ELECTRICAL FIRE CAUSES
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". In fact, it is actually quite difficult to start a fire with electricity. 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.
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.
Another 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.
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 (Vol. 2, No.2 May 1993). Fires have also been known to initiate this way in electrical boxes which become damp or wet.
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 (Vol. 1, No. 13 Third Quarter 1990).
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.