Through a large area alarm accident of fire alarm controller circuit in a building, the causes of fire alarm are analyzed through multi-party investigation.
It is determined that the failure of fire alarm system controller is caused by short circuit of fire alarm circuit to ground, and the treatment is carried out. A fire alarm system with 8 circuits and 2015 points is installed in a famous farmer’s market which integrates agriculture, trade, department stores and KTV entertainment. It includes alarm part and linkage part. It undertakes the important task of monitoring the electric fire in the farmer’s market. The system was built in July 2010, because of strict management, operation and maintenance has been in accordance with the norms, 24 hours a day with professional staff, stable operation of the system, there has never been a large area of failure.
However, one day this summer, a large area of system failure suddenly occurred, resulting in the paralysis of the whole system. I am fortunate to participate in the process of fault analysis and maintenance. The indoor vertical control cabinet of the fire alarm device consists of a main alarm unit, a fire linkage cabinet, a multi-wire linkage cabinet, a power supply cabinet, a fire emergency broadcasting cabinet and a fire telephone cabinet. There are three shifts of 2 people on duty 24 hours a day. At 10:30 a.m. on August 17, 2001, the fire control room automatic alarm system of the large-scale agricultural market suddenly appeared a large-area fault alarm. Four of the eight circuits had a large-area fault, and the number of fault alarm points was at least 800 (Note: the upper limit of the total number of circuit fault alarms stored in the main fire alarm machine was 200, and the total number of four circuits must exceed 800).
The system was paralyzed and needed to be established. That is to say, disposal. Decide to check whether the main engine is working properly, find that the fire alarm main engine voltage is normal, and preliminary troubleshooting of the main engine. Due to the primary elimination of the main engine fault, the fault point is very likely to appear in the various loops. Measuring the working voltage of each circuit with a multimeter, it is found that the voltage value is 20V DC, and the voltage shows normal. Then inquiring the load of the alarm host, we can get the original number of detection linkage points and the actual effective number of detection linkage points of each circuit, as shown in Table 1. In Table 1, the value of circuit 1 is 215 original detection linkage points, but only 5 detection linkage points are effective, that is, 210 failure reports; in the same circuit 2, 5, 6, 7 and 8, there are a large number of detection linkage points and alarm mainframe failure reports. The 3,4 loop is normal. At the same time, many times operation query alarm host load, found that the effective value is not fixed, such as 1 loop sometimes effective value is 5, sometimes 8, 30, and so on. Preliminary analysis of the cause of the fault is caused by the line fault. According to the above analysis, turn off the main power supply, remove the loop bus from the main machine, and preliminarily measure the ground insulation resistance of the loop bus with a multimeter. The measured value is shown in Table 2. The data show that the ground resistance values of all 8 circuits are too small to meet the requirements of the code GB50166-2007 for construction and acceptance of automatic fire alarm system (Note: Requirements are greater than 20M). In addition, because the line has been equipped with detectors and other equipment, measuring the insulation resistance of the circuit to the ground with Megohmeter can easily damage the equipment because the output voltage of Megohmeter can reach 500V. Therefore, measuring the insulation resistance of the alarm circuit to the ground with multimeter can simply and quickly judge the quality of the line in practical application. If the alarm circuit of the above meter is measured by megaohmmeter, the insulation resistance of the alarm circuit to the ground is 0, but it should be noted that in order to avoid damage to the equipment easily by 500V voltage, the equipment must be removed to measure. Although the above method of measuring voltage preliminarily eliminates the main engine fault, it still needs rigorous experimental confirmation. In the control room, several alarm devices are connected into a series of wires separately. The address codes of the devices are set according to the address codes of the fault points displayed in the circuit, and are directly connected to the alarm host. The experimental results show that the address code of the original fault point works normally, and the insulation resistance of the loop bus is measured. The value is infinite, which indicates that the host works normally, thermostatic element and finally eliminates the possibility of failure of the alarm host. In summary, it is very likely that the reason of the fault is the short circuit of insulating resistance of wires in multiple alarm loops, which results in large area fault reporting of the main engine. 3,4 circuit is normal at present, but the working state is unstable, so it is decided to re-examine all alarm circuit buses. There is water in the pipeline. The solution is to lay new pipes and replace the wires in this part. The base of the field detector is wet and dusty. The solution is to clean the detector and dry it with a dry cloth. The alarm linkage device touches the ground through the feedback device. The solution is to isolate the linkage and feedback devices by using intermediate relays. After 20 days of checking and sorting out the lines, the automatic alarm controller returns to normal. Strict inspection and control of equipment and materials entering the field. According to the specifications, unqualified materials are strictly forbidden to enter the construction site, and the material and wall thickness of wire pipes should meet the requirements of the specifications; the insulation layer thickness and insulation performance of alarm circuit line (red and blue twisted pair) are checked and tested before threading. The standard thickness of RVS2 *1.5mm2 twisted pair insulation layer is 0.8 mm, and the insulation performance should not be less than 500 M/100m, and it is forbidden to use if it does not meet the requirements. The burrs in the broken pipe of the steel pipe should be milled with a file, the metal pipe should be put into the box, the lock mother should be sleeved on the outside of the box, and the guard should be installed on the inside. The water and dust in the buried pipe must be blown and cleaned by air compressor before threading. Sealing treatment should be done at the orifice of the pipe and the joint of the pipe in dusty or wet places. When threading, the application force is uniform, and no hard pull is allowed to prevent damage to the insulating layer of the wire.
When installing the base, the dust around the base and the base of the detector should be cleaned. Connections of conductors should be welded in junction boxes or connected with terminals. Waterproof adhesive tape and black adhesive tape should be used for two layers of insulation at welding junction, and three layers of insulation should be used in wet places. The methods are waterproof adhesive tape, black adhesive tape and waterproof adhesive tape. Article 3.2.14 of the Code for Construction and Acceptance of Automatic Fire Alarm System stipulates that after laying wires for automatic fire alarm system, 500V megohm meter should be used to measure the insulation resistance of the circuit wires to the ground, and the insulation resistance should not be less than 20M, so it is necessary to test the grounding insulation resistance of each circuit carefully according to the code. In addition, the grounding resistance test shall be carried out after each layer, each circuit is threaded and the equipment is installed, and any problem found shall be solved immediately. N is the total number of branches of the loop. N is the total number of layers in the loop. In order to eliminate hidden dangers and ensure that the insulation resistance of the circuit bus measured at the alarm host can meet the requirement of Specification no less than 20M. For the water flow indicator, safety signal valve, pressure switch of automatic sprinkler fire extinguishing system, smoke exhaust outlet and air supply outlet of mechanical smoke prevention and smoke exhaust system, various fire valves of mechanical smoke prevention and smoke exhaust system, feedback signal of curtain door, etc., and fire alarm linkage module, it is necessary to check whether the feedback device is grounded before wiring. Only when it is confirmed that it is not grounded can it be connected with the linkage module.
Since there are about 2 k_-8 K resistances between the alarm bus of general alarm equipment and the 24V power supply wire, while the 24V power supply ground wire and the 220V power supply are common ground, when measuring the insulation resistance of the alarm bus to the ground, the 24V power supply wire should be removed from the alarm controller and then measured. The ground fault analysis and removal process is complex and time-consuming, and the lessons are profound. It warns us that we need to further strengthen the supervision and management of the construction team and strictly follow the standard construction, so as to make the grounding resistance of the alarm circuit line not less than 20M, meet the requirements of the construction and acceptance code GB50166-2007 for automatic fire alarm system, maintain the good operation of the automatic fire alarm system, and lay a solid foundation for ensuring the safety of people’s property.