsoindicated an electrical problem. Therefore, he should have told the homeowner to call an electrician. If hehad done so without cutting the coaxial cable, the fire would not have occurred. It was also noted that thecable technician violated OSHA requirements by cutting the coax (i.e. working on energized equipmentwithout wearing personal protective equipment).

SIDEBAR: Importance of the Neutral Conductor

SIDEBAR: Importance of the Neutral Conductor

Electrical current typically flows from an electric utility transformer into a residential dwelling unitspanelboard. From there, it flows through individual branch circuits to end-use receptacles. For 120V loads,the return path back to the utility transformer is the neutral conductor. If the neutral conductor is severed,the current seeks other paths to return to the transformer. One path is through the earth, since the neutralat the panelboard is connected to a grounding electrode conductor that is connected to a ground rod. Ofcourse, the electric utility transformer also has its neutral connected to a ground rod at the utility pole.

Another available current path is the shield on a coaxial cable, which is also connected to ground rod at thehouse as well as to the neutral on the electric utility pole.

The neutral provides a 0V reference. When the neutral is severed, this reference is lost, and voltages canexceed 120V depending upon the loads connected to each branch circuit. As in this case, this situationcan cause some lights in the house to be brighter and some to be dimmer. Changes in higher loads, suchas refrigerators or air conditioners turning on and off, can also cause lights to brighten and dim. In addition,higher voltages on the branch circuits can damage appliances

Discuss this Article 2jceson Jul 23, 2014

Why did the fire start after cutting the coax cable?

BrusselsSprouton Jul 24, 2014

To be completely clear, any current from 120V loads on one leg that are not balanced by loads on the otherappear on the neutral AND any other paths between the home and transformer center tap.

If the loads on each leg are nominally equal, very little current capacity may be required at any one moment onthese paths.

Since the transformer center tap is bonded to a ground rod, the other conductors with parallel paths to the

neutral include earth bonded connections such as metal water lines, bonds to ground rods, shield conductors oncoax cables, and any bonded metal structures in contact with the ground or buried in concrete.

Disaster strikes when the imbalanced current is high AND the main current path of the neutral conductor isdisabled. If the current limit of these remaining paths is exceeded excessive voltage drop will occur and thesepaths may fail in a cascade further increasing the voltage drop. This voltage drop allows a shift in the neutralaffecting only the single leg loads. Loads on a heavily loaded leg will see low voltage. Devices on the lightlyloaded leg will see excessive voltage often leading to device failures. Each device failure cascades into moreexcess voltage and an increase in the likelihood of external damage/fire.

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