Electrical Fires: How Appliances Cause Fires and How to Reduce the Risk

PAT testing is often associated with preventing electric shock, but a good inspection and testing programme is just as much about identifying defects that may lead to electrical fires.

Electrical fires are often described simply as being caused by a “short circuit” or an “overload”. This can be misleading. In reality, the more important problem is localised heating at a poor connection, arcing from damaged conductors, tracking across contaminated insulation, or heat build-up caused by blocked ventilation or poor maintenance.

Poor connections and thermal cycling

Poor connections are one of the most common causes of electrical appliance fires. A poor connection can occur anywhere current passes from one part to another, including plug terminals, socket contacts, fuse clips, switches, relays, crimped joints, spade connectors, detachable lead connections and internal appliance wiring.

A good electrical connection has low resistance and remains cool under normal load. A poor connection has higher resistance at the contact point. When current passes through that resistance, heat is produced. Because the contact area may be very small, the heating can be extremely concentrated.

Thermal cycling can make this worse. When an appliance is switched on, conductors, terminals and contacts heat up and expand. When it is switched off, they cool and contract. Repeated heating and cooling can create small movements at the contact point, reduce contact pressure, increase oxidation or contamination, and gradually increase contact resistance.

This can become a self-reinforcing fault. A slightly poor connection produces heat. The heat causes further expansion, relaxation or deterioration at the contact. The contact resistance increases, producing more heat. Over time this can lead to discolouration, melting, carbonisation, arcing or ignition of nearby material.

This is one reason why a visual inspection is so important. Discolouration, melting, distortion, cracking, heat marks or a burning smell around a plug, socket, connector or appliance inlet may indicate a poor connection.

Arcing faults

An arc is an electrical discharge through air, or across a damaged insulating surface. Arcing can produce very high local temperatures and may eject hot particles. It can occur where conductors are damaged, where insulation has broken down, where a connection is loose, or where a cable has been repeatedly flexed until conductor strands begin to break.

There are two broad types of arcing fault that are relevant to appliance fires:

Series arcing occurs in one conductor, often because of a loose connection, broken conductor strand or damaged flex.

A series arc can be particularly difficult to detect because the current is limited by the appliance load. The appliance may continue to operate, and the current may not be high enough to blow the fuse. However, the arc can still carbonise insulation, damage nearby plastic parts and ignite combustible material.

Parallel arcing occurs between conductors at different potentials, such as line and neutral, and is often caused by damaged insulation, contamination or carbonised material between the conductors.

A parallel arc is more likely to produce a higher fault current, but it may still not result in immediate disconnection if the arc is intermittent, limited by the fault path, or occurs through carbonised material rather than a solid metallic short circuit.

Tracking across damaged or contaminated insulation

Electrical insulation is intended to prevent current from flowing where it should not. However, insulation can become less effective if it is damp, dirty, overheated, cracked, contaminated or carbonised.

Tracking occurs when current begins to flow across the surface of an insulating material. This may start as a small leakage current across moisture, dust, grease or degraded plastic. Over time, heating and arcing can carbonise the surface, making it more conductive. The more conductive it becomes, the easier it is for further current to flow.

This process can gradually turn an insulating surface into a fire risk. It is particularly relevant to appliances used in damp, dirty or dusty environments, or appliances where contamination can collect around terminals, switches, heating elements, circuit boards or cable entries.

Signs that may suggest tracking or insulation breakdown include black marks, burning smells, crackling noises, visible carbon deposits, repeated tripping, intermittent operation, or evidence that the appliance has been wet or contaminated.

Damaged flexible cables

The flexible cable is one of the most vulnerable parts of an electrical appliance. It may be pulled, twisted, crushed, trapped, kinked, driven over, exposed to heat or damaged where it enters the plug or appliance.

Damage to the cable can create a fire risk in several ways. If conductor strands break, the remaining strands may have to carry the same current through a smaller cross-sectional area, causing local heating. If insulation is damaged, live conductors may arc to each other or to accessible metal parts. If the protective conductor is broken, the appliance may also become dangerous from an electric shock point of view.

Overload heating in plugs, cables and extension leads

Overload heating occurs when a plug, cable, adaptor or extension lead carries more current than it is designed to carry safely. As current increases, the heating effect in the conductors and connections also increases. If that heat cannot dissipate, the cable, plug or socket can become hot enough to soften insulation, damage plastic parts or ignite nearby material.

The fuse in a UK plug is designed to protect the appliance cable from overheating if too much current flows. However, a fuse does not operate the moment its rated current is exceeded. A 13A fuse is designed to carry 13A continuously, and it can carry higher currents for a period of time before operating. For a BS 1362 plug fuse, the non-fusing current is 1.6 times the rated current, so a 13A fuse must be capable of carrying 20.8A for at least 30 minutes without operating.

This means that a cable, plug or extension lead can become hot before the fuse operates, especially if it is overloaded, coiled, covered, damaged or poorly ventilated. The fuse also cannot detect a poor connection, weak socket contact or loose terminal if the overall current remains within the fuse rating.

High-power appliances such as kettles, heaters, microwaves, tumble dryers, washing machines and dishwashers should normally be plugged directly into a suitable wall socket, rather than being run through multi-way adaptors or lightweight extension leads. The total load connected to an extension lead must not exceed its rating.

Cable drum extension leads are a particular risk if they are used while still coiled. Current flowing through the cable produces heat, and a coiled cable cannot dissipate that heat properly. Cable drums should be fully unwound before use, especially when supplying higher-power equipment.

A fuse should never be uprated simply because it keeps blowing. Repeated fuse operation usually indicates a fault or overload that needs to be investigated. We explain this in more detail in our article on BS 1362 fuse operation characteristics.

Blocked ventilation and heat build-up

Not all electrical appliance fires are caused by an electrical fault. Many appliances produce heat during normal operation and rely on airflow, thermal controls or safe positioning to keep temperatures under control.

If ventilation is blocked, filters are not cleaned, cooling fans fail, or an appliance is covered, normal operating heat can build up to unsafe levels. The appliance may then ignite nearby combustible material or overheat its own internal components.

Common examples include chargers covered by bedding or clothing, power supplies used in confined spaces, computers or projectors with blocked vents, heaters placed too close to furniture, tumble dryers with blocked filters, and extension leads or cable reels covered by mats, carpets or stored materials.

Appliances that produce heat, or contain power supplies, should be positioned so that heat can escape. Ventilation slots should be kept clear, filters should be cleaned as instructed, and appliances should not be covered unless the manufacturer specifically says this is safe.

Tumble dryers, washing machines and white goods

Large appliances such as tumble dryers, washing machines, dishwashers, fridges and freezers can cause serious fires. These appliances may contain motors, heaters, compressors, relays, switches, electronic controls, internal wiring and plastic components. Some also operate for long periods or are left running unattended.

Tumble dryers are a particular concern because lint and fibres can build up inside the appliance. Lint is combustible and can be ignited by overheating, a faulty heater, restricted airflow or an internal fault. Dryer filters should be cleaned in accordance with the manufacturer’s instructions, and the appliance should not be used if it smells hot, takes longer than normal to dry, makes unusual noises or shows signs of overheating.

Washing machines and dishwashers have different risks. Fires may involve internal wiring, cable insulation, motors, door locks, control boards, pumps or plastic casing. Any sign of burning, repeated tripping, leakage onto electrical parts, unusual noise or intermittent operation should be investigated.

Fridges and freezers should have adequate ventilation around them. Dust build-up around rear components can reduce cooling and contribute to overheating. Appliances should also be registered with the manufacturer where possible, so that owners can be contacted if a safety recall is issued.

Chargers, power supplies and battery appliances

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Chargers and power supplies are often left connected for long periods. They can become a fire risk if they are counterfeit, poor quality, damaged, overloaded, used with the wrong appliance or covered while operating.

Only use chargers and power supplies that are suitable for the appliance and supplied or recommended by the manufacturer. Cheap counterfeit chargers may have inadequate insulation, poor separation between mains and low-voltage circuits, poor temperature control, undersized components or unsafe plugs.

Battery-powered products also need care, particularly where lithium-ion batteries are involved. Charging should be carried out on a stable surface where heat can escape. Avoid charging phones, tablets, tools, e-bikes, scooters or battery packs on beds, sofas, under pillows or in escape routes. If a battery becomes swollen, unusually hot, damaged, starts leaking or gives off an unusual smell, stop using it and follow the manufacturer’s safety advice.

You can read more about the risks of poor quality chargers in our article on fake and counterfeit chargers.

Unsafe products and recalls

Some fires are caused by product defects rather than wear and tear. A product may have a design fault, manufacturing defect, unsafe plug, inadequate cable, poor internal connections, insufficient thermal protection or components that fail in service.

This is why product recalls matter. A recall or safety notice may be issued because a product presents a risk of fire, electric shock, overheating or burns. Owners should register appliances where possible and check recall information, especially for white goods, heaters, chargers, battery products and second-hand appliances.

We cover this topic further in our article on electrical product recalls.

How user checks and PAT testing help reduce fire risk

PAT testing, more correctly known as in-service inspection and testing of electrical equipment, can help reduce the risk of appliance fires by identifying defects that may lead to overheating, arcing or insulation failure. However, many fire risks can be found before any electrical test is carried out.

The most important part of the process is usually the visual inspection. A visual inspection can identify damaged plugs, incorrect fuses, loose cord grips, damaged cables, heat damage, cracked casings, blocked vents, contamination, unsuitable equipment and poor repairs.

User checks are also important because an appliance can be damaged at any time. Users should be encouraged to report burning smells, hot plugs, scorch marks, damaged cables, crackling noises, tripping, blocked ventilation or unusual operation. A simple check before use can prevent damaged equipment being used until the next formal inspection. More detailed guidance is available in our article on user checks for electrical equipment.

Electrical testing can also help identify fire risks, particularly where insulation has deteriorated or contamination has created a leakage path. An insulation resistance test can reveal breakdown or contamination of insulation that may not be visible externally. Leakage current tests can be useful where an insulation resistance test is not suitable, or where electronic components may affect the test method.

PAT testing should not be treated as a guarantee that an appliance will remain safe until the next test date. It is part of a wider system that includes safe use, user checks, maintenance, suitable equipment selection and prompt action when damage or overheating is found. For a general guide to the inspection and testing process, see our article on how to PAT test electrical equipment.

Summary

Electrical appliance fires usually start because heat is produced where it should not be, or because normal operating heat cannot escape. The most important causes include poor connections, arcing, tracking across damaged insulation, damaged flexible cables, overheating, overloading, faulty components and incorrect use.

It is important not to think only in terms of “short circuits” and “overloads”. A loose connection, weak socket contact or damaged internal connector can produce enough localised heat to ignite nearby material without necessarily blowing a fuse or tripping a circuit breaker.

Many risks can be reduced by simple precautions: buying safe equipment, using it correctly, keeping it clean and ventilated, avoiding overloads, replacing damaged leads, carrying out user checks and arranging inspection and testing where appropriate.

PAT testing is not just about preventing electric shock. A good inspection and testing programme can also help identify defects that may lead to overheating and fire, particularly through visual inspection and suitable insulation or leakage tests. However, it must be supported by user checks, safe working practices and prompt action whenever damage, overheating or unusual operation is found.