Due to the relatively small number of fault arc detector products in China, there is some confusion in how to implement this specification. The construction drawing review unit is required to implement according to the specifications. However, due to the designer's lack of understanding of the fault arc protection and the scarcity of products that meet the specifications, this regulation is difficult to implement. With the introduction of relevant product manufacturing standards, the design, production and use conditions of the Arc Fault Protector (AFDD) are now available.
The author tries to start from the cause of fire caused by arc fault, the characteristics of arc fault protection device and the status quo of domestic and foreign technology development, and put forward reference for its application in engineering, for the reference of the majority of designers.
Protection methods and limitations of existing building electrical fires
1 Building electrical fire protection methods
Building electrical fire protection methods are mainly divided into the following two categories.
a. Building fire protection system: including automatic fire alarm system, emergency lighting and evacuation indication system, smoke control system, fire hydrant and automatic fire extinguishing system.
Most of the facilities focus on the escape and fire extinguishing in the early stage of the fire. The electrical fire detection and alarm system mainly uses the residual current electrical fire detector system and the temperature electrical fire detector.
b. Building electrical protection switch: including fuses, circuit breakers , residual current protectors, etc. The residual current protector (including the residual current detector) protects against personal electric shock and fire by detecting the residual current. Fuse and circuit breaker are mainly used to protect the circuit when the circuit is short-circuited or severely overloaded, to prevent the occurrence of fire caused by high temperature caused by over-current.
2 Limitations of existing distribution protection switches for arc fault protection
The residual current protector (RCD) can effectively reduce the risk of fire by detecting leakage currents in electrical devices and grounding arcs caused by electrical tracking currents.
In reality, however, RCDs, fuses, or miniature circuit breakers (MCBs) do not reduce the electrical fire hazard caused by series or parallel arcing between live conductors. When a series arc occurs, the RCD cannot detect such a fault because no ground leakage current is generated, and the fault impedance of the series arc reduces the load current so that the current is lower than the MCB or the trip threshold of the fuse.
When a parallel arc is generated between the phase conductor and the neutral conductor, the current is limited by the impedance of the device, and the most serious case is the sporadic gap arc. The conventional circuit breaker is not designed for this purpose, and it is difficult to protect such a fault. .
The fault current of the arc is usually below 100 A, and the fault current of the continuous stable arc is more than 10 A. For this type of fault current, the conventional overcurrent protector based on the current amplitude is not fully protected.
Taking the conventional terminal outlet branch in modern residential buildings in China as an example, the expected short-circuit current at the socket is generally between 150 and 500 A. The protection switch is usually equipped with a miniature circuit breaker with a rated current of 16 A and a C-curve with RCD. In the case of no ground leakage, when the loop fault current is less than 160 A, it is difficult to ensure that the circuit breaker cuts the fault current instantaneously within 0.1 s, which leaves a hidden danger for the arc fault.
The use of fault arc protection devices abroad is not long. The fault arc circuit breaker (AFCI), which was first used in the United States, is only about 20 years old. After being used in residential buildings, AFCI is considered to be effective for electrical fire protection. In 1999, UL in the United States developed and promulgated the first standard for fault arc protection: UL 1699 (Fault Arc Circuit Breaker Standard).
The National Electrical Code (NEC) 2002 Edition establishes mandatory regulations for the use of fault arc protection (NEC 2002 - 210. 12 A, B):
1 All faulty arc breakers shall be installed in the branch road installed in the bedroom to protect the entire branch;
2 The single-phase power cord plus plug-connected indoor air conditioner needs to be equipped with an arc protection device. NEC 2008 (210.12B) expands the use of fault arc breakers, requiring fault arc fault breakers to be installed in all branch circuits.
In 2003, the National Fire Protection Association (NFPA) concluded the installation of a fault arc circuit breaker: the fault arc circuit breaker can protect against electrical line fires of 75% to 80%.
After August 1, 2004, the United States requires that household air conditioners sold in the market must have a power plug with arc protection. In 2007, the National Electrical Manufacturers Association (NEMA) published a white paper titled “Arm Fault Circuit Breakers to Improve Home Safety,†which considers arc fault circuit breakers to be able to prevent them from occurring at the beginning of a fire, a “precautionary†technology. .
In 2014, UL 474 in the United States required that dehumidifiers sold in North America after 2017 must be equipped with a protector with fault arc protection.
Compared to the US application of fault arc breakers, it was not until 2013 that the European International Electrotechnical Commission introduced the protection standard for fault arcs: IEC 62606 - 2013 "General requirements for arc fault detection devices". In many countries in Asia, research on arc detection and protection has started relatively late, and technical standards for fault arc fire protection are being drafted.
With the introduction of the design standard GB 50116 - 2013 "Fire Automatic Alarm System Design Specification", China's relevant departments have successively formulated and released two national standards related to fault arc detection:
GB / T 31143 - 2014 "General Requirements for Arc Fault Protection Devices (AFDD)" (implemented on April 1, 2015);
GB14287. 4 - 2014 "Electrical Fire Monitoring System Part 4: Fault Arc Detector" (implemented on June 1, 2015).
The technical performance and related testing methods of the above standards are currently applicable to AFDD products with rated voltage not exceeding 240 V, rated current (In) not exceeding 63 A, and terminal installation.
In foreign countries, the United States first proposed arc protection technology. Eaton, Siemens, Texas Instruments, Schneider, and GE also introduced fault arc protection products (AFCI), but these products are only applicable to the North American power grid AC120 60 Hz, and the products High false positive rates have also hampered the development of fault arc products.
South Korea's KESCO has launched AFCI products, but it only produces products for 120 V 60 Hz lines, which is not suitable for China's national conditions.
In China, research on fault arc protection products for the Chinese market has just started. More than a dozen companies are developing products. Although most of them have not yet matured products, some companies have started from the cause of fault arcs. Through the research and test of fault arc detection signal source, low-voltage distribution line system transmission function, fault arc signal attenuation theory, reliability, sampling transformer, etc., the effective criterion of fault arc is found and high reliability is developed. Sexual AFDD products.
Take Tianjin Zhongli's products as an example. The company's FADD Arc Detection Device is developed according to national standards and drawing on international standards (IEC). FADD is divided into two categories: secondary distribution arc monitoring and final distribution arc protection.
The products of the last-level distribution arc protection can be subdivided into two types: single protection type and multiple protection type. Single-protective products feature arc protection and multiple protection products offer multiple protection features. FADD shape is shown in Figure 2.
FADD with secondary distribution arc monitoring function is mainly installed in the power distribution cabinet or distribution box to protect the entire back-end line, meet the requirements of GB 50116 - 2013 "Fire Automatic Alarm System Design Specification", with convenient installation and remaining The current monitoring is more accurate, the system can be expanded, and the management function is strong. An example of installing FADD in a distribution box for fault arc detection in a secondary distribution box is shown in Figure 3.
FADD with the final distribution arc protection function has the characteristics of high prediction accuracy, multiple protection functions, strong versatility, wide application range and full current specifications. An example of the installation of the end loop with FADD in the distribution box is shown in Figure 4.
In order to prevent electrical fire accidents caused by fault arcs, enhance the ability to prevent electrical fires, and conduct in-depth research and popularization of fault arc monitoring and protection devices, it has important practical significance. The author invites domestic and foreign enterprises and scientific research institutions to vigorously To develop high-reliability arc fault protection products, the relevant regulatory and standard-setting units are also urged to add corresponding clauses to standardization standards, especially application and regulatory standards, to promote the application of fault arc protection devices and to ensure the safety of people's lives and property.
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