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.

         Ignition by Electrical Energy.

          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.

          Resistance Heating.

          General. Whenever electric current flows through a conductive material, heat will be produced. See 14 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.

          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.