Very often we find that fire caused by an electrical fault in for instance a wall outlet, earlier stopped in the outlet, while it nowadays will spread to the building construction. This may be as a result from using plastic products which burn or melt more easily.
SINTEF NBL issued two reports in 2006 on behalf of Norwegian Directorate for Civil Protection and Emergency Planning about this subject. The reports NBL A06121 Fires due to electric installation material and NBL A06122 Overheating in electric installation materials and equipment as an ignition source in buildings are published in Norwegian. A continuation of these two projects is planed for 2007 and 2008. It will be carried out laboratory tests to learn more about how electrical material and lighting can cause fire in buildings.
LABORATORY EXPERIMENTS AT SINTEF NBL
During 2007 SINTEF NBL hooked up an electrical system in the laboratory, which simulates an electrical installation in a residence. The system consists of wall outlets, connection boxes, terminal clamps, cables, 12 V and 220 V downlight, luminous tube etc. in two 16 A electric circuits. The system is installed onto a realistic wall construction with insulation and combustible materials like for instance wall paper, wood, pollution and so on.
The test is continuous with a maximum amperage is 18 - 20 A . By introducing electric defects in the system, SINTEF NBL may expose how such local faults can cause a building construction to catch fire.
Connections where for instance cables and different electrical equipment are connected are the weakest point of an electrical system. We have seen that considerable overheating may arise from such loose connections. This can result in glowing connection points (1200 - 1300 °C) or arcing fault (2000 - 6000 °C) in the opening between coupling-screw and copper wire. If the cable insulation is damaged a short circuit arc (2000 - 6000 °C) may arise or series arc, without releasing the current protection.
TEST RESULTS
The tests at SINTEF NBL started just before the summer holidays this year. The temperature in well tightened coupling points in electric outlets and terminal clamps soon stabilized at approximately 60 °C and a little more than 110 °C in flush mounted 12 V and 220 V down light in a ceiling in an insulated roof construction. The first mentioned temperature is to low to cause pyrophoric material. However, 110 °C is enough to cause pyrophoric material and may cause a fire after long-lasting thermal stress. These temperatures were relatively stable for months.
The figure show the temperature development during a little more than three days in a single electric outlet (black curve), terminal clamp in a wall (red curve), two dual outlets (yellow and light blue curve) and on the outer surface of a halogen lamp (brown curve)
Loose coupling points did not cause any temperature growth worth mentioning, not during the first 2 - 3 weeks anyway.
By introducing weak vibrations, the temperature quickly increased to 400-450 °C in the coupling point in an electric outlet (see blue curve in the graph). This caused the metal to melt and the coupling point melted together like a well screwed coupling.
This caused the temperature to decrease to a little more than 100 °C (see the blue curve). Thereafter this temperature rose gradually within a couple of days, until a sudden rise which made the current protection to release.
The red curve shows the temperature in a badly screwed "screw clamp connector". The temperature in the coupling point fluctuated between 200 - 350 °C, which is sufficient for wood and paper and similar materials to ignite.
The fact that loosely screwed coupling points and vibrations causes overheating was first pointed out by professor J. Slettbakk et al. at NTNU in Trondheim, Norway, in a scientific article in 1991.
SINTEF NBL wants to study the conditions causing such electric faults and how they induce ignition and fire in dwellings.
