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 The three main groups of fire causes are accidental, natural and
incendiary. Of the accidental fire causes, electricity often plays a
major part. Electricity can also be involved in incendiary and
natural causes. Fire investigators are faced with difficulty in
determining the cause of a fire, they have often relied on the
phrase "the fire was electrically caused" as a "catch all" to avoid
the more appropriate phrase "the fire cause was undetermined". This
article discusses the various ways in which electricity can be
involved in the cause of fires and recent developments in
investigation techniques.
Three elements must be present in order for a fire to initiate.
These are oxygen, fuel and heat. Not only do these elements have to
be present together at the same time, but the fuel must be
conditioned in a way that the heat source can initiate an exothermic
oxidation reaction. Electricity can play an important role in this
process by providing the heat source.
Short Circuits
One example of an electrical heat source is a short circuit. There
exists two main types of short circuits; a dead short circuit and a
limited short circuit.
A dead short occurs when a live wire comes in contact with a common
or ground wire (or the positive and negative wires are connected in
a DC circuit) and the circuit is subsequently energized. In properly
fused circuits this will cause the fuse to blow and the circuit to
de-energize. This type of situation does not create sufficient heat
to ignite combustibles. However, it is possible that the circuit is
not fused properly. If this occurs, the current can continue to pass
through the wires causing them to significantly overheat. This type
of situation can ignite surrounding combustibles causing a fire.
Limited Short Circuit
The other type of short circuit is a limited short circuit. In this
case, wires come in contact such that the volume of material through
which the current flows is smaller than the fusible link. This will
create a spark or flash and result in melting of the copper of the
wiring. Characteristic beading of the copper wire is normally
observed. This situation can also cause ignition of combustibles
provided the mass of the combustibles contacting the heat source is
small enough that the heat source can cause it to reach ignition
temperatures and initiate a self-sustaining exothermic oxidation
reaction or fire. It is difficult to ignite concentrated, solid
combustibles such as wood, plastic and even paper with this type of
heat source. However, cotton products, sawdust, wood chips and
combustible gases can be ignited.
Overloaded Circuits
A nother electrical heat source can be created when a circuit is over
fused. Over fusing of a circuit can result in high current flow
through the wires overloading the circuit. Although electrical
wiring is designed to carry current at much
higher than its rated
capacity, increasing current above this rated capacity causes the
wire to generate excess heat.
This is not a problem as long as the
heat can be dissipated from the wire. However, if the wire is
enclosed within a small insulated space, such as the holes through
which wires run through floor joists in a home, the heat in these
areas may not be able to dissipate as quickly as it is being
generated. As a result, the surrounding combustibles can pyrolize
and eventually ignite, causing a fire.
Leakage Current
Fires can also be caused electrically through what is called leakage
current. Leakage current occurs when water is in the presence of
electricity. Exposed wiring, which exists primarily at connectors
and switches, can come in contact with water. Since water conducts
electricity, a current will flow through the water between contacts
or from the live to ground or common. Over time, the water will
accumulate salts which increases its ability to conduct a current.
This current can eventually develop to a point where it generates a
significant quantity of heat which begins to pyrolize and carbonize
the combustibles in the area. This can eventually result in a
situation where a carbon bridge is formed, creating a continuous arc
or significant generation of heat. Ignition of surrounding
combustibles can result in a fire. This mechanism was involved in
the Inglis refrigerator recall discussed in a previous Leading Edge
article. Fires have also been known to initiate this way in
electrical boxes which become damp or wet.
Electrical Contacts
Electrical contacts can also fail resulting in uncontrolled heating.
Each time a contact is opened or closed, a small spark is generated.
This causes degradation of the surface of the contact. Contacts can
fail "open" in which case the circuit simply becomes inoperable.
However, they can also fail "closed" and weld together resulting in
uncontrolled heating. A previous Leading Edge article concerning
Proctor-Silex coffee makers discusses this mechanism.
 Electrical Spark
A simple spark can initiate a fire or devastating explosion if a
combustible gas/air mixture is located at the position of the spark.
As discussed previously, a spark is usually created whenever a
contact is opened or closed. For this reason, specially designed
switches and contacts are required for installation in an
environment in which you can reasonably expect combustible gaseous
mixtures to be present. The requirements for this equipment are
listed in the Building and Fire Codes.
Assessment of Liability
The investigators' job does not end when they determine that the
fire was caused electrically. The mechanism of the cause must also
be determined to assess liability. Essentially, the investigation
shifts to a failure assessment. For example, an overload situation
can be created when a wire is damaged or pulled loose from a
connector. A damaged wire can result in a loss of mass at a specific
point where a hot spot can be formed. This type of damage is
generally related to misuse, however, if the manufacturer did not
provide sufficient strength to withstand normal expected use, which
caused the damage, it can be argued that he design was insufficient
for the product's intended use. A wire can also be pulled partially
free from connector such as that which exists within a plug. This
can also result in a loss of mass through which current is passed
between the wire and the plug connector creating a hot spot. This
situation can be created through misuse, insufficient design and/or
manufacturing defects. History of fire related problems, recalls and
examination of other similar units may enable the investigator to
determine the mechanism of the cause of the failure and thus assess
liability.
A New Technique
The most common sign of an electrical failure is the balling of
copper created as a result of electrical arcing. Investigators have
been confronted with the problem of whether the electrical arcing
evidence observed caused the fire or was caused by the fire. A new
analytical technique called auger Analysis can now assist in
answering some of these questions. Auger Electron Spectroscopy (AES)
profiles the chemical elements found below the first 50 to 100
Angstroms of the surface of arc residue. The analysis assesses the
products of combustion found within a bead of copper created by an
electrical arc. If the arcing occurred in the presence of combustion
products only associated with the insulation and combustibles
located at the immediate location of the failure, the electrical arc
may have caused the fire. However, if the arcing occurred in the
presence of combustion products associated with combustibles located
away from the electrical arc, the failure is probably a result of
the fire. This analysis can be particularly useful in ruling out
possible
electrical causes in the case of subrogation and even arson. Auger
Analysis results have reportedly been accepted in courts in the
states of California, Washington and North Carolina.
When assessing a possible electrical cause, the investigator must
not only determine that the electrical malfunction caused the fire,
but also complete an assessment of the mechanism of the cause.
Research and testing can provide the information required to answer
these questions and permit a full assessment of liability.
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