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
(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
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
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
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
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
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
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
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,
(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
For more information, contact:
The NFPA Library at (617) 984-7445 or e-mail firstname.lastname@example.org
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
Used by permission.