SAMAC Articles

Electrical Fire Causation

by Dave Hines P.Eng.(EE)1

Within the insurance industry, the investigation of property and vehicle fires is a major forensic investigation activity. Fire investigators, with fire department background or an engineering background with relevant experience, are normally retained first. The next most frequent expert required is a forensic electrical engineer.

With few exceptions, every fire scene contains electrical evidence which must be examined and either confirmed or ruled out as the cause of fire. In some cases, this may be within the scope of expertise of the fire investigator. However, in other cases, the expertise of the forensic electrical engineer will be required.

This article discusses the types of electrical problems which may lead to a fire or component failure. Future articles are planned that will discuss the techniques used to investigate the cause of electrical fires and illustrative case studies. Overall, fire causes fall into three main categories:

  1. Accidental: unattended flame/candles, poor storage of chemicals, electrical loads and wiring, appliances.
  2. Natural: Lightning strikes, high winds which down power lines
  3. Incendiary: Arson using accelerants, electrical tampering, or other intentional means.

Electrical fires are typically considered accidental, although natural and incendiary causes do occur. Determining whether an electrical condition caused or contributed to a fire can be a difficult task, and requires expert investigation techniques. Even when the evidence at a fire scene has been wholly or partially destroyed, there will normally still be some physical data that can point to the likelihood electrical factors contributing to the incident.

Modes Of Ignition

The three main modes of ignition leading to electrical fires are arcing, excessive ohmic heating, and external heating. These ignition modes are discussed below and apply to all electrical components, including large transformers, residential wiring, electronic circuit boards, and batteries.

  1. Arcing: An arc (also known as an electrical discharge or spark) occurs when electrical current passes through a normally non-conductive medium such as an air gap. The electricity is able to bridge the gap by electrically breaking down the gases in the air gap, producing an ongoing plasma discharge. This plasma discharge reaches temperatures of 3000°C to 10,000°C, and therefore can very easily cause ignition of materials, such as wire insulation, near the arc. Arcs can occur in two ways: series arcs which decrease the flow of current in the circuit (Figure 1) and parallel arcs, which increase the flow of current in the circuit (Figure 2).

    Figure 1 Series arc

    Figure 1 Series arc

    Figure 2 Parallel arc

    Figure 2 Parallel arc

    There are many causes of arcing, but the three most common are; arcing-through-char, ionization of air, and short circuits.

    1. Char: Arcing-through-char refers to arcs crossing insulation due to carbonization of the insulation. Carbon is a conductive material and can build-up on insulation in many ways. Once a carbonized conductive path is established, conditions for sustained arcing and ignition exist.
    2. Ionization: The ionization of air is affected by pre-existing arcs and flames. If there have been large arcing events in a system, significant amounts of ionized gases have been emitted. These ionized gases have the potential to affect other circuits and aid new arcing events. Flames also contribute to the ionization of air and additional arcing. The creation of this additional arcing creates a self-perpetuating scenario for arcs to continue during a fire, as long as the electrical wiring is energized. This makes the science of arc mapping2 a very important tool in determining whether arcing caused the fire or whether the arcs were a result of the fire.
    3. Shorts: Shorts may occur due to improper wiring, wiring abrading against vehicle components, nails puncturing a wire, external heating, etc. Short circuits tend to cause a lower resistance path from the origin of the short, back to the source. This lower resistance path causes increased currents and arcing. Two types of short circuits can be created:
      1. An arcing short occurs when there is momentary contact between two conductors. The wires are then pushed apart by resulting forces of the initial contact. If the conductors remain close together, arcing across the air gap can occur at a level that will not trip overcurrent protection. This may cause ignition of materials, or ionization of the air to create the possibility of other arcing in the system.
      2. A bolted short happens when there is a sustained metal to metal connection. The heating due to a bolted short is not localized. All the current is carried through the entire length of the circuit from the short back to the source. This can cause ohmic heating at any point along the circuit path. However, the increased current often trips the overcurrent protection at the panel, which de-energizes the circuit and stops the current flow.

    Evidence of arcing at a fire scene can be separated into three main categories; notching, beading and high energy splatter. To differentiate between arcing and fire damage, it is often necessary to use tools such as stereomicroscopes, x-rays and metallurgical analysis. Through the use of expert analysis, often a clearer picture of what has taken place in the fire can be obtained.

    Figure 3 Electrical arcing

    Figure 3 Electrical arcing

    Figure 4 Arcing evidence: notching

    Figure 4 Arcing evidence: notching

    Figure 5 Arc severing & subsequent beading

    Figure 5 Arc severing & subsequent beading

    Figure 6 High energy splatter due to arcing

    Figure 6 High energy splatter due to arcing

  2. Ohmic Heating3: Excessive ohmic heating becomes dangerous in a system when the thermal load begins to exceed the thermal capacity of the conductor insulation, or materials around it.

    The five major causes or contributing factors to ohmic heating are:

    1. Poor Connections: These create progressively deteriorating conditions. Proper electrical connections are mechanically tight and of low resistance. If a connection is loose, the result is a higher resistance connection due to smaller points of contact and air gaps. This higher resistance connection generates heat as current passes through it. This causes oxidation and creep in the metal, further loosening the connection and increasing the resistance of the connection even higher. As resistance increases, more heat is given off and the connection may become glowing hot (Figure 7). At that point, any nearby combustibles are in danger of igniting. Poor connections are dangerous because they do not raise the current to a level that overcurrent protection can respond.

      Figure 7 Glowing connection (arrow)

      Figure 7 Glowing connection (arrow)

    2. Excessive Thermal Insulation: The current carrying capacity of conductors is dependent on adequate cooling space for the wires. Overheating occurs when the heat in the conductors is generated faster than can be dissipated. Common causes of this are; running electrical cords under carpets, energizing an electrical cord while it is coiled onto itself, and excessive insulation in wiring pathways.
    3. Leakage Current: This happens when current is able to flow through unintended paths. Ground faults are the most common type of leakage current. The intended ground path in a circuit provides a very low resistance path to ground. As well, if a current carrying circuit contacts the intended ground path the current will spike, due to the low resistance, and trip the overcurrent protection device. However, when abraded conductors make contact with items such as metal siding, or roofing gutters, there may not be a clean, low resistance path to ground, and the current may be able to leak along that path. This can cause problems through ohmic heating when the combination of leakage and load current do not exceed the overcurrent protection level. This current flow will cause heat increase and, depending on the current and resistance level, ignition may occur.
    4. Excessive Overcurrent: A dangerous current overload can occur when a circuit is overloaded to a level where the current exceeds the conductor rating and leads to ignition. This problem is easily shown through testing scenarios in labs, but it is rare in structures built to normal safety codes. Lab tests have shown that 3 to 7 times the rated current load is required in a conductor to cause ignition. Since conductors are installed with overcurrent protection for the circuit (e.g. a 15A breaker), both a faulty protection device and an extreme current overload are required to cause ignition.
    5. Overvoltage: This is a problem for ohmic heating because, with a higher voltage through a load, higher currents result. If the level of overvoltage is high enough, ignition can occur. Three main ways of applying overvoltage to a circuit are; lightning strikes, accidental application of high voltage to low voltage wiring, and floating neutral.
      Floating neutral problems are illustrated in Figure 3. In this schematic, Rx expects to see 120 V presented to it. However, if a break in the neutral occurs, it will be presented with a voltage that can range from slightly above 0, up to almost 240 V. The exact voltage level is determined by the other loads on the system, R1 and R2. Any combination of resistances which supply overvoltage to a load presents a potential for ignition.
  3. External Heating: External heating can cause breakdown of the insulation on conductors, and can cause arcing between wires in circuits. The majority of external heating problems are caused by a pre-existing fire and thus the conductors are typically “victims” of a fire rather than a cause. However, there are some situations where external heating can contribute to the initial cause of the fire. Vehicles, and other machines using internal combustion engines, have exhaust paths for the waste products of the combustion. Wires can sometimes be routed too closely to the exhaust piping and the heat from the piping will compromise the properties of the insulation and expose the conductors. This leaves the conductors vulnerable to shorting, arcing and leakage currents. External heating can also be a factor near heating elements such as baseboard heating and lighting above stovetops.

Pratical Issues

It is common for investigators to find several causes of fire ignition. Many fires are caused by conductors initially overheating, and then arcing and igniting nearby combustibles. It has been proven that, once a conductor’s temperature has been raised above 200°C to 300°C, significant charring occurs in the conductor insulation. This makes the insulation able to ignite more quickly and at lower temperatures.

The following is the most common sequence of events during an electrical fire:

  1. Poor connection and overcurrent
  2. Thermal degradation of insulation
  3. Release of ionizing gases
  4. Formation of carbon internally and externally on insulation
  5. Arcing
  6. Ignition

There are a number of factors which can play a role in any fire. Expert analysis will often be required to obtain accurate and supportable results during an electrical fire investigation. Fires often involve very large levels of financial loss, and in some cases, fatalities. Careful application of scientific principles and process, combined with a clear understanding of how electricity and fire behave, is the best way to minimize the risk of misdiagnosis.

(1) Dave Hines is an electrical expert with SAMAC Engineering (www.samaceng.com) in Calgary. He specializes in failure analysis and product liability investigations of electrical equipment and components; electrical fire investigations; vehicle fires; residential, commercial & industrial equipment failures; and code reviews.
(2) Arc Mapping: the process of finding the location of energised electrical events or arcs within a defined area to locate or “map” an area of origin. Used logically and interpreted appropriately, it is a powerful analytical tool. Results are strong and often definitive and can be used when investigating any type of fire.
(3) Ohmic heating refers to the generation of heat as a current passes through a medium with resistance. “Ohm” is the unit measurement for resistance.