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NFPA 921 Sections 14-1 and 14-9 through 14-12.2
Electricity and Fire

[interFIRE VR Note: Tables and Figures have not been reproduced.]

14-1. Introduction. This chapter discusses the analysis of electrical systems and equipment. The primary emphasis is on buildings with 120/240-volt, single-phase electrical systems. These voltages are typical in residential and commercial buildings. This chapter also discusses the basic principles of physics that relate to electricity and fire.

Prior to beginning an analysis of a specific electrical item, it is assumed that the person responsible for determining the cause of the fire will have already defined the area or point of origin. Electrical equipment should be considered as an ignition source equally with all other possible sources and not as either a first or last choice. The presence of electrical wiring or equipment at or near the origin of a fire does not necessarily mean that the fire was caused by electrical energy. Often the fire may destroy insulation or cause changes in the appearance of conductors or equipment that can lead to false assumptions. Careful evaluation is warranted.

Electrical conductors and equipment that are appropriately used and protected by properly sized and operating fuses or circuit breakers do not normally present a fire hazard. However, the conductors and equipment can provide ignition sources if easily ignitible materials are present when they have been improperly installed or used. A condition in the electrical wiring that does not conform to the National Electrical Code might or might not be related to the cause of a fire.

14-9. Ignition by Electrical Energy.

14-9.1. General. For ignition to be from an electrical source, the following must occur:

(a) The electrical wiring, equipment, or component must have been energized from a building's wiring, an emergency system, a battery, or some other source.

(b) Sufficient heat and temperature to ignite a close combustible material must have been produced by electrical energy at the point of origin by the electrical source.

Ignition by electrical energy involves generating both a sufficiently high temperature and heat (i.e., competent ignition source) by passage of electrical current to ignite material that is close. Sufficient heat and temperature may be generated by a wide variety of means, such as short circuit and ground fault parting arcs, excessive current through wiring or equipment, resistance heating, or by ordinary sources such as lightbulbs, heaters, and cooking equipment. The requirement for ignition is that the temperature of the electrical source be maintained long enough to bring the adjacent fuel up to its ignition temperature with air present to allow combustion.

The presence of sufficient energy for ignition does not assure ignition. Distribution of energy and heat loss factors need to be considered. For example, an electric blanket spread out on a bed can continuously dissipate 180 W safely. If that same blanket is wadded up, the heating will be concentrated in a smaller space. Most of the heat will be held in by the outer layers of the blanket, which will lead to higher internal temperatures and possibly ignition. In contrast to the 180 W used by a typical electric blanket, just a few watts used by a small flashlight bulb will cause the filament to glow white hot, indicating temperatures in excess of 4000°F (2204°C).

In considering the possibility of electrical ignition, the temperature and duration of the heating must be great enough to ignite the initial fuels. The type and geometry of the fuel must be evaluated to be sure that the heat was sufficient to generate combustible vapors and for the heat source still to be hot enough to ignite those vapors. If the suspect electrical component is not a competent ignition source, other causes should be investigated.

14-9.2. Resistance Heating.

14-9.2.1. General. Whenever electric current flows through a conductive material, heat will be produced. See 14-2.13 for the relationships of current, voltage, resistance, and power (i.e., heating). With proper design and compliance with the codes, wiring systems and devices will have resistances low enough that current-carrying parts and connections should not overheat. Some specific parts such as lamp filaments and heating elements are designed to become very hot. However, when properly designed and manufactured and when used according to directions, those hot parts should not cause fires.

The use of copper or aluminum conductors of sufficient size in wiring systems (e.g., 12 AWG for up to 20 A for copper) will keep the resistance low. What little heat is generated should be readily dissipated to the air around the conductor under normal conditions. When conductors are thermally insulated and operating at rated currents, enough energy may be available to cause a fault or ignition.

14-9.2.2. Heat-Producing Devices. Common heat-producing devices can cause fires when misused or when certain malfunctions occur during proper use. Examples include combustibles placed too close to incandescent lamps or to heaters or coffee makers and deep-fat fryers whose temperature controls fail or are bypassed. (See Chapter 18.)

14-9.2.3. Poor Connections. When a circuit has a poor connection such as a loose screw at a terminal, increased resistance causes increased heating at the contact, which promotes formation of an oxide interface. The oxide conducts current and keeps the circuit functional, but the resistance of the oxide at that point is significantly greater than in the metals. A spot of heating develops at that oxide interface that can become hot enough to glow. If combustible materials are close enough to the hot spot, they can be ignited. Generally, the connection will be in a box or appliance, and the probability of ignition is greatly reduced. The wattage of well-developed heating connections in wiring can be up to 30-40 W with currents of 15-20 A. Heating connections of lower wattage have also been noted at currents as low as about 1 A.

14-9.3. Overcurrent and Overload. Overcurrent is the condition in which more current flows in a conductor than is allowed by accepted safety standards. The magnitude and duration of the overcurrent determines whether there is a possible ignition source. For example, an overcurrent at 25 A in a 14-AWG copper conductor should pose no fire danger except in circumstances that do not allow dissipation of the heat such as when thermally insulated or when bundled in cable applications. A large overload of 120 A in a 14-AWG conductor, for example, would cause the conductor to glow red hot and could ignite adjacent combustibles.

Large overcurrents that persist (i.e., overload) can bring a conductor up to its melting temperature. There is a brief parting arc as the conductor melts in two. The melting opens the circuit and stops further heating.

In order to get a large overcurrent, either there must be a fault that bypasses the normal loads (i.e., short circuit) or far too many loads must be put on the circuit. To have a sustained overcurrent (i.e., overload), the protection (i.e., fuses or circuit breakers) must fail to open or must have been defeated. Ignition by overload is rare in circuits that have the proper size conductors throughout the circuit, because most of the time the protection opens and stops further heating before ignition conditions are obtained. When there is a reduction in the conductor size between the load and the circuit protection, such as an extension cord, the smaller size conductor may be heated beyond its temperature rating. This can occur without activating the overcurrent protection. For an example, see 14-2.16.

14-9.4. Arcs. An arc is a high-temperature luminous electric discharge across a gap. Temperatures within the arc are in the range of several thousand degrees depending on circumstances including current, voltage drop, and metal involved. For an arc to jump even the smallest gap in air spontaneously, there must be a voltage difference of at least 350 V. In the 120/240-V systems being considered here, arcs do not form spontaneously under normal circumstances. (See Section 14-12.) In spite of the very high temperatures in an arc path, arcs may not be competent ignition sources for many fuels. In most cases, the arcing is so brief and localized that solid fuels such as wood structural members cannot be ignited. Fuels with high surface-area-to-mass ratio, such as cotton batting and tissue paper and combustible gases and vapors, may be ignited when in contact with the arc.

14-9.4.1. High-Voltage Arcs. High voltages can get into a 120/240-V system through accidental contact between the distribution system of the power company and the system on the premises. Whether there is a momentary discharge or a sustained high voltage, an arc may occur in a device for which the separation of conductive parts is safe at 240 V but not at many thousands of volts. If easily ignitible materials are present along the arc path, a fire can be started. Lightning can send extremely high voltage surges into an electrical installation. Because the voltages and currents from lightning strikes are so high, arcs can jump at many places, cause mechanical damage, and ignite many kinds of combustibles. (See 14-12.8.)

14-9.4.2. Static Electricity. Static electricity is a stationary charge that builds up on some objects. Walking across a carpet in a dry atmosphere will produce a static charge that can produce an arc when discharged. Other kinds of motion can cause a build-up of charge, including the pulling off of clothing, operation of conveyor belts, and the flowing of liquids. (See Section 14-12.)

14-9.4.3. Parting Arcs. A parting arc is a brief discharge that occurs as an energized electrical path is opened while current is flowing, such as by turning off a switch or pulling a plug. The arc usually is not seen in a switch but might be seen when a plug is pulled while current is flowing. Motors with brushes may produce a nearly continuous display of arcing between the brushes and the commutator. At 120/240-V ac, a parting arc is not sustained and will quickly be quenched. Ordinary parting arcs in electrical systems are usually so brief and of low enough energy that only combustible gases, vapors, and dusts can be ignited.

In arc welding, the rod must first be touched to the workpiece to start current flowing. Then the rod is withdrawn a small distance to create a parting arc. If the gap does not become too great, the arc will be sustained. A welding arc involves enough power to ignite nearly any combustible material. However, the sustained arc during welding requires specific design characteristics in the power supply that are not present in most parting arc situations in 120/240-V wiring systems.

Another kind of parting arc occurs when there is a direct short circuit or ground fault. The surge of current melts the metals at the point of contact and causes a brief parting arc as a gap develops between the metal pieces. The arc quenches immediately but can throw particles of melted metal (i.e., sparks) around. (See 14-9.5.)

14-9.4.4.* Arc Tracking. Arcs may occur on surfaces of nonconductive materials if they become contaminated with salts, conductive dusts, or liquids. It is thought that small leakage currents through such contamination causes degradation of the base material leading to the arc discharge, charring or igniting combustible materials around the arc. Arc tracking is a known phenomenon at high voltages. It has also been reported in experimental studies in 120/240-V ac systems.

Electrical current will flow through water or moisture only when that water or moisture contains contaminants such as dirt, dusts, salts, or mineral deposits. This stray current may promote electrochemical changes that can lead to electrical arcing. Most of the time the stray currents through a contaminated wet path cause enough warming that the path will dry. Then little or no current flows and the heating stops. If the moisture is continuously replenished so that the currents are sustained, deposits of metals or corrosion products can form along the electrical pathway. That effect is more pronounced in direct current situations. A more energetic arc through the deposits might cause a fire under the right conditions. More study is needed to more clearly define the conditions needed for causing a fire.

14-9.5. Sparks. Sparks are luminous particles that can be formed when an arc melts metal and spatters the particles away from the point of arcing. The term spark has commonly been used for a high voltage discharge as with a spark plug in an engine. For purposes of electrical fire investigation, the term spark is reserved for particles thrown out by arcs, whereas an arc is a luminous electrical discharge across a gap.

Short circuits and high-current ground faults, such as when the ungrounded conductor (i.e., hot conductor) touches the neutral or a ground, produce violent events. Because there may be very little resistance in the short circuit, the fault current may be many hundreds or even thousands of amperes. The energy that is dissipated at the point of contact is sufficient to melt the metals involved, thereby creating a gap and a visible arc and throwing sparks. Protective devices in most cases will open (i.e., turn off the circuit) in a fraction of a second and prevent repetition of the event.

When just copper and steel are involved in arcing, the spatters of melted metal begin to cool immediately as they fly through the air. When aluminum is involved in faulting, the particles may actually burn as they fly and continue to be extremely hot until they burn out or are quenched by landing on some material. Burning aluminum sparks, therefore, may have a greater ability to ignite fine fuels than do sparks of copper or steel. However, sparks from arcs in branch circuits are inefficient ignition sources and can ignite only fine fuels when conditions are favorable. In addition to the temperature, the size of the particles is important for the total heat content of the particles and the ability to ignite fuels. For example, sparks spattered from a welding arc can ignite many kinds of fuels because of the relatively large size of the particles and the total heat content. Arcing in entry cables can produce more and larger sparks than can arcing in branch circuits.

14-9.6. High-Resistance Faults. High-resistance faults are long-lived events in which the fault current is not high enough to trip the circuit overcurrent protection, at least in the initial stages. A high-resistance fault on a branch circuit may be capable of producing energy sufficient to ignite combustibles in contact with the point of heating. It is rare to find evidence of a high-resistance fault after a fire. An example of a high-resistance fault is an energized conductor coming into contact with a poorly grounded object.

14-10 Interpreting Damage to Electrical Systems

14-10.1. General. Abnormal electrical activity will usually produce characteristic damage that may be recognized after a fire. Evidence of this electrical activity may be useful in locating the area of origin. The damage may occur on conductors, contacts, terminals, conduits, or other components. However, many kinds of damage can occur from nonelectrical events. This section will give guidelines for deciding whether observed damage was caused by electrical energy and whether it was the cause of the fire or a result of the fire. These guidelines are not absolute, and many times the physical evidence will be ambiguous and will not allow a definite conclusion. Figure 14-10.1 illustrates some of the types of damage that may be encountered.

14-10.2.* Short Circuit and Ground Fault Parting Arcs. Whenever an energized conductor contacts a grounded conductor or a metal object that is grounded with nearly zero resistance in the circuit, there will be a surge of current in the circuit and melting at the point of contact. This event may be caused by heat-softened insulation due to a fire. The high current flow produces heat that can melt the metals at the points of contact of the objects involved, thereby producing a gap and the parting arc. A solid copper conductor typically appears as though it had been notched with a round file. [See Figure 14-10.2(a).] The notch may or may not sever the conductor. The conductor will break easily at the notch upon handling. The surface of the notch can be seen by microscopic examination to have been melted. Sometimes, there can be a projection of porous copper in the notch.

The parting arc melts the metal only at the point of initial contact. The adjacent surfaces will be unmelted unless fire or some other event causes subsequent melting. In the event of subsequent melting, it may be difficult to identify the site of the initial short circuit or ground fault. If the conductors were insulated prior to the faulting and the fault is suspected as the cause of the fire, it will be necessary to determine how the insulation failed or was removed and how the conductors came in contact with each other. If the conductor or other metal object involved in the short circuit or ground fault was bare of insulation at the time of the faulting, there may be spatter of metal onto the otherwise unmelted adjacent surfaces.

Stranded conductors, such as for lamp and appliance cords, appear to display effects from short circuits and ground faults that are less consistent than those in solid conductors. A stranded conductor may exhibit a notch with only some of the strands severed, or all of the strands may be severed with strands fused together or individual strands melted. [See Figure 14-10.2(b).]

14-10.3.* Arcing Through Char. Insulation on conductors, when exposed to direct flame or radiant heat, may be charred before being melted. That char, when exposed to fire, is conductive enough to allow sporadic arcing through the char. That arcing can leave surface melting at spots or can melt through the conductor, depending on the duration and repetition of the arcing. There often will be multiple points of arcing. Several inches of conductor can be destroyed either by melting or severing of several small segments.

When conductors are subject to highly localized heating, such as from arcing through char, the ends of individual conductors may be severed. When severed, they will have beads on the end. The bead may weld two conductors together. If the conductors are in conduit, holes may be melted in the conduit. Beads can be differentiated from globules, which are created by nonlocalized heating such as overload or fire melting. Beads are characterized by the distinct and identifiable line of demarcation between the melted bead and the adjacent unmelted portion of the conductor. [See Figures 14-10.3(a), (b), and (c).]

The conductors downstream from the power source and the point where the conductors are severed become de-energized. Those conductors will likely remain in the debris with part or all of their insulation destroyed. The upstream remains of the conductors between the point of arc-severing and the power supply may remain energized if the overcurrent protection does not function. Those conductors can sustain further arcing through the char. In a situation with multiple arc-severing on the same circuit, arc-severing farthest from the power supply occurred first. It is necessary to find as much of the conductors as possible to determine the location of the first arcing through char. This will indicate the first point on the circuit to be compromised by the fire and may be useful in determining the area of origin. In branch circuits, holes extending for several inches may be seen in the conduit or in metal panels to which the conductor arced.

If the fault occurs in service entrance conductors, several feet of conductor may be partly melted or destroyed by repeated arcing because there is usually no overcurrent protection for the service entrance. An elongated hole or series of holes extending several feet may be seen in the conduit.

14-10.4.* Overheating Connections. Connection points are the most likely place for overheating to occur on a circuit. The most likely cause of the overheating will be a loose connection or the presence of resistive oxides at the point of connection. Metals at an overheating connection will be more severely oxidized than similar metals with equivalent exposure to the fire. For example, an overheated connection on a duplex receptacle will be more severely damaged than the other connections on that receptacle. The conductor and terminal parts may have pitted surfaces or may have sustained a loss of mass where poor contact has been made. This loss of mass can appear as missing metal or tapering of the conductor. These effects are more likely to survive the fire when copper conductors are connected to steel terminals. Where brass or aluminum are involved at the connection, the metals are more likely to be melted than pitted. This melting can occur either from resistance heating or from the fire. Pitting also can be caused by alloying. (See 14-10.6.3.)

14-10.5.* Overload. Currents in excess of rated ampacity produce effects in proportion to the degree and duration of overcurrent. Overcurrents that are large enough and persist long enough to cause damage or create a danger of fire are called overloads. Under any circumstance, suspected overloads require that the circuit protection be examined. The most likely place for an overload to occur is on an extension cord. Overloads are unlikely to occur on wiring circuits with proper overcurrent protection.

Overloads cause internal heating of the conductor. This heating occurs along the entire length of the overloaded portion of the circuit and may cause sleeving. Sleeving is the softening and sagging of thermoplastic conductor insulation due to heating of the conductor. If the overload is severe, the conductor may become hot enough to ignite fuels in contact with it as the insulation melts off. Severe overloads may melt the conductor. If the conductor melts in two, the circuit is opened and heating immediately stops. The other places where melting had started may become frozen as offsets. This effect has been noted in copper, aluminum, and Nichrome conductors. (See Figure 14-10.5.) The finding of distinct offsets is an indication of a large overload. Evidence of overcurrent melting of conductors is not proof of ignition by that means.

Overload in service entrance cables is more common than in branch circuits but is usually a result of fire. Faulting in entrance cables produces sparking and melting only at the point of faulting unless the conductors maintain continuous contact to allow the sustained massive overloads needed to melt long sections of the cables.

14-10.6. Effects Not Caused by Electricity. Conductors may be damaged before or during a fire by other than electrical means and often these effects are distinguishable from electrical activity.

14-10.6.1. Conductor Surface Colors. When the insulation is damaged and removed from copper conductors by any means, heat will cause dark red to black oxidation on the conductor surface. Green or blue colors may form when some acids are present. The most common acid comes from the decomposition of PVC. These various colors are of no value in determining cause because they are nearly always results of the fire condition.

14-10.6.2. Melting by Fire. When exposed to fire, copper conductors may melt. At first, there is blistering and distortion of the surface. [See Figure 14-10.6.2(a).] The striations created on the surface of the conductor during manufacture become obliterated. The next stage is some flow of copper on the surface with some hanging drops forming. Further melting may allow flow with thin areas (i.e., necking and drops). [See Figure 14-10.6.2(b).] In that circumstance, the surface of the conductor tends to become smooth. The resolidified copper forms globules. Globules caused by exposure to fire are irregular in shape and size. They are often tapered and may be pointed. There is no distinct line of demarcation between melted and unmelted surfaces.

Stranded conductors that just reach melting temperatures become stiffened. Further heating can let copper flow among the strands so that the conductor becomes solid with an irregular surface that can show where the individual strands were. [See Figure 14-10.6.2(c).] Continued heating can cause the flowing, thinning, and globule formation typical of solid conductors. Magnification is needed to see some of these effects. Large-gauge stranded conductors that melt in fires can have the strands fused together by flowing metal or the strands may be thinned and stay separated. In some cases, individual strands may display a bead-like globule even though the damage to the conductor was from melting.

Aluminum conductors melt and resolidify into irregular shapes that are usually of no value for interpreting cause. [See Figure 14-10.6.2(d).] Because of the relatively low melting temperature, aluminum conductors can be expected to melt in almost any fire and rarely aid in finding the cause.

14-10.6.3.* Alloying. Metals such as aluminum and zinc can form alloys when melted in the presence of other metals. If aluminum drips onto a bare copper conductor during a fire and cools, the aluminum will be just lightly stuck to the copper. If that spot is further heated by fire, the aluminum can penetrate the oxide interface and form an alloy with the copper that melts at a lower temperature than does either pure metal. After the fire, an aluminum alloy spot may appear as a rough gray area on the surface, or it may be a shiny silvery area. The copper-aluminum alloy is brittle, and the conductor may readily break if it is bent at the spot of alloying. If the melted alloy drips off the conductor during the fire, there would be a pit that is lined with alloy. The presence of alloys can be confirmed by chemical analysis.

Aluminum conductors that melt from fire heating at a terminal may cause alloying and pitting of the terminal pieces. There is no clear way of visually distinguishing alloying from the effects of an overheating connection. Zinc forms a brass alloy readily with copper. It is yellowish in color and not as brittle as the aluminum alloy.

14-10.6.4.* Mechanical Gouges. Gouges and dents that are formed in a conductor by mechanical means can usually be distinguished from arcing marks by microscopic examination. Mechanical gouges will usually show scratch marks from whatever caused the gouge. Dents will show deformation of the conductors beneath the dents. Dents or gouges will not show the fused surfaces caused by electrical energy.

14-11. Considerations and Cautions. Laboratory experiments, combined with the knowledge of basic chemical, physical, and electrical sciences, indicate that some prior beliefs are incorrect or are correct only under limited circumstances.

14-11.1. Undersized Conductors. Undersized conductors, such as a 14 AWG conductor in a 20-A circuit, are sometimes thought to overheat and cause fires. There is a large safety factor in the allowed ampacities. Although the current in a 14 AWG conductor is supposed to be limited to 15 A, the extra heating from increasing the current to 20 A would not necessarily indicate a fire cause. The higher operating temperature would deteriorate the insulation faster but would not melt it or cause it to fall off and bare the conductor without some additional factors to generate or retain heat. The presence of undersized conductors or overfused protection is not proof of a fire cause. (See 14-2.16.)

14-11.2. Nicked or Stretched Conductors. Conductors that are reduced in cross section by being nicked or gouged are sometimes thought to heat excessively at the nick. Calculations and experiments have shown that the additional heating is negligible. Also, it is sometimes thought that pulling conductors through conduit can stretch them like taffy and reduce the cross section to a size too small for the ampacity of the protection. Copper conductors do not stretch that much without breaking at the weakest point. Whatever stretching can occur before the range of plastic deformation is exceeded would not cause either a significant reduction in cross section or excessive resistance heating.

14-11.3. Deteriorated Insulation. When thermoplastic insulation deteriorates with age and heating, it tends to become brittle and will crack if bent. Those cracks do not allow leakage current unless conductive solutions get into the cracks. Rubber insulation does deteriorate more easily than thermoplastic insulation and loses more mechanical strength. Thus, rubber insulated lamp or appliance cords that are subject to being moved can become hazardous because of embrittled insulation breaking off. However, simple cracking of rubber insulation as with thermoplastic insulation does not allow leakage of current unless conductive solutions get into the cracks.

14-11.4.* Overdriven or Misdriven Staple. Staples driven too hard over nonmetallic cable have been thought to cause heating or some kind of faulting. The suppositions range from induced currents because of the staple being too close to the conductors to actually cutting through the insulation and touching the conductors. A properly installed cable staple with a flattened top cannot be driven through the insulation. If the staple is bent over, the edge of it can be driven through the insulation to contact the conductors. In that case, a short circuit or a ground fault would occur. That event should be evident after a fire by bent points of the staple and by melt spots on the staple or on the conductors unless obliterated by the ensuing fire. A short circuit should cause the circuit overcurrent protection to operate and prevent any further damage. There would not be any continued heating at the contact, and the brief parting arc would not ignite the insulation on the conductor or the wood to which it was stapled.

If a staple is misdriven so that one leg of the staple penetrates the insulation and contacts both an energized conductor and a grounded conductor, then a short circuit or ground fault will result. If the staple severs the energized conductor, a heating connection may be formed at that point.

14-11.5. Short Circuit. A short circuit (i.e., low resistance and high current) in wiring on a branch circuit has been thought to ignite insulation on the conductors and allow fire to propagate. Normally, the quick flash of a parting arc prior to operation of the circuit protection cannot heat insulation enough to generate ignitible fumes even though the temperature of the core of the arc may be several thousand degrees. If the overcurrent protection is defeated or defective, then a short circuit may become an overload and, as such, may become an ignition source.

14-11.6. Beaded Conductor. A bead on the end of a conductor in and of itself does not indicate the cause of the fire.

14-12. Static Electricity.

14-12.1. Introduction to Static Electricity. Static electricity is the electrical charging of materials through physical contact and separation and the various effects that result from the positive and negative electrical charges formed by this process. This is accomplished by the transfer of electrons (negatively charged) between bodies, one giving up electrons and becoming positively charged and the other gaining electrons and becoming oppositely, but equally, negatively charged.

Common sources of static electricity include the following:

(a) Pulverized materials passing through chutes or pneumatic conveyors

(b) Steam, air, or gas flowing from any opening in a pipe or hose, when the steam is wet or the air or gas stream contains particulate matter

(c) Nonconductive power or conveyor belts in motion

(d) Moving vehicles

(e) Nonconductive liquids flowing through pipes or splashing, pouring, or falling

(f) Movement of clothing layers against each other or contact of footwear with floors and floor coverings while walking

(g) Thunderstorms that produce violent air currents and temperature differences that move water, dust, and ice crystals creating lightning

(h) Motions of all sorts that involve changes in relative position of contacting surfaces, usually of dissimilar liquids or solids

14-12.2. Generation of Static Electricity. The generation of static electricity cannot be prevented absolutely, but this is of little consequence because the development of electrical charges may not in itself be a potential fire or explosion hazard. For there to be an ignition there must be a discharge or sudden recombination of the separated positive and negative charges in the form of an electric arc in an ignitible atmosphere.

When an electrical charge is present on the surface of a nonconducting body, where it is trapped or prevented from escaping, it is called static electricity. An electric charge on a conducting body that is in contact only with nonconductors is also prevented from escaping and is therefore nonmobile or static. In either case, the body is said to be charged. The charge may be either positive (+) or negative (-).

*A-14-9.4.4 Additional information on arc tracking is found in Campbell, Flashover Failures from Wet-Wire Arcing and Tracking, and in Cahill and Dailey, Aircraft Electrical Wet-Wire Arc Tracking.

*A-14-10.2 For more information, see Beland, Considerations on Arcing as a Fire Cause, and Beland, Electrical Damages Cause or Consequence?

*A-14-10.3 For more information, see Beland, Consideration on Arcing as a Fire Cause, and Beland, Electrical Damages - Cause or Consequence?

*A-14-10.4 For more information, see Ettling, Glowing Connections.

*A-14-10.5 For more information, see Beland, Examination of Electrical Conductors Following Fire.

*A-14-10.6.3 For more information, see Beland et al., Copper-Aluminum Interactions in Fire Environments.

*A-14-10.6.4 For more information, see Ettling, Arc Marks and Gouges in Wires and Heating at Gouges.

*A-14-11.4 For more information, see Ettling, The Overdriven Staple as a Fire Cause, and Ettling, Ignitability of PVC Electrical Insulation by Arcing.

For more information, contact:
The NFPA Library at (617) 984-7445 or e-mail

Taken from NFPA 921Guide for Fire and Explosion Investigations 1998 Edition, copyright © National Fire Protection Association, 1998. This material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.

Used by permission.

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