ELECTRICAL ENGINEERING & COMPUTER SCIENCE NEWSLETTER Volume

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1 ELECTRICAL ENGINEERING & COMPUTER SCIENCE NEWSLETTER Volume A Treatise on Grounding of AC Electrical Systems, and an Atypical Electrocution Case Study i Author: Noshirwan K. Medora, SMEE, P.E. Reviewer: Ashish Arora, MSEE, P.E. Introduction This article presents information on grounded and grounding conductors in single-phase service panels and also discusses potential reasons for ineffective grounding that could pose an electrocution hazard. The article further presents an unusual case study of an inadvertently ungrounded electric air compressor that was alleged to have resulted in an electrocution. Review of the initial expert report and preliminary electrical tests conducted by the authors indicated a different probable cause cause. Subsequently, the authors performed electrical tests using an IEC human body model (HBM) and confirmed that the alleged air compressor was not the likely cause of the electrocution. Further investigation revealed a different electrical source as the probable cause of the electrocution. National Electrical Code (NEC) The term grounded is as old as the electrical and electronics industry itself, referring to the lowest and safest potential level. The British use the terms earth and earthing. In the United States, grounded and grounding are defined by the National Electrical Code (NEC). The ground is applied by an equipment grounding conductor. The NEC specifies color codes for what is grounded and grounding. NEC 2008 [1], in Article 100, defines a Grounded Conductor as, A system or circuit conductor that is intentionally grounded. Article 100 also defines a Grounding Conductor as, A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

2 Identification of Equipment Grounding Conductor Article of the NEC 2008 includes Sections A through C on this subject. However, the most common identification of an equipment grounding connector is the following: a. Wire size 6 AWG or smaller: bare, covered, or insulated. Covered or insulated conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes. b. Wire size larger than 6 AWG: Insulated or covered conductor identified at each end and where accessible by stripping the covering or insulation where exposed, coloring the insulation or covering green at the terminations, marking the insulation or covering with green tape or green labels at the termination. Receptacle A duplex receptacle (defined by NEC Article 406.2) for a branch circuit is shown in Figure 1 and is used to illustrate the color coding of wires. In Figure 1, the black wire, which is the live, ungrounded 120-V conductor, would be connected to the dark (brass colored) screws on the receptacle. The white wire, which is the grounded, zero-potential conductor, would be connected to the shiny (silver colored) screws. The green wire, which is the equipment grounding conductor, would be connected to the grounding screw that grounds the metal parts of the receptacle, the metal box, and the faceplate. In some cases, a bare copper wire is used instead of the green wire. Figure 1. New 120-Vac duplex receptacle for a branch circuit, showing the hot, neutral, and ground terminal screws. Black arrows show the brass screws for the hot wires (black), white arrows show the silver-colored screws for the neutral wires (white), and green arrows show the green ground screw for the ground wire (green). Single-Phase Service Panel A typical single-phase service panel is supplied by a utility transformer and a three-conductor service cable. The service cable consists of a neutral conductor, a +120-V conductor and a 120-V conductor. The neutral, the transformer core, and the tank are connected to a grounding electrode at the pole.

3 The designations V and 120 -V refer to the AC voltages at the ends of the secondary winding of the transformer. A partially installed 120/240-Vac circuit-breaker panel with single-phase branch circuits is shown in Figure 2. Two typical branch circuits are also shown. The diagram of Figure 2 does not show the utility meter. The service panel typically contains the following pertinent parts: Main circuit breaker: Two-pole for the +120-V and 120-V conductors that supply the two bus bars. Two bus bars: Rated ±120-V to neutral, 240-V between bus bars. Usually designed for one side of the branch circuit breakers to clamp onto the bus bar. Neutral terminal block: Terminations for the neutral service conductor. The neutral terminal block is connected to the utility neutral conductor. Ground terminal block: Connected to the neutral terminal block. Connected by a bonding conductor to the metal enclosure and to the grounding conductor to earth ground (e.g., water pipe or ground rod). Branch circuit breakers: 2-pole 240-V circuit breakers, 1-pole 120-V circuit breakers, and GFCI (ground-fault circuit interrupter). Each 120-V branch circuit is supplied by a single-pole circuit breaker from the +120 V or 120 V bus bar. It contains the following three conductors to supply ac power to receptacles similar to that shown in Figure 1: Black conductor: ungrounded, hot 120-V, from circuit breaker White conductor: grounded, neutral, from neutral terminal block Green (or bare) conductor: equipment grounding, from ground terminal block The 240-V branch circuit is supplied by a 2-pole circuit breaker from both bus bars. It contains the following four conductors: Black conductors (two) or one black and one red conductor: ungrounded, live 120-V to neutral, 240-V between conductors, supplied from a 2-pole circuit breaker. White conductor: grounded, neutral, from neutral terminal block. Not provided if only 240-V is required. Green (or bare) conductor: equipment grounding, from ground terminal block.

4 Figure 2. A partially installed 120/240-Vac panel. Yellow arrow shows the 200-A main circuit breaker. White arrow shows the neutral terminal block. Red arrows show the +120-V and 120-V bus bars. Blue arrows show the single-pole branch circuit breakers. Green arrows show the green grounding conductor. Orange arrows show 2-pole branch circuit breakers with the red and black wires. The purple arrows show the service cables from the utility, and the gray arrows show the sockets for the utility meter.

5 Electrical Accidents Accidents leading to electric shocks or an electrical fire occur for a variety of reasons. Examples include the following: 1. In Figure 2, the grounding conductor is connected to a metal water pipe, with the expectation that the pipe proceeds to earth. However, a change from metal to a plastic section of the pipe insulates the pipe from earth, and it can prevent all of the metal parts of the electrical system from being grounded by the equipment grounding conductors. When a subsequent undetected short circuit or insulation failure occurs, a person who is grounded, by being in contact with the metal parts, may receive an electric shock. 2. A typical panel may have a 200-A main circuit breaker and up to forty 15-A or 20- A branch circuit breakers. In the event that the main breaker or its conductors are overloaded and the breaker does not trip, a fire could result. 3. Some appliances are "double insulated" and do not require a ground pin in the plug. If the appliance gets wet, the insulation may be compromised, causing the equipment housing to become electrically hot and leading to an electrocution hazard. 4. In Figure 1, the most important elements of the receptacle are the ground pin socket and the green equipment grounding conductor connected to the earth ground. A person using an electric appliance in contact with the earth is typically protected from an electric shock if the appliance cord has a ground pin. However, a damaged or removed ground pin can lead to an electrocution hazard. This is further addressed in the following sections. Equipment Ground for Appliances The human body is susceptible to electric shock, and consequently, electrical equipment, tools, and appliances need to be adequately protected. One of the principal means of protection is a safety ground wire connected to a known earth ground. Electrical equipment is grounded for several reasons: 1. To prevent electric shock if a fault occurs in the equipment 2. To provide a path for the fault current to operate a protection element such as a fuse or a circuit breaker 3. To prevent the buildup of electrostatic charge that can result in errors in computers and data processing equipment 4. To prevent the buildup of electrostatic charge that can cause a spark and possible explosions in a hazardous environment. Typically, the grounding is accomplished by a green safety wire in the cord, or by a separate grounding jumper, or by the metallic conduit that carries the wires to the equipment. As mentioned earlier, the NEC provides specific requirements for grounding.

6 Ineffective Ground A Safety Hazard Grounding of equipment can be ineffective for a number of reasons. These include, but are not limited to, the following: 1. The green safety wire is disconnected at the equipment 2. A two-wire cord is used with no green safety wire 3. The three-prong plug is plugged into a two-socket receptacle, using an adapter, and the grounding pigtail of the adapter is not connected 4. The ground socket of the receptacle is not grounded within the receptacle 5. The green safety wire is broken in the cord 6. The ground connection at the circuit breaker panel is intermittent or has a poor connection An ineffective safety ground can further lead to a fault condition, possibly resulting in an electrocution hazard. The fault condition can be due to a number of different reasons, including, but not limited to: 1. Frayed or damaged insulation within the equipment, resulting in exposed energized conductors making contact with the metal enclosure 2. The ground wire connected to the metal enclosure of the appliance is broken, and the frayed conductors make contact with the hot terminal of the ac supply, energizing the metal enclosure 3. The presence of a conductive foreign object such as an unsecured metal screw bridging the gap between the hot terminal and the metal enclosure 4. Conductive contamination on an insulating member that creates a conduction path from the hot terminal to the metal enclosure. Figure 3 shows the buildup of conductive contamination between the hot and the ground terminals on a terminal strip used in a 115-Vac filter network. Consequently, when an electric tool or appliance loses its safety ground connection, an internal fault can cause the metal housing to become electrically hot, resulting in a safety hazard. Figure 3 (left). Arrow shows build-up of conductive contamination on a three-terminal, lug-type terminal strip used in a 115-Vac filter network, in a shipyard environment. The two outer lugs were also connected to the hot and neutral wires. The center terminal was bolted to the metal housing [2].

7 Case Study Investigation of an Alleged vs. Actual Cause of Electrocution In numerous electrocution cases that this author has investigated, two conditions were present: (1) the ground terminal was inadvertently or purposely broken or disconnected, and subsequently, (2) the grounded metallic housing of the subject equipment was accidentally connected to the hot terminal. This connection could result from: (a) electrically conductive contamination, (b) a metallic foreign object, or (c) frayed or damaged insulation within the equipment that cause exposed energized conductors to make contact with the metal enclosure. This case study involved a 120-Vac, 3-wire electric air compressor connected to a 3-wire extension cord. The subject extension cord was plugged into a three-terminal wall outlet. The air compressor was alleged to have caused an electrocution. Available information indicated that, on the morning of the incident, the air compressor was being used to power several pneumatic tools (including a pneumatic nut driver). The tools were being used to install a high-power 12-Vdc electric recovery winch to a pickup truck. The truck was parked adjacent to a chain-link fence, in a patch of tall, wet grass. The individual working at the site with these tools was discovered in an unresponsive condition, and the coroner s report stated death by electrocution. Figure 4 shows an exemplar electric air compressor similar to the one that was being used at the time of the incident. Figure 5 shows an exemplar 3-wire extension cord. The police report and photographs showed that the individual was found slumped on the ground with one hand on a lower section of the chain-link fence and the other hand on the side of the pickup truck. Initial Theory of Electrocution Multiple Problems in the Electric Air Compressor The initial expert report indicated that the electrocution occurred due to: (1) a missing ground pin in the 3- wire cord of the electric air compressor, and (2) high leakage current in the electric air compressor due to conductive contamination. The combination of the missing ground pin and high leakage current was alleged to have resulted in the air compressor housing being at approximately 120-Vac with respect to earth ground. It was further alleged that, because the air compressor had metal supporting feet and was placed in the bed of the pickup truck, the truck housing was now at an elevated potential of approximately Vac. This fault condition was alleged to have resulted in the electrocution. The expert report listed electrical tests that were conducted and indicated that, when the air compressor was operating with the missing ground pin on the extension cord, the compressor housing was at approximately 89-Vac with respect to earth ground, resulting in an electrocution hazard.

8 Figure 4. Exemplar electric air compressor with the original molded 3-wire, 120-Vac plug. Figure 5. Exemplar 3-wire extension cord with the ground pin cut off. The expert s report concluded that the evidence indicated that the deceased was standing in the wet grassy area and was in contact with the grounded chain-link fence. Further, it stated that, in making contact with the body of the pickup truck, the deceased had inadvertently completed the electric circuit from the electrically hot faulted air compressor to the grounded chain-link fence, and also through his feet to the wet grass and to earth ground.

9 Root-Cause Analysis by Exponent An Exponent engineer reviewed the expert report and inspected the subject electric air-compressor. He also performed electrical tests on an exemplar new compressor and determined that, when the ground pin was floating, the open-circuit voltage measured by a digital multimeter (DMM) between the air compressor metal housing and earth ground was approximately 86.5-Vac. This measured voltage agreed with the initial measurements noted in the expert report. However, further tests revealed that this voltage was not due to a high leakage current, but rather, was primarily due to the parasitic capacitance between the motor windings and the enclosure. Further tests were conducted with the ground pin floating and using an IEC human body model ii (HBM) [3] between the exemplar air compressor housing and earth ground. The measured voltage was approximately Vac. Calculations indicated that the current through the HBM was less than 1mA. Subsequently, electrical tests on the subject air compressor also showed that the leakage current using the HBM was less than 1mA. Figure 6 presents information on the IEC HBM. The fibrillation current (1 sec contact) for a typical adult is approximately ma [2]. The measurements indicated that the faulted system was not capable of supplying sufficient current to result in an electrocution, indicating that the speculated fault scenario was not the cause of the incident. Annex E of IEC [3] states in part: Figure 6. Circuit configuration of the HBM in IEC [3]. Resistor R S = 1.5 kω and capacitance C S = 0.22 µf are used to model the total skin impedance of two points of contact. The internal impedance of the human body is modeled by Resistor R B = 500 Ω.

10 Further Investigation A site inspection determined that the chain-link fence was located at the edge of the property, and adjacent to the property was a neighbor s yard. Subsequent inquiry revealed that the neighbor had been using an electric weed wacker on the morning of the incident. The available information indicated that the neighbor had used an old 3-wire extension cord plugged into a non-gfci receptacle to supply ac power to the weed wacker on the morning of the incident. Due to the length of the extension cord, the excess cord was draped over the chain-link fence. An inspection of the extension cord revealed a frayed area where the electrically hot copper conductors were visibly exposed. iii X-ray and optical microscope photographs revealed mechanical damage to numerous strands of the electrically hot copper conductors of the frayed extension cord. The investigation determined that the electrocution likely occurred due to the energized but frayed extension cord being draped over the chain-link fence. The energized, exposed conductors inadvertently made contact with the metal chain-link fence, resulting in the fence voltage rising to 120-V. Although the chain-link fence was installed in the ground, the relatively high ground resistance permitted the fence to reach a high voltage without tripping the 20-A branch circuit breaker. Contact with the electrically energized fence resulted in current flow from the victim s hand to his chest cavity. The return path to ground was most likely through the truck, which was standing in the tall, wet grass, and also through the victim s legs to ground. Conclusions 1. A grounding conductor can be identified by Article of the NEC However, the most common identification is the following: a. Wire size 6 AWG or smaller: bare, covered, or insulated. Covered or insulated conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes. b. Wire size larger than 6 AWG: Insulated or covered conductor identified at each end and where accessible by stripping the covering or insulation where exposed, coloring the insulation or covering green at the terminations, marking the insulation or covering with green tape or green labels at the termination. 2. An IEC human body model can be used when determining if a hazardous potential on the metal housing of equipment results in an electrocution hazard. 3. The absence of a ground pin or damage to wire insulation can result in a safety hazard.

11 For Further Information, Contact: Noshirwan K. Medora, P.E. Senior Managing Engineer References 1. NFPA NFPA 70, National Electrical Code, 2008 Ed. National Fire Protection Association, Quincy, MA. 2. Medora, N.K., Connection Technology. In: P. Martin (Ed.,), Electronic Failure Analysis Handbook, Chapter 17, pp McGraw Hill. 3. IEC International Electrotechnical Commission (IEC) 60990, Second Ed., , Methods of measurement of touch current and protective conductor current. i This article is extracted from a technical paper by Noshirwan K. Medora and Alexander Kusko, titled, A Treatise on Grounding of AC Electrical Systems and an Atypical Electrocution Case Study, presented at the 2008 IEEE Symposium on Product Compliance Engineering, IEEE Product Safety Engineering Society (PSES), Austin, TX, October 20 22, 2008, and also approved for publication in the IEEE PSES 2008 Conference Proceedings. ii IEC 60990:1999, pg. 63 states, The network of figure 4 simulates body impedance and provides weighting to follow the frequency characteristics of the body for current causing involuntary reaction. It has been assumed that the shape of the frequency characteristic is the same for reaction and perception, and the data establishing the frequency characteristic was actually obtained through tests on the threshold of perception. iii The neighbor indicated that the frayed portion was probably due to the extension cord accidentally being run over by the electric lawn mower on previous occasions.