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High-Impedance Neutrals Author: Brian D’Andrade, Ph.D., P.E., CFEI In 2013, John Hall wrote a report on electrical fires i that was published by the National Fire Protection Association. ii The report stated: In 2011, an estimated 47,700 home structure fires reported to U.S. fire departments involved some type of electrical failure or malfunction as a factor contributing to ignition. These fires resulted in 418 civilian deaths, 1,570 civilian injuries, and $1.4 billion in direct property damage. In 2007–2011, home electrical fires represented 13% of total home structure fires, 18% of associated civilian deaths, 11% of associated civilian injuries, and 20% of associated direct property damage. Electrical distribution or lighting equipment accounted for 6% of 2007–2011 home structure fires, ranking fourth among major causes behind cooking equipment, heating equipment, and intentional. Given those statistics, it is not surprising that Exponent consultants are regularly involved in a variety of electrical fire investigations. In particular, consultants in Exponent’s Electrical Engineering and Computer Science practice are the key to fire investigations when there is involvement of an electrical device because they are both professional electrical engineers and certified or trained as fire and explosion investigators. The combination of the two skills is highly useful when complex failures may appear to be unresolved at first glance. Fire investigations involving electrical equipment and wiring can be complex for many reasons, including scenarios where the cause is not near the origin of the fire patterns, and extensive fire damage to the suspect electrical device or system hampers electrical characterizations. Some electrical failures are obvious, but the insidious ones are most appropriately investigated using a systematic and scientific approach, as detailed in the National Fire Protection Association’s Guide for Fire and Explosion Investigations (“NFPA 921”). A highimpedance neutral is one potentially insidious fault, because the cause of the fire may not be located at its origin, and an effort to thoroughly investigate the building wiring, for example, can be economically daunting. An electrical fault that is hard to determine when there is extensive fire damage, is an increased resistance (or high impedance) at a connection or at some point along a current path. Areas of increased resistance are prone to overheating due to basic resistive losses, which is similar to the heating mechanism in a toaster oven. If there is minor, localized damage at a plug for example, this type of fault can be identified by a process of elimination or with basic visual inspection; however, electrical characteristics of the suspected fault would likely not provide meaningful insight to the original electrical state of the system before the fire. If the fire damage is substantial, it will be difficult to locate the point of the overheating or determine exactly what overheated, because of the inability to perform basic electrical characterizations (e.g., resistance measurement) caused by residual char or changes to the damaged electrical system. Detecting a high-resistance fault is complicated further because it affects the voltages across devices. In such a scenario, the fire origin may be at a different location from the high-resistance fault. A highly simplified multi-wire branch circuit is shown in Figure 1 to help provide an intuitive understanding of how increasing resistance along the neutral conductor, which is typically the grounded conductor iii in a residence, affects the voltage across equipment such as a lamp bulb. Power from a grounded power source is fed into the circuit via two lines and a neutral. Ideally, the resistance, X, of the neutral between the main bonding jumper iv and the node at VA is very low, such that VA = 0 V. If X increases, VA will become increasingly negative in the circuit shown in Figure 1. For example, VA has a nominal –10 V when X = 18 ohms, so the voltage across the bulb will be 130 V = 120 V – (–10 V). Figure 2 shows more values of VA for various values of X. www.exponent.com The increased resistance may be caused by a variety of factors, including corrosion, loose connections, and damaged conductors, for example. Figure 1. A simplified circuit diagram of a multi-wire branch circuit where the resistance along the neutral is X ohms between the main bonding jumper connection and the node where the electrical devices are connected. Figure 2. The voltage at VA, shown in Figure 1, is a function of the resistance, X, along the neutral between the main bonding jumper and electrical devices. The graph illustrates the value of VA for various values of resistance X. www.exponent.com In the above scenario, it is uncertain that the high-resistance neutral fault will be found if the bulb fails when 130 V, instead of 120 V, is applied across the bulb and if a fire occurs at the location of the bulb. There may be some observable overheating of the neutral wire, but the location of the overheating may be hidden in the building structure. The simplified circuit in Figure 1 is only for pedagogical purposes, but it provides quantitative insight into the magnitude of resistance that may be considered high resistance for this type of electrical fault. Broken neutrals, which occur when X has a magnitude of thousands or millions of ohms, are well known electrical faults, but this example raises the awareness of electrical faults that occur when neutrals are not broken but still have high resistance. An even more precarious situation can occur if the high-resistance area is located along the utility neutral or the service neutral between the main bonding jumper and the grounded source (which is typically a utility transformer for residences). In this second scenario, the magnitude of the voltage at the main bonding jumper may increase above 0 V. v Cable TV, telephone, and satellites are all required to be bonded to the building ground terminal, so there is the possibility that increased voltage at the main bonding jumper will affect telecommunication electrical systems that are bonded to ground. Occasions do arise when the wiring at a fire scene can be energized and electrical measurements can be made to provide useful data for fire investigators. In those cases, one may be able to measure branch circuit electrical characteristics with an AC load tester to determine whether the circuits have adequate performance characteristics, such as low resistance of circuit conductors. ANSI/ASHRAE/IES 90.1-07 - 2010 is an energy standard for buildings, except low-rise residences; it provides requirements for voltage drop in feeders and branch circuits. vi Also, the National Electrical Code (NEC), 2014 Edition, FPN 4 to rule 210.19 and FPN 2 to rule 215.2, discusses voltage drops of 2% (feeders) + 3% (branch circuits). Note that informational notes in the NEC are not rules. In residences, it is common to measure voltage drops greater than 10% with a 15-amp load. Such a large voltage drop is typically due to long lengths of wire used to form the branch circuit. Additionally, one can use a ground clamp meter to get an estimate of the utility and service neutral resistances. A ground clamp meter placed around the grounding electrode conductor measures the total resistance in a circuit path, and thereby simultaneously measures the ground contact resistance plus the utility neutral resistance plus the service neutral resistance. In Section 250.53 of the NEC, ground contact resistance must be less than 25 ohms; otherwise, there shall be a supplemental ground electrode. Therefore, one typically expects to measure less than 25 ohms when the ground clamp measurement is performed at a site investigation. www.exponent.com This article has focused on one of the more complex and not readily identified causes of electrical fires that require engineering expertise to identify. Causes of other electrical fires could be readily visible or easier to identify. These fires, in most cases, are preventable by following the guidelines offered by the NFPA on how to prevent more common electrical fires, including: • • • • • • Replace or repair damaged or loose electrical cords. Avoid running extension cords across doorways or under carpets. In homes with small children, make sure your home has tamper-resistant (TR) receptacles. Consider having additional circuits or outlets added by a qualified electrician, so you do not have to use extension cords. Follow the manufacturer's instructions for plugging an appliance into a receptacle outlet. Avoid overloading outlets. Plug only one high-wattage appliance into each receptacle outlet at a time. • • • • If outlets or switches feel warm, if fuses blow or breakers trip frequently, or if flickering or dimming lights are observed, call a qualified electrician. Place lamps on level surfaces, away from things that can burn, and use bulbs that match the lamp's recommended wattage. Make sure your home has ground fault circuit interrupters (GFCIs) in the kitchen, bathroom(s), laundry, basement, and outdoor areas. Arc-fault circuit interrupters (AFCIs) should be installed in your home to protect electrical outlets. Summary Electrical fires account for a significant number of home fires and can be complex. A most likely cause based on reasonable engineering investigative procedures may be appropriate when losses are small, but a more extensive investigation should be considered when the losses are high or when there is significant injury or death. Exponent electrical engineers provide technical support in electrical fire-related matters, including matters involving improper wiring and highresistance neutrals. Contribution Authors Brian D'Andrade, Ph.D., P.E., Senior Managing Engineer 202.772.4907 • [email protected] • Bio Dr. D’Andrade has a diversified set of expertise in electrical, electronic, computer engineering, software engineering, optical engineering, and microelectronic systems. www.exponent.com i Hall, J.R. Jr. 2013. Home electrical fires. National Fire Protection Association: Fire Analysis & Research. ii An electrical fire is defined in Hall (2013, ibid) as a structure fire that involved some type of electrical failure or malfunction as a factor contributing to ignition. iii A grounded conductor is defined as “a system or circuit conductor which is intentionally grounded” (NFPA 70, Article 100, 2014). iv A main bonding jumper is defined as the “connection between the grounded circuit conductor and the equipment grounding conductor at the service” (NFPA 70, Article 100, 2014). A grounding electrode conductor is defined as a “conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system” (NFPA 70, Article 100, 2014). A grounding electrode is defined as a “conducting object through which a direct connection to earth is established” (NFPA 70, Article 100, 2014). v This is due in part to fact that the contact resistance between earth and the ground electrode is not zero. vi 8.4.1.1 Feeders. Feeder conductors shall be sized for a maximum voltage drop of 2% at design load. 8.4.1.2 Branch Circuits. Branch circuit conductors shall be sized for a maximum voltage drop of 3% at design load. www.exponent.com