Beware Thor’s Hammer
Hidden hazards in nonmetallic cable installations.
Eighty-three percent of homes inspected during construction had code violations related to nonmetallic (NM) electrical cable, according to the National Electrical Manufacturers Association’s (NEMA) document on stapling violations. NM cable misinstallations are common according to Underwriting Laboratories’ (UL) final report, “The Influence of Damage and Degradation on Breakdown Voltage of NM Cables.” The report confirmed what has long been suspected: Hammer strikes during NM cable installations can cause degradation in breakdown voltage, and key contributors to electrical fires include wire degradation, damage, and improper installation.
Use of NM cable has been increasing, and estimates are that sales reached more than eight billion feet in one year alone. This article discusses NM cable and the UL report, and reviews a recent civil case that covered the consequences of NM cable breakdown from hammer misstrikes by suggesting steps to identify inadvertent blows from “Thor’s Hammer.”
Cables and Mythology
There are different types of electrical wiring used in residential homes in the U.S. Electrical wiring can be broken down into three types: armored cable (AC), separated conductors, and NM cables. Armored cable, manufactured by General Electric and trademarked “BX,” includes an aluminum bonding strip under steel armor. Separated conductors used in building electrical systems require a metal conduit or metal wire for wire protection. You first install the tubes or conduits, then feed electrical wires through them.
NM cable, the most common type used in the U.S., originally had individual conductors insulated by being wrapped in a cotton braid with a moisture protection substance. Later, NM cable had a PVC jacket with a 90 C-rated individual conductor and a 75 C-rated outer jacket. No grounding was originally required for NM, but that was changed so that equipment grounding was required at all branch circuits.
Residential power starts at the electric power meter. The meter is connected to the service panel box, where wiring is connected to the rest of the home. Power is distributed to the rooms through circuit breakers in the service panel box via installed NM cables. Because NM cable runs throughout the rooms of a home and can be longer than five feet, they must be secured. Generally, NM cable runs are secured by a hammer and staples, manual stapler, or electric stapler.
Probably the most famous hammer is that of Thor. According to Germanic mythology, Thor is a god who uses the power of his hammer to control electricity and even create lightning. However, like any tool, a hammer must be used carefully, and mistakes can be costly. NM cable installation errors due to hammer misstrikes—including damage from hammer impacts, punctures, and overcompression—have long been suspected as the cause of electrical failures and, in some cases, fires. While Thor strikes with force and accuracy—he is a god, after all—that is not always the case for earthly installers whose imprecise blows damage cables. Those homes with damaged NM cables are subject to sporadic voltage surges in the electrical system. Other degradation issues affecting NM cables include high humidity exposure, high temperature exposure if overloading, salt and pollutant contamination, and UV exposure.
Cable Damage and Degradation
The lack of research on degradation in electrical performance where NM cables are used spurred the UL’s report, which stated that its research investigation “was undertaken to study the influence of damage (i.e., damage occurring during installation of the electrical wiring) and subsequent degradation of the dielectric breakdown voltage of the cable insulation.” The report focused on assessing mechanical damage conditions found when installing NM cables to include, among other issues, accidental hammer blows and poor stapling.
Several manufacturers of NM cables were included in the report. The NM cables were subject to voltage surges representative of what might occur in the wiring of a typical home. Different types of insulation materials were assessed for varying breakdown voltages. Based on field inspection reports, UL set up various damage scenarios to include misaligned staples, inadvertent hammer strikes, and overdriven staples.
Staples, which are used to secure NM cables, were driven by electric stapler, manual stapler, and a hammer. The results showed that the cables were compromised by staple punctures, hammer impacts, and overdriven staples.
During hammer installation, the user first taps a staple into place before hitting it harder to secure it. This procedure is less likely to cause a puncture of the NM cable. However, using a staple gun blocks the user’s view and can cause staple misalignment, damaging the conductor by perforating the protective outer cover. Punctured NM cables then were assessed for breakdown voltage. Manual staplers with an alignment notch were less likely to misalign than an electric stapler without a notch for staple alignment.
Overdriven staples that caused compression of NM cable jackets and insulation also were evaluated. Some staples have a plastic coating; others are metal only. Staples with a plastic coating may provide additional overcompression protection to NM cable. Overcompression was evaluated by using a manual stapler, electric stapler, and hammer for installation. Sometimes overcompression could be seen; other times it could not be visibly identified. Insulation breakdown voltage of overdriven NM cables was tested. The testing revealed that hammer and electric stapler overdrive reduced the breakdown voltage of the NM cable. Moreover, UL assessed that same condition when the NM cable also was aged.
Hammer installation is commonly used for NM cables in the construction industry. Hammer strikes were conducted on the NM cable outer jacket. A hammer impact can cause the NM cable dielectric strength to fall well below the 6,000-volt threshold value required for safe installation. UL testing found that the greatest breakdown resulted from direct hammer strikes to the cable jacket during installation. The report noted that these hammer strikes can significantly reduce breakdown voltage of insulation materials, potentially leading to catastrophic failure.
Fancher Monterey Inc. v. Avila Design
The problems identified by the UL report on NM cables were explored in Fancher Monterey Inc. v. Avila Design. This civil case involved a fire that destroyed a commercial building in Monterey, Calif., in February 2007 and resulted in multiple lawsuits. Electrical conductors were involved, and consultants were retained to examine the fire scene. An area of origin was identified by a private fire investigator, and an electrical expert was brought in to examine the conductors running through the area of origin. A combustion expert then was brought in to evaluate whether the NM cables in the area of origin were the most likely cause of the fire.
One way an electrical NM conductor can cause a fire is by arc tracking. Arc tracking can be defined as a conductive path formed across an insulating surface. The carbon path provides a short circuit route through which current can flow. According to Canadian-based Electrical Safety Authority’s “2008 Ontario Electrical Safety Report,” arc tracking is the fifth most common cause of electrically ignited fires.
Arc tracking can be divided into two failure scenarios: wet tracking and dry tracking. Wet tracking happens when moisture forms a conductive film over the surface of an insulator. Normally, this requires a wet environment. However, for PVC, it can occur in a dry environment. Dry tracking starts with any event that carbonizes electrical insulation, creating a layer of char, which is a semi-conductive substance. Carbonization can be caused by improper use of heating equipment, overheating electrical connections, breakdown of the material due to overvoltage, and fire contacting the insulation. The Fancher Monterey action involved a breakdown of the polymer due to an overvoltage condition. Excessive temperatures will aide carbonization. The most common is an overcurrent condition in a wire or cable, which can raise its temperature.
The UL report noted that NM cable insulation can break down from mechanical abuse, such as overcompression due to improper installation, raised temperatures, and voltage surges. Those conditions can occur for various reasons at the same time. The report also found that even a barely visible dimple from a hammer miss can lead to lowering the breakdown voltage from 20,000 V to as low as 80 V. Insulation failures due to overcompression by improper installation have been repeatedly found in the field by NEMA inspectors. Even properly installed NM cables have normal resistance with current flowing that causes some resistance heating. But improperly installed NM cable causes higher temperatures that shorten the life of the cable by causing abnormal resistance heating that negatively impacts its life as well as adjacent surrounding materials.
Other failure modes impacting NM cable include moisture getting on its surface, which accelerates tracking of electrical current; surges created by lightning; switching operations; and equipment failures. Surges need to be expected in any electrical system and appropriate safeguards taken to protect against them.
Surges Related to Wiring Conditions
Normally, 6,000 V will cause a sparkover—a disruptive electrical discharge—of clearances in building wiring. Sparkover clearances are detectable since most outlets will be destroyed. If there is a surge below 6,000 V, nondefective outlets will not be destroyed. Cords, cables, and other equipment with breakdown voltage below 6,000 V will suffer dielectric breakdown if the surge voltage exceeds the withstand voltage of the item.
As noted in the UL report, NM cable with a defect in the PVC insulation can carbonize. At first, this will not cause a fire. Sometimes the circuit breaker will trip and reset, suggesting nothing is wrong. However, because the circuit is energized, arc tracking continues. That process can continue for a long time as carbonization develops and grows. Examples have been found of arc tracking occurring four months to two years after the triggering event. Fires that occur in the exact timing are hard to pinpoint.
In Fancher Monterey, the consulting expert testified that the building fire was consistent with arc tracking as the cause. NM cable arc tracking requires a localized defect and voltage surge. Most likely, a mishit defect was due to overcompression or a hammer mishit. Other potential causes were ruled out by other consultants.
As the UL report established and the Fancher Monterey case confirmed, damaging NM cables by overcompression or mishits can create conditions favorable to arc tracking, and expected voltage surges and catastrophic fires can result. When evaluating NM conductor failures in an area of fire origin, ensure your consultants consider that installers, unlike Thor, may have had hammer mishits. Otherwise, the true culprit may go undiscovered.