The aspirating smoke detection (ASD) system can give early alarms when detecting the early smoke in the Electrical room. The early smoke can’t be seen yet by human eyes, for it is generated from solids sublimated by heating when an electrical fire occurs. By detecting the electrical fire early and putting it out on time, people can protect their property and personal safety, and reduce the harm from fire.
1.Introduction to Electrical Fire
The main reason for the occurrence of electrical fire is that the faulty parts of the electrical devices are heated for a long time. It causes a further degradation in insulation, which eventually leads to short circuits in the loops, resulting in fires. Or the arc or spark generated at the faulty parts releases heat instantly, causing short circuits in the loop and leading to a fire.
The electrical devices always heat up during operation for the following reasons:
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The current generates heat on the resistance of the conductor;
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The iron core is worn out and generates heat;
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The insulating medium is worn out and generates heat;
Usually, the heat generation and heat dissipation of electrical equipment can balance at a rather low temperature. If the temperature doesn’t exceed the continuous service temperature of the electrical equipment, there will be no dangerous situation occurs. However, when the operation of the electrical equipment is abnormal, it will keep heating up and can’t dissipate heat in time, making the temperature increase sharply and causing a fire.
Gas or liquid insulating materials can have sparks and arcs when heating. After the insulation is damaged, an arc might occur between the cracks, and the voltage of it is 30kV/cm, which will lead to a damage between the two conductors. The arc will generate high temperature, for example, the arc current of 2~20A can generate a high temperature of 2000℃~4000℃. Therefore, an arc that has a current of 0.5A is enough to cause a fire. As for the spark, it can be seen as an unstable, short-duration arc, whose temperature is also high. The second ignition sources generated by sparks and arcs are more dangerous.
Analysis of Electrical Fire
Under Heating
Heat the PVC insulated cable at a heating rate of 20ºC per minute. When at 85ºC, its insulation begins to soften. At 159ºC, its insulation starts to discolor, and a little gas appears. At 240ºC, a small amount of organic gas appears and the insulation begins to crack. At about 288ºC, a large amount of organic gas appears. At 328ºC, the weight of the PVC insulated cable drops quickly, and the released gas contained HCI. At 385ºC, there are HCl, plasticizers, and carbon-containing oxides in the gas. At around 477ºC, the insulation is completely carbonized, causing a fire.
Under Overload Operation
When the temperature is 23°C, run a new 1.0mm² BV wire (rated load 15A, limit load 20A) under a load current of 28.5Ato test its temperature. When the operation is 90% overloaded, after a period of time, the temperature of the wire is stable at 96.1 ℃ and it begins to soften. There is a slightly pungent odor, indicating the occurrence of gas.
Under Short Circuit
At normal temperature, use a large current generator and increase the current of the 1.0mm² BV wire to 5-time the rated current (75A), making a short-circuit simulation experiment. The wire’s temperature rises sharply and much white smoke appears instantly. After a few minutes, the insulating layer is carbonized and peeled off along the entire line, with high-temperature molten droplets dripping down. If change into the 2.5mm² BLV wire and do a short-circuit experiment with a 5-time rated current (140A), you will find that the wire core breaks at the thin point, producing molten droplets close to 300ºC and igniting combustibles. In the actual operation, the short-circuit current is hundreds or thousands of times larger than the rated current, which is more dangerous.
Development of Electrical Fire
The development of fire has four stages: smoldering period, visible smoke, smoke flame and open flame.
Most electrical fires are caused by overheating equipment, shorted circuits or electrical discharges. Once a fire reaches the open flame stage, the fire will spread faster and cause serious damage. If smoke is detected early in a fire, there will be more time to control the dangerous situation.
2.Application of Smoke Detection System in Electrical Room
Application of Point-type Smoke Detector in Electrical Room
The point-type smoke detector is the most common detector used in Electrical rooms. They are usually installed on the ceiling. There are three factors that affect the detection speed of a point-type smoke detector: the path of the smoke from the fire source to the detector, the dilution of the smoke in the process of transmission, and the sensitivity of the detector. Due to the high voltages, the power distribution systems are usually enclosed in cabinets. There is a cooling fan or air conditioning system installed in the cabinet, and it can keep the temperature of the equipment within a safe range. These cabinets can affect the smoke detectors installed in the Electrical rooms in two negative ways:
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The cabinet will affect the flow of smoke, prolonging the time it reaches to the detectors mounted on the ceiling.
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The cooling fan or air conditioning system will dilute the smoke, making it hard to rise to the ceiling, and it will prolong the detecting time of the point-type smoke detectors.
If there is smoke inside the cabinet, it will be hard to get out. Even if there is a little smoke leaking into the room, it will spread, and the point-type smoke detectors installed on the ceiling are usually hard to detect.
The working method of the aspirating smoke detection system is to use the air pump inside the detection panel, connect the sampling pipe, extract the air sample inside the cabinet and send it back to the panel for detection. When the panel detects smoke, it will give a fire alarm to warn people.
Installing the aspirating smoke detection system in cabinets is the most effective way to protect the electrical equipment. It reduces the influence of air pressure and air contamination on the detection system.
Aspirating smoke detection system Installation from the top of the cabinet
The sampling pipes’ installation of the aspirating smoke detection system is quite flexible. They can be installed on the top of the cabinet, and the air inside the cabinet can be extracted by using the pipes and the opening holes above the cabinet. The sampling pipes can also be installed under the cabinet to draw the air samples.
Aspirating smoke detection system Installation from the bottom of the cabinet
Aspirating smoke detection system Installation example
The AW-AS100 aspirating smoke detection system is an extremely early smoke detection device that collects air samples through air sampling pipes and analyzes the concentration of smoke particles in air samples to give alarms.
Its main features are as fellow:
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Sensitivity: The AW-AS100 aspirating smoke detection system is extremely sensitive, it can detect small smoke particles in the early stage of combustion.
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Accurate alarm: The AW-AS100 adopts the principle of laser cavity detection and is extremely accurate in monitoring smoke particles.
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Large detection area: The detection area of each double-pipe detector can reach 1000 square meters.
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Smoke content visualization: After the intelligent terminal connects to the device, it can view the smoke content waveform of the device, so that people can take immediate action to the dangerous situation.
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Multi-level alarm function: The alarm is divided into five stages: alarm, early warning, patrol, fire alarm 1, and fire alarm 2. The parameter values at each stage can be set according to customer needs.
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Alarm query function: It can save 30,000 previous alarms.
