ELECTRICAL APPLIANCE FIRES

by Tony Cafe

While most electrical appliance manufacturers need convincing that their products can cause fires, the experience of most fire investigators is that electrical appliances frequently do cause fires. One of the most common reasons for appliance fires is through misuse by the owner, however in my experience another prevalent factor is component fatigue. Design faults are also a common fire cause and this is reflected by the significant number of recall notices issued for television sets, heaters and cooling fans in recent years.

Introduction

The aim of this article is to outline some of the ways that fires can originate from electrical appliances and to describe evidence the investigator should look for at the scene. There have been very few articles published dealing with appliance fires and apart from a recent article by Terry Casey there has been virtually nothing published here in Australia. Maybe some of our highly qualified electrical engineers (who are only too willing to turn up in court claiming to be fire experts) should put pen to paper and share some of their expertise...

Background

Identifying whether an electrical appliance has caused a fire firstly requires a thorough interview with the persons who were last at the premises before the fire. The most important questions that should be asked are: what appliances were supplied with power before the fire; and most importantly were there any recent problems or incidents with these appliances. These questions when asked of the occupants soon after they have learnt of the fire will often yield little information of value because they are still in a state of shock. However once they have settled down and begin to ask themselves what may have caused the fire their minds will begin to sift through the days and weeks before the fire and some useful information could come to light.

When examining the fire scene, the most reliable physical indicators of an electrical appliance fire cause are: the burn patterns indicate the area of origin being near the appliance; the appliance itself suffers severe fire damage; and arc damage is found on the conductors either inside or near the appliance.

Two of the major causes of appliance fires are moisture contacting the electrics or a readily combustible material coming into close contact with a heat emitting part of an appliance. Appliances such as dishwashers, washing machines, hot water heaters and outdoor appliances have a high probability of fire caused by moisture, while heat emitting appliances such as heaters, transformers and clothes dryers have a high probability of fire caused by ignition of a nearby combustible material. Appliances that draw a large amount of current for an extended period of time such as heaters and air conditioners have a high probability of fire being caused by a hot connection or an overloaded circuit. A fire hazard associated with heavy appliances such as freezers or refrigerators is a short circuit at the power cord caused by some recent moving that left the appliance sitting on top of the cord.

The following notes on appliance fires should be viewed as a general guide because the design of appliances is continually being improved to reduce fire risks, so some of the faults discussed below may not be relevant to current models. For example, refrigerators once employed electrical heating elements around door seals to prevent icing up but now utilise reticulated waste heat, which not only reduces fire risk, it is also much more energy efficient.

Air conditioners that are operating continuously in a dusty environment can cause fires because the dust can settle over electrical components such as capacitors and cause tracking faults. The top plastic surface of the capacitor can also break down thermally leading to a tracking fault.

Clothes dryers have a large element running around either the rear or the front of the drying drum and any combustible material that comes into contact with the element can ignite and cause a fire. The element is surrounded by non-combustible materials however combustible materials can reach the element in certain circumstances and cause a fire. If the lint filter is not cleaned on a regular basis then lint can blow back into the interior of the dryer cabinet and settle across the base and eventually reach the element and ignite.

Clothes dryers have a flexible bearing that the drying drum rests upon and the bearing is a composite of PVC and other synthetics or cotton. If this bearing comes out of position it can contact the element and ignite. Because PVC retards fire spread, these flexible bearings can smoulder for very long periods before they ignite the clothes inside the drum and the fire is then discovered. This can lead to a fire originating from a clothes dryer many hours after the dryer has been disconnected from power. For an excellent technical explanation of the causes of dryer fires, see: http://www.pathcom.com/~dryerfyr/Ext.Link.

Dishwashers. As previously mentioned, the major cause of fire from dishwashers is moisture contacting the conductors. The controller for most dishwashers is at the top of the door and moisture can reach this area if the interior of the door leaks. The plastic components that release the detergent or finishing agents on the interior of the door can become brittle over time and then crack and start to leak. When the door is opened to a horizontal position, the water inside the door can reach the controller at the top of the door and cause a fire.

Another fire cause is the wiring loom passing through the base of the door becoming stressed due to the door opening and closing over time and the insulation breaking down, resulting in a short circuit.

Dishwashers can also cause fires if a combustible material contacts the element during the drying cycle. Combustible materials such as plastic cups, paper plates or a towel accidentally left inside the machine can ignite.

Fax machines. The older generation of fax machines generate a lot of heat on standby mainly because they have a thermal print head. If the fax machine is covered with a combustible material and the heat from the print head cannot dissipate a fire could occur.

Hot water heaters. The main cause of fires from hot water heaters is moisture contacting the electrics. Because of corrosion, heaters can leak from several areas such as around the elements or from the water inlets and outlets. In many instances the polyurethane insulation that is blown into the heater jacket during manufacture is found very close to the electrics and provides a nearby readily combustible material that can ignite and then smoulder for hours. The presence of polyurethane in close proximity to the electrics should be of some concern to the authorities particularly when these heaters are installed inside a house.

Heaters are one of the greatest causes of fires, although it should be pointed out that electrical heaters are one of the safest forms of heating. Heaters use large currents and so the fire risks associated with large currents apply.

Electric heaters that are fan forced can cause a fire if some combustible material such as plastic wrap is sucked into the heater. The element can become detached through rough handling and later ignite the plastic casing. If the inlet air is partially restricted and the over-temperature sensor is not affected by the restriction the element can overheat and ignite the plastic casing.

The fire risks associated with radiant heaters are that a combustible material is left too close to the heater or the heater falls over and ignites the floor coverings.

Television sets and computer monitors. There have been several recalls for television sets and computer monitors in recent years mainly because of soldering defects and faults in the transformers. These soldering defects are called dry joints and are caused by the solder failing to run correctly during manufacture, which leaves a high resistance pathway which progressively worsens with use. Dry joints are a major problem for the electronics industry.

Television sets are manufactured using fire rated plastics that do not easily ignite. However because so many different plastics are used in the manufacture of a television set and their fire ratings vary, it is still possible for electrical faults to cause a television set to catch fire.

Electrical chip cookers (deep fryers) are normally found in food outlets however there are smaller models that can be used in the home. The temperature of the cooking oil is normally controlled by at least one thermostat, however if this thermostat fails the oil can then overheat and ignite. The evidence that indicates the oil has overheated and caused the fire is a charred or blackened residue found in the cooking vat.

Failure of the thermostat can be due to a number of reasons, however one of the most common is that the capillary line connecting the temperature probe in the vat to the thermostat has broken, either through corrosion or through over-vigorous cleaning of the vat prior to the fire. Manufacturers could reduce the risk of fire by designing their appliances so that they 'failed safe' in the event of the capillary breaking. This could be achieved by cutting off power to the cooking elements when pressure in the thermostat is lost.

Refrigerators & freezers. As previously mentioned, freezers and refrigerators can cause fire if they have been left sitting on their power leads. For this reason it is important to ask the occupants if they recently moved these appliances. Also previously mentioned was the fire hazard associated with the door seal elements. If water does penetrate the door seal then a short circuit can be created resulting in a hot spot that can ignite the plastic door seal. Evidence of hot spots in the element can sometimes be detected by the occupant before the fire because the unusual odours produced by the pyrolysing door seal inside the refrigerator led them to believe that some food had been going off.

One of the major causes of moisture problems with refrigerators is that the drain tube from the freezer becomes blocked. If the tube is blocked and the freezer goes into the defrosting cycle, the water can then run out of the front of the refrigerator and reach the electrics underneath and cause a fire.

Modern refrigerators use waste heat to evaporate the defrost water on a tray mounted above the motor. Older refrigerators sometimes have a plastic tray embedded with elements to evaporate the defrost water. If the drain valve becomes blocked this tray can over heat and over a long period of time can start to crack and expose the elements. When the blockage clears, water fills the tray and a short circuit and fire can occur because of the exposed elements.

Water jugs and urns. Failure of the safety cut-out device on water jugs and urns is the most common reason these appliances cause fires. Another problem is, as the electrics are normally located underneath the water reservoir any leaks will allow water to reach them, possibly causing a fire.

Power strips and extension leads. The larger the current flowing through a power strip or extension lead, the greater the risk of fire because of poor connections or due to thermal breakdown of the insulation. Extension leads exposed to moisture at a point well away from the power socket can cause a fire back at the socket. If extension leads are exposed to an outdoor environment, rain water can run back along the lead and reach the power socket and cause a fire.

Lights emit heat that can ignite some nearby combustible material and lead to a fire. High wattage incandescent lights generate significant heat that can damage the socket and associated wiring causing a short-circuit and possibly a fire. The ballast in fluorescent lights can overheat due to internal short-circuiting and ignite combustible ceiling materials. Quartz halogen lights have a filament that is very hot and if the globe is not installed horizontally or there is finger grease present, the globe can break and the hot filament may ignite combustible materials below. Down lights also generate significant heat and if the top of the down light is covered by insulation the insulation could ignite. Down lights installed in bathrooms are susceptible to moisture accumulating at the terminals which can lead to short-circuits.

Microwave ovens. A recent fire hazard associated with wheat bags heated in microwave ovens was reported in Firepoint magazine. Microwave ovens emit radiation that will excite water molecules and if there is no moisture available, other less volatile molecules will absorb the radiation and start to heat. Therefore any material which has dried out can overheat and ignite in a microwave oven. Also, any metal objects inside a microwave oven can cause severe sparking which could lead to a fire.

Cooling fans. There have been several recalls of cooling fans in recent years, one of which involved a hazard associated with a capacitor that overheated and caused the plastic casing near the capacitor to ignite. Problems with fan motors are common, however fires originating from motors are rare because the motors are well ventilated and the heat generated from a fault is readily dissipated.

Sink disposal units do not normally present a fire hazard because many are equipped with overload devices that cut the power if the motor becomes seized. However, the power for most "sinkerators" is supplied from a power outlet installed under the sink and these sockets are susceptible to moisture, particularly if there is a leak at the back of the sink.

Stoves and ovens are normally made of non-combustible materials that significantly reduce the fire hazard. However, if a combustible material such as a towel or some cooking oil is left on the stove or in the oven and there is a fault with a controller, fire can occur.

Electric blankets continually emit heat and do not have any over-temperature sensors. If the heat cannot escape because the bed is too well insulated, the bedding material can ignite. Another fire hazard associated with electric blankets is when the bed is made the leg of the bed could be placed onto the power lead or the controller, causing a short circuit and eventually a fire.

Waterbed heaters rely on good thermal conductivity between the heater and the waterbed to prevent them from overheating. If an insulating material such as a sheet is placed between the heater and the bed, the sheet can then ignite.

(This article is a new paper currently awaiting print publication. If anyone would like to make a suggestion, add to the paper or referee, please contact the author)