Chris W. Korinek, 8/24/2001
The full article, including photographs, has been published by NAFI at : http://www.nafi.org/archives%202/archive0015.htm
Electrical overheating can occur in so many different ways that it can be difficult to organize an investigation for an electrical cause. In an investigation, many electrical devices may need to be analyzed. In addition, each device is generally capable of causing undesired heating in more than one way. During the investigation various electrical overheating modes may be ruled out and if the evidence is sufficient, an electrical overheating mode may be identified in the fire causation hypothesis. Section 6 of NFPA 921, Guide for Fire and Explosion Investigations, 2001, Electricity and Fire is a very helpful section that describes phenomena that explain how electrical current can overheat and cause a fire. In Figure 6.10.1, Guide for interpreting damage to electrical wires provides a good start for classifying the damage done by certain electrical overheating modes. This article further organizes the information in NFPA 921into a complete classification system for undesired electrical overheating modes.
This classification system has been developed to provide a method for identifying modes of electrical overheating damage that have similarities, regardless of the specific device being analyzed. This system enables those dealing with electricity to better understand electrical overheating and fires, communicate with each other about how electricity causes fires, and to design better protection devices to effectively decrease the number of electrical fires. This classification system adheres to the scientific process; it allows investigators to better organize the analysis of data that has been collected and form and test the hypothesis.
We all benefit when a particular fire cause has been found and published. We can then be on the lookout for additional instances of this cause. However, we, as individuals involved in fire investigation, are well advised not to be biased as to previously found fire causes, especially those well-publicized. When hearing of fires involving particular appliances that have been identified as having similar problems, it is tempting to narrow the focus and give more attention to the problems previously found. Each fire investigator must keep an open mind and make a conclusion based only on solid evidence. This is the best defense against incorrect conclusions. We must be vigilant for less publicized or less-familiar overheating modes. For example, we have all heard of fires in fluorescent light fixtures that were caused by overheating ballasts. However, I have also seen two fires where the evidence of fire causation pointed to poor pin connections on fluorescent light bulbs.
It has been seen that even though two electrical circuit components are significantly
different, they can cause undesired electrical overheating with certain similarities.
In a current carrying circuit, for example, a loose screw on a receptacle acts
similar to a severed solid conductor inside nonmetallic cable: both components
are capable of generating heat and high temperatures at a small location. These
conditions can be readily created in the lab. The heat generated and electrical
insulation degradation may be visible after a short time.1, 2
Other overheating modes appear quite independent from the above example. Take for example a well-insulated non-metallic (NM) cable carrying excessive current. The explanation of how this overheating mode affects a circuit and what evidence may be found after the fire are quite different from the example of the loose screw given previously. The heating pattern from excessive current can be seen over the entire length of the wire that has the same current and thermal insulation, as opposed to the very localized heating pattern of a loose screw. In both examples, the loose screw and excessive current, the electrical insulation is intact during the initial overheating phase.
In yet another overheating mode, current can flow through degraded electrical insulation; this is a very different condition than that discussed regarding the previous overheating modes.
The undesired overheating modes discussed in this article are defined to be malfunctions of the electrical conduction and/or electrical insulation systems. This is different than when an electrical component, such as a heating element, in a device operates as designed, but without the proper control. Two examples of this are a coffee maker or dryer that does not shut off after it reaches the acceptable high temperature level. The latter condition is usually due to the malfunction of the control and high-limit thermostat protection. Even though these fires do occur, have well-documented causes, and deserve our attention as engineers and investigators, they are not within the scope of this article. This article focuses on failures of the electrical conductors and/or electrical insulation systems themselves: it is aimed at better organizing and classifying undesirable electrical overheating modes. For the sake of brevity, the term overheating modes will be used in this article to refer to the undesired electrical type.
Electrical Heating and Overheating
Overheating, as used in this article, is defined as permanently destructive heating that has the potential to ignite a fire. Overheating is heating to or beyond the degradation temperature of the electrical insulation, conductor, or other material immediately adjacent to the heating. It is important to note that all electrical conductors and devices cause some heating in their operation. There is no one temperature limit that these items must reach to be considered severe enough to cause a fire, under which it is considered 100% safe. Some general guidelines are available, for example, the temperature rating of NM cable is 90 deg. C and use above this temperature may cause degradation; however, each specific material and fire requires that individual factors be considered. All heating situations, including electrical heating, occur in degrees, and many factors can cause one situation to progress to a fire and another to remain safe. Each situation needs to be looked at individually for the heat generation, heat transfer, temperatures reached, time of heating, fuel present, and other aspects specific to the fire.
The Four Overheating Classifications:
I have found that every undesired overheating mode I have witnessed, demonstrated,
read about, or heard about fits into one of four classifications: Overheating
due to Poor Connection (OPC), Overheating due to Excessive Current (OEC), Overheating
due to Insulation Breakdown (OIB), and Overheating due to Induction (OI). These
classifications are independent from one another and have various sub-classifications
to further classify the overheating modes. For clarity, the names of the classifications
and sub-classifications of overheating modes were chosen to be self-explanatory
and different from already-defined modes of overheating found in NFPA 921 and
other literature. Abbreviations have been added for faster reference in future
discussion. Within the four classifications, all of the particular overheating
modes have the same names as in NFPA 921.
One of the major outcomes from setting up these classifications is clarifying how certain overheating modes are similar to and different from others. The emphasis in this article will be on how electrical conductors and electrical insulation systems cause overheating to occur, as opposed to the physical configuration of the particular conductor and electrical insulation system and what kind of device or appliance it was used on. Any device or appliance can experience any of the overheating modes, if the conditions are right. You will most likely find that the information contained in this article is not a new discovery; it is a reorganizing of existing knowledge to better visualize the scope of overheating modes, resulting in a more efficient and organized investigation.
Overheating due to Poor Connection (OPC) The OPC class of excessive heat generation
can occur when current, acceptable for the rating of the wire, passes through
a poor connection. The water analogy to an OPC is when unwanted restriction
causes excessive erosion immediately at the piping restriction, such as at a
partially closed valve. When an OPC occurs, current passes through the increased
resistance of a poor connection that is connected in series with the desired
load. Heat is generated and the temperature increases, causing degradation of
the surrounding combustibles at a focused point or small area. For an OPC, it
is as if the connection itself is a small heating element dissipating the unwanted,
abnormal heat. On an outlet, a loose screw carrying acceptable current can result
in a glowing connection that can reach temperatures of 400C (750F). The electrical
insulation can be intact in the early stages of this overheating mode. Examples
that may become OPCs in NFPA 921 are poor and overheating connections, high-resistance
faults, and severed wires.3 Per NFPA, poor connections and overheating connections
are identical: both are connections in the conductors designed to be current
carrying. A high-resistance fault first requires an electrical insulation breakdown
and conductor-to-conductor contact. Electrical insulation breakdown and conductor-to-conductor
contact does not necessarily cause charge flow, heating, or overheating. After
a new path for charge flow is established, charge can flow through the undesired
path to either a neutral or ground conductor(s). If there is sufficient localized
resistance in this new path, overheating in the form of an OPC can occur without
the overcurrent protection device (OCPD) tripping. Fig. 1 shows a picture of
OPC, a glowing loose screw that can reach over 400C and give off 35 Watts at
120VAC carrying 14 Amps, if the conditions are right.
Fig. 1. A glowing screw, one example of overheating due to a poor connection (OPC)
Series Parting Arc (OPC-SPA) This mode of overheating occurs when conductors in series with a load move apart after contact, causing an arc.4 Since an arc is several thousands of degrees, there can be melting or vaporization of metals at the arc. This type of parting arc has a load in series that limits the current of the arc. This occurs whenever a switch is opened and normally does not cause overheating. However, if switch contacts open and close frequently, such as in a chattering relay, the contacts can overheat, weld shut, or ignite nearby combustibles.
2. Overheating due to Excessive Current (OEC) The OEC class of excessive heat generation occurs when excessive current passes through conductors. The water analogy of OEC is too much water flow through a pipe that causes excessive erosion along the entire length of the pipe. The temperature may be high enough for conductors to glow, or cause degradation, or pyrolysis of electrical insulation. Excessive current through the normally minimal resistance of the wires generates heat; the increased temperature of the conductor causes degradation of the surrounding combustibles more or less equally along expanses of the wire with similar thermal insulation. For the OEC, the length of conductor is literally the heating element dissipating the unwanted, abnormal heat. As with the OPC, this overheating can occur initially while the electrical insulation is still intact. According to NFPA 921, examples that may become OECs include overload, undersized conductors, an open (or floating) neutral, and a short circuit.5, 6 A short circuit is a special case of an OEC as it requires an electrical insulation breakdown to occur first; however, the danger of overheating of the conductor comes from OEC. Another overheating mode is triplen harmonics, a problem that can occur in a neutral conductor7. A voltage surge or open neutral condition can create overheating due to excessive current, however, the other OEC modes of overheating are due to too little resistance in the circuit, causing too high a current for the conductor.
Short Circuit (IB-CC-SC leading to OEC) This mode of overheating needs to be thought of as a sequence of events. The first is an electrical insulation breakdown (IB). Then energized and non-energized (ground or neutral) conductors contact each other (CC). A short circuit (SC) can then occur because the current takes a shortcut around the load following the path of least resistance. A short circuit derives its heating characteristics from the OEC mode.8 This overheating mode requires that the OCPD be faulty, undersized, or nonexistent. The significant aspect of this overheating mode is that it can generate high wattage because the voltage across the short circuit is the full line voltage (such as 120 Volts for a typical North American residential circuit). Fig. 2 shows an example of an OEC, a short circuit carrying 120 Amps on a #18 wire with a 12 V DC battery circuit with no fuse of circuit breaker to protect the wires. The yellow wire provides a shortcut for current from the 12 Volt wire to the ground wire.
Fig. 2. A short circuit, one example of overheating due to excessive current (OEC).
3. Overheating due to Insulation Breakdown (OIB) The general class of OIB occurs when the electrical insulating properties of the wire degrades or is removed entirely. The water analogy of an OIB is when the pipe leaks or breaks and allows water to flow into areas that are damaged by getting wet. Electrical insulation consists of a material that purposely inhibits current or charge flow between conductors or between a conductor and a person. Electrical current through a channel outside the normal path of the conductors results in heat generated and increased temperature, and can lead to glowing and/or flaming combustion. For an OIB, it is as if the insulation itself is a small heating element dissipating the unwanted, abnormal heat. This mode of heating can be broken into two sub-classifications: Conductor-to-Conductor Contact (CC) and Degraded Insulation System (DIS). Following is a discussion of these two sub-classifications and their division into other sub-classifications.
Conductor-to-Conductor Contact (CC) This occurs when the electrical insulation system is no longer present and conductors make direct contact.
Parallel Parting Arc (OIB-CC-PPA) This mode of overheating happens when an arc occurs due to conductors (in parallel with a load) moving apart after contact in a short circuit condition. This can happen in certain cases, such as when conductors that normally are insulated from one another touch and then separate. A parallel parting arc can occur, especially at high current and voltage levels. An example of OIB-CC-PPA is when current carrying conductors contact grounded conductors (ground fault parting arc).8 Fig. 3 shows a parallel parting arc using a 12 V DC battery source, #12 wire, and no fuse
Fig. 3. A parallel parting arc (PPA)
Sparks (OIB-CC-SPK) This mode of overheating occurs when glowing particles, caused by arcing, fly away from the arc.9
Degraded Insulation Systems (DIS). This sub-classification indicates that the
electrical insulation has degraded, but is still present. Electrical insulation
can be thought of as the combination of three relatively high-resistance, parallel
paths to ground or neutral. These paths can be
1. through air,
2. across the surface of the insulating material, or
3. through the insulating material.
If any of these three paths becomes conductive, the electrical insulation system is considered degraded, as current will then flow along this path. For instance, if current along the surface of an insulator is present due to contaminated moisture, the current can further heat and degrade the electrical insulation material and form pyrolyzed conductive material. The conductive material can conduct more current through the material and thus generate even more heat.
Leakage Current (OIB-DIS-LC) Leakage current is an overheating mode that involves current through a partially degraded electrical insulation system that can cause further degradation. We may hear or see contaminated water sizzling as it conducts current. Once degradation starts, it creates a condition that continually worsens with time.10
Arc tracking (OIB-DIS-AT) Arc tracking is defined as arcing that may occur on surfaces of nonconductive materials if they become contaminated with salts, conductive dusts, or liquids.11 Though the arcing may be small in size, it may have the potential to ignite flammable gasses being produced from the surface of a pyrolyzing combustible. Even though much has been written on this subject, more research is needed to understand the details of this mode of overheating and to clearly define the conditions for causing a fire. 12, 13, 14 Fig. 4 shows an example of arc tracking at 120 V AC that can happen to NM cable if salt water contacts both the hot and ground or neutral conductors, even though the NM cable has no appliance operating on the circuit.
Fig. 4. Arc tracking induced by salt water on electrified NM cable.
High Voltage Arc (OIB-DIS-HVA)-Lightning, arcing from a high-voltage service, and static electric discharges are examples of this form of arcing.15 Arcing through air requires a practical limit of 350V to start the arc; however, lower voltages are required to sustain the arc once it has been established. Fig. 5 shows an example of how a high voltage arc can be created with 12,000 Volts across the surface of wood, moistened with distilled water.
Fig. 5. High voltage arcing (HVA) simulating a neon sign fire.
Semiconductor (OIB-DIS-SEMI) If damaged, a semiconductor device such as a Metal Oxide Varistor (MOV) in a surge suppressor might conduct current at voltage levels below that which it was designed.16 This can cause further degradation of the device and can result in ignition of nearby combustibles. This is considered a unique form of OIB as the electrical insulation degrades; however, the conduction path through the semiconductor is initially the normal path designed to carry current, when the voltage level is above the threshold voltage.
4. Overheating due to Induction (OI) This rare class of excessive heat generation can occur when conductors with non-canceling unbalanced currents are placed in proximity to conductive materials. These conductive materials are not intended to be current carrying. Induced currents or hysteresis heating within the conductive materials can generate heat, raise the temperature, and degrade surrounding combustibles.17 Cords with conductor pairs running together allow the canceling of the magnetic fields due to the currents in opposite directions. For this reason, cords wound into a bundle that have been known to overheat are not thought to overheat due to inductive heating, but rather due to excessive thermal insulation that does not allow heat to escape from the cord conductors. The OI overheating mode is different from the other heating modes discussed in that the charge is flowing due to a changing magnetic field instead of direct conductor contact. To cause OI, current levels are thought to be very high, possibly hundreds to thousands of Amperes. The heating effect is undoubtedly dependent on the material and physical dimensions of the conducting materials in proximity to the conductors.
Fig. 6, the chart Classification of Overheating or Damage Modes groups the various types of overheating or damage modes into classifications, along with identifying other aspects of the classifications and modes. The chart is further color coded to help visualize the groupings and the rationale for the classifications. With only one exception, all types of OPC (blue) can begin with acceptable levels of current and good quality insulation. With only one exception, all types of OEC (yellow) can begin with acceptable connections and insulation. All types of OIB can begin with acceptable connections and current levels. Other aspects of these classifications and modes are also shown in this table, including the protection device with the best chance of stopping the overheating mode or damage. The decision to use sub-classifications was based on whether or not the sub-classification was critical in the immediate area of overheating. If an event, such as an insulation breakdown, occurred in a different location than the immediate area of overheating, the event was described in the chart instead as a sequence of events necessary to cause the overheating mode. At our website, www.synergytech.net, under publications, we have a handy checklist that can be used to help investigate and evaluate multiple devices or appliances at the scene or lab.
Phenomena that do not fit into the Overheating Mode Classifications:
This system of classifying overheating modes was designed to identify overheating with the potential to ignite a fire. For reasons of clarity, lower levels of heating that do not result in material degradation or combustion, or control and high limit thermostat failures are not considered in these electrical overheating modes. Examples of non-electrical overheating modes include, but are not limited to the following:
1. A series-parting arc in a well-designed switch that does not cause degradation of materials after the arcing occurs and is quenched.
2. An overcurrent situation that trips a properly sized circuit breaker as the overcurrent situation is stopped before it caused permanent damage.
3. Induction heating in certain appliances such as a voltage transformer, if temperatures remain at an acceptable level.
4. An electric heating element that is functioning as it was designed to, but does not shut off when the maximum temperature is reached, thus causing a fire. This problem would be identified as a heating element without proper thermal control or limitation. An example of this would be a coffee maker that does not turn off because the control or high-limit thermostats do not properly function.
5. Arcing through char is a well-known result of a fire. Major arcing damage can occur to energized conductors and conduit that is caused by arcing through char. The damage can include beading and severing of wires and melting holes in steel conduit. Arcing through char can produce bright flashes of light and spattering of molten metal. However, this phenomenon is thought to always be a result and never the cause of a fire.18
6. Mechanical gouges, nicked or stretched conductors have been proven to be phenomena that do not cause a temperature rise in damaged conductors to the extent that they present a fire hazard (unless in extreme cases).19
7. Deteriorated or cracked electrical insulation has been shown to not be a fire hazard, unless moisture is present. A shock hazard, however, may be present with this condition.20
8. Melting by fire if the conductor reaches or exceeds its melting point. If melting by fire damages copper or other conductors, the conductor size will generally change gradually along the length and the conductor may be severed with the ends narrowing to a point. This is in contrast to an arc bead where a sudden change in conductor shape can be observed from the round pre-fire shape.21
9. Alloying of the conductor with other metals such as aluminum or brass can allow the alloy to melt at a lower temperature than either metal.22
Initial Overheating Mode
A very important goal of the investigation that uses the classifications of electrical overheating modes described in this article is to determine the sequence of events that have occurred to start the fire. Often, there are signs of two, three, or even more electrical overheating modes that can be observed at a fire scene. There may be evidence of a poor connection, tripped circuit breakers or fuses, and beaded wires. Some evidence, such as arc tracking can easily be destroyed in the fire; however, eyewitness testimony may help confirm that it was present. The challenge then is to try to determine the sequence of electrical events that led to the fire. Determining the sequence of electrical events can be done by first gathering all evidence. Then, based on the evidence, the initial overheating mode that started the sequence, along with the sequence of subsequent overheating modes may be identified. The analysis can continue to potentially determine why the initial overheating occurred and what actions were responsible without confusion by the subsequent overheating mode damage.
A good way to think of the events in a fire is like a game of dominos. When properly placed, once the first domino falls, the other dominoes have no choice but to fall. Similarly, if an initial electrical overheating mode of adequate heating capability occurs in the presence of combustibles, it may cause a sequence of further thermal and electrical damage, and a free-burning fire.
Another way of stating the value of finding the initial heating mode is that certain overheating modes do not typically result in flaming combustion, initially. If we cause a glowing connection with a loose screw on a receptacle, the receptacle does not typically burst into flames by itself. An OPC or OEC condition may result in decomposition such as pyrolization, in its first stage. Then, leakage current can occur, leading to arc tracking. Arc tracking can then ignite the gases evolved from the combustibles and progress to flaming combustion.
Sequence of Electrical Events
By collecting physical and interview evidence, we may be able to determine that the initial overheating mode was, for example, OEC. If someone replaced a 15-amp circuit breaker with a 30-amp circuit breaker and then insulated the attic where the conductors were run, the following sequence is plausible:
1. Overheating due to excessive current (OEC) Higher than rated current causes well-insulated electrical insulation to exceed its temperature rating and pyrolyze forming conductive deposits along sizable lengths of the cable.
2. Leakage current (OIB-DIS-LC) As the electrical insulation begins to degrade into conductive material, a small amount of leakage charge flows through the material. This creates more heating and more conductive material.
Arc tracking (OIB-DIS-AT) Once the degradation reaches a certain level, the
conductive material on the electrical insulation begins arc tracking. This heating
mode ignites the gas from pyrolyzing electrical insulation and nearby cellulose
insulation. The ensuing fire eventually ignites other nearby electrical insulation
and wooden structural members.
Heating due to overcurrent. A short circuit causes overcurrent through remaining energized wires and then the circuit breaker trips. No additional degradation of materials is caused by this last instance of overcurrent.
Identifying this sequence gives us the best method to explain, within a reasonable degree of scientific certainty, what occurred to create the evidence we have found at the scene. It can be noted that the heating significant enough to be called overheating led up to ignition during arc tracking. The latter heating modes were not involved with starting the fire; however, they were the result of the fire. Heating due to excessive current was interrupted by the circuit breaker and did no additional damage. All the damage is then accounted for. Even if the entire sequence is not identified, as some will be difficult or impossible to identify, the goal should be to piece back together as much of the sequence as possible to reconstruct the events leading up to the fire.
OPC and OEC, in general, are frequently seen initial overheating modes. However, OIB-DIS-AT is thought by many to be a major cause of the start of flaming combustion. Glowing connections or glowing wire can also ignite combustibles into flaming combustion without OIB-DIS-AT. Unless electrical insulation breaks down by itself due to age, moisture, or contaminants and OIB-DIS-AT occurs, there often is a different mechanism of heat generation such as OPC, OEC, or another mode that causes electrical insulation breakdown and the possible ensuing OIB-DIS-AT.
This concept of sequences follows the idea of the first fuel ignited discussed in Kirks Fire Investigation.23 The most complete electrical fire engineering analysis results from not only identifying the general origin and cause of the fire, but pinpointing the exact origin of initial heating, the initial overheating mode, and the reason this overheating mode began. After these steps are complete, the investigation team has the best chance to determine the actions leading up to this initial overheating mode and the party legally responsible for the fire.
There are more types of heating possible: dielectric, microwave, etc. However, these modes of heating are not known via literature or experience to have been the cause of fires.
The classifications of overheating modes set forth in this article help to organize the discipline of engineering analysis of electrical fires into a more scientific endeavor than has previously been available. There are only four basic overheating classes and they are easier to remember than the numerous specific modes. The classification of overheating modes also makes it easier to piece together the sequence of heating events in order to explain the evidence found at the fire scene. Think of these four classifications as the genus or highest level of electrical overheating modes. Each can be broken down into more detail and further classified, but each instance of overheating typically fits into one of the four main classifications. The water analogies given help us to visualize what is happening in an electrical circuit. This classification system increases our scientific knowledge base and benefits personnel in the fields of fire investigation. Using this scientific approach gives us the best chance of identifying the cause of fires, means of prevention, and saving property damage and lives.
It is hoped that these classifications will be adopted and used to standardize discussions of the mechanisms of electrical fires.
To help bring the field of electrical fire analysis to an even more scientific level, it is suggested that future studies be done on the following topics:
1. Further development of detailed heating mechanisms for each overheating mode (such as arc tracking)
2. Additional material analysis aspects (copper, steel, aluminum, electrical and thermal insulation systems, carbon, printed circuit board materials, etc) involved with the overheating mechanisms and process
3. Better-understood variations of AC, DC, solid versus stranded wire, current and voltage levels and thresholds, etc.
4. Better quantification of forensic markers left behind that can be scientifically explained by the overheating modes/mechanisms (standard tests and qualitative and quantitative measurement standards for particular overheating modes)
1. Meese, William J. and Beausoleil, Robert W., Exploratory Study of Glowing Electrical Connections, National Bureau of Standards, October 1977.
2. Ettling, Bruce V., Glowing Connections, Selected Articles, IAAI 1992 Edition, Article dated March 1983.
3. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 126.96.36.199, 6.10.4, 6.9.6, and 6.11.8.
4. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 188.8.131.52.
5. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 1.3, 6.9.3, 6.10.2, 6.10.5, 6.5.1, 6.11.4, and 6.11.9.
6. Sanderson, Jack and Finneran, James, "Floating Neutral Problems,"
Vol. 3, No. 1, WINTER 1995. Pp. 9-11.
7. Fink, Donald G., Beaty, Wayne H., Standard Handbook for Electrical Engineers, McGraw-Hill, 13th Edition, 1993, pp. 18-110.
8. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 1.3, 184.108.40.206, 6.10.2.
9. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, 6.9.5.
10. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 6.10.2.
11. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 220.127.116.11.
12. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 18.104.22.168.
13. Yearance, Robert A., Electrical Fire Investigation, Charles C. Thomas, 1987, pp. 132-138.
14. Beland, Bernard, Arc Tracking in Relation to Fire Investigation, Fire and Arson Investigator, March 1995.
15. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 1.3, 6.12.
16. Olson, P.E., David B., "Surge Suppressors: Real Protection or Potential
Hazards," FIRE FINDINGS,
Vol. 3, No. 2, SPRING 1995,pp. 1-4.
17. Fink, Donald G., Beaty, Wayne H., Standard Handbook for Electrical Engineers, McGraw-Hill, 13th Edition, 1993, pp. 2-42 to 2-44.
18. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 6.10.3.
19. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 6.11.5.
20. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 6.11.7.
21. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 22.214.171.124.
22. NFPA, NFPA 921: Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2001, Section 126.96.36.199.
23. DeHaan, John D., Kirks Fire Investigation, Brady-Prentice Hall, 1997, P. xiii, preface.
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