An important topic within the fire safety issue is the combustibility of plastic material used in electrical installations.
SINTEF NBL was asked by DSB (Norwegian Directorate for Civil Protection and Emergency Planning) to investigate whether an electrical defect in a component like for example a wall outlet is able to cause a fire in the component, and also further in the building.
The reliability of one single component in an electrical installation, for instance a socket outlet, is actually rather high. Despite this, the police have proven that an average of 70 residential fires annual are due to some electrical defect or error in installation material during the period from 1994 to 2004. This is explained with the large amount of, for instance, wall outlets in Norwegian homes.
A study of the statistics from DSB show that this number has a rising tendency from 60 fires in 1995 to approximately 80 in 2004.
This survey shows huge uncertainty when it comes to establishing which processes that cause fire in electrical installations, and that we need more research into the matter.
The objective of this project
- The objectiv of the project was to find, on the basis of a literary study, how faults in electical installations may cause fire in housings. It is well known that electrical faults can cause heavy temperaturs, which may constitute an ignition source for eletrical insulating materials and other materials in buildings.
- The project has concentrated the attention on the status regarding important electrical causes of fire in installation material in buildings. We have espesially emphasized on describing how electric conductors/wires, cables, plugs and socket outlets may be the cause of fire in buildings.
An analysis of the statistic from Norwegian Directorate for Civil Protection and Emergency Planning (DSB) show that the number of fires in homes due to electric installation material have increased during the ten year period from 1995 to 2004.
Fires in domestic electric appliances had the opposite development.
Wires and cables causes the majority of the fires in installation material, while fires in wall outlets causes second-most fires. Then follows fires in connection boxes/conductor joints and fuse material. Switch buttons and other installation material have the lowest rate when it comes to causing fire in residences in Norway according to the statistics from the Norwegian Directorate for Civil Protection and Emergency Planning (DSB).
Ignition sources in electrical installations
Eletrical material and equipment which are correctly used and protected with adequate dimensioned fuses or circuit breakers, will normally not represent any fire hazard. Such marerial and equipment may nevertheless constitute an ignition source if inflammable materials are present, or if the electrical material and equipment are incorrectly installed or applied.
In order to constitute an ignition source, the electric material must be supplied with voltage/current which causes that high enough temperature arises locally. Such locally hot spots in electrical installations (wiring) may cause ignition of combustible material. To prevent electrical caused fires, we need more knowledge about how and why such fires occur. Sufficient energy to ignite combustible material close to electrical equipment may according to the literature, arise from one of the following phenomena:
- voltaic arc
- resistance heating
- external heat sources
In some cases this may happen as a result of a combination of all three mechanismes.
Voltaic arc is a luminous high-temperature electric discharge across a gap or a joint. The temperature in an arc can be several thousand degrees, dependent on current, voltage and the type of metal. A voltaic arc can be a serial or a parallel arc (short circuit arc). A serial arc cannot exist without current load and will disappear when the current is turned off. Fire may break out if the arc get in touch with combustible material, due to heat radiation to materials close to the arc, from gases generated by the arc or because of hot pieces of metal.
The causes to resistance heating can be devided into:
- heavy overcharge
- to much insulation
- leakage current and earth fault
- poor binding post (contact failure)
Ignition due to overcharge will usually need an amperage in the region of 3 - 7 times the amperage that the cable is dimensioned for. Since electric circuits normally are shielded with 10 - 20 A fuses, heavy overcharge must be considered to be a fairly rare cause of fire in cables.
Cables which are neither damaged nor exposed to overcharge, can still cause fire. One just need to coil up the cable a sufficient number of times, or use too much insulation around the cable, or both. Laboratory tests have shown that ignition of combustible material may easily occur in such cases. At one such incident it was sufficient to coil up the cable three times and cover the coil in clothing.
Leakage current occur when circumstances causes the current to go where it not was intended to go. Earth fault is an example of leakage current. Such phenomena can arise if the cable is worn or damaged and the conductor is in contact with metal. It is documented that an amperage of just 5 A was sufficient for igniting a PVC-insulated cable which had contact with a galvanized steel roof.
Everything indicate that over-voltage is a relatively rare cause to ignition in a limited part of an electric circuit. The material used in cables are generally able to withstand natural voltage ripples in the power grid. For a fire to break out, a stroke of lightning or an accidental voltage peak in a low-voltage lead caused by an error in the network must occur.
Connection points in electrical installations (wiring) are a weak spots in the system, and fire due to electric errors can break out due to contact failure in such junction point. Weak spots are often where conductors and cables are connected, or where cables and electric outlets, switches or plugs are connected. Such connection spots may be exposed to moisture, dust and dirt, or that the connectors may loosen over time due to vibration or movements in the building. If the connection in a junction point is poor, the increased resistance can result in heating of the connection point. A so-called progressive error may lead to high resistance building up in the connection spot over time. High resistance will result in local heating, which can promote oxidation and saging of metal. The oxide conduct current, but the resistance in the oxide coating is considerable higher than in the metal in the conductor. This may result in high enough temperatures in the connection point to make the connection point glow.
Conductors and cables
The best known factors that result in ignition of cables are:
- manufacturing defects
- extreme amperage
- too much insulation (in combination with overcurrent)
- local heating due to fracture in a stranded conductor
- local heating due to damage to a conductor as a consequence of a tack/nail
- local heating due to contact failure between conductor and junction point
The following types of electric fault can arise in plugs:
- short circuit/voltaic arc flashover in a conductor
- oxidation and corrosion of conductor because of heat generation due to contact failure
- leakage current and voltaic arc flashover between the pins in a plug
Plugs with screw-couplings may be exposed to overheating because of contact failure.
Mechanical damage often occur in plugs as a consequence of pulling and bending a conductor a great number of times. This can lead to breakage of the conducting wires in the plug. This may cause voltaic arc, overheating and fire.
Heat generation due to vibrations and poorly tighted coupling-screwes which result in overheating, is the most frequent cause to fire in outlets.
The torsional moment in a screw clamp connection in an outlet matters a great deal for the temperature growth in the outlet. At low twisting moment one achieved temperatures up to 90 °C in the screw clamp.
When the conector was moved forwards and bachwards one achieved temperatures up to 300 °C. Thus it is great danger for ignition and fire in the outlet.
The fire hazard increases in a poorly screwed clamp connection (low twisting moment) if it is combinded with movements in the clamp connection.
Laboratory tests of interest
Several electric equipments with contact failure, damage and high current load will be simulated in the same electric circuit. The most interesting faults will be picked and installed in an elektric curcuit with fluctuating and high consumption of electric current.
During the tests, which will last for a long time, temperatures will be measured in junction points with contact failure and in conductors with mechanical or thermal damage. Continuous video of junction points with clear temperture growth will be recorded.
The objective doing such tests are the following:
- try to find out and understand how electric faults due to contact fault in a socket outlet, arises and develops over time
- study the nature and physical properties of the fault
- study to what extent an electric fault is able to ignite combustible material in an outlet/building construction
For more information, the reports NBL A06121 and NBL A06122 are available in Norwegian.