【 Weak current College 】 electric fire causes and solutions-Power By 【 China power house network 】

1 preface
In recent years, the number of the national fire accidents on the rise. From the 1980s onwards, the electric fire is gradually becoming the protagonist of "fire." The data show that by 1999, the national electrical fire occurred more than doubled in 10 years ago, up to 1 million, representing various types of fire of 30 percent. By 2002, account for all types of fire for more than 35 per cent. In rural fire cause, a second electrical column; why the city fire, electrical, was ranked the first place. Frequent cause of electrical fires?

2 the main cause of electrical fires

Short circuit arc of grounding 2.1 — the most dangerous and multiple electrical fire hazard

Electrical short circuit can be divided into white shorts and short circuit single-phase grounded. Interphase circuit generally able to produce larger short circuit current, the short-circuit current so overcurrent protection devices, power off time, fewer short circuit arc resistance occurs. Single-phase short circuit can be divided into metallic shorts and short circuit arc resistance. Metal fire danger of short circuit. Mainly due to the short circuit current, overcurrent protection device in the short circuit current of short time cut off the power. But the arc of the short circuit as the point of failure, poor contact is not melting and spurt arc or spark. Because of the short circuit arc-fault point impedance is large, it's not short circuit current, circuit breaker, hard-to-action (fuse General not fuse), so that the arc persist. According to the survey, only slightly larger than the current generation of 0.5A arc temperature can be up to 2000 ° c--3000 ° c is sufficient to ignite any fuel, and the maintenance of low-voltage arc-20V to enable continuous and stable arc, it is difficult to put out there. This short circuit arc often become the electric fire sources of ignition. Therefore, short circuit arc of Earth is the most dangerous and multiple cause of electrical fires.

2.2 electrical lines of harmonic currents--of emerging electrical fire hazard

As a result of electrical technology, non-linear loads increasing the electrical equipment, such as gas discharge lamps, television sets, computers, microwave ovens, frequency control devices, etc. This class of device load current contains more harmonic current, the harmonic current into the public power grid can cause power voltage distortion, waveform distortion and loss, and may make the electrical lines (especially the neutral conductor N) overload fever, accelerated aging and fire hazards exist. Neutral heating overload because the 3-phase balance the load of harmonic generation (9, 15-harmonic, etc) in each phase of the component is in phase with each other, in neutral line not cancel each other out but mutually superimposed, (the other being ordered, negative sequence harmonic components in neutral line can offset each other,) superposition of neutral currents may exceed the line current, even reached nearly 2 times the neutral current, resulting in a neutral wire overheated and buried electrical fire hazard. If the three-phase load imbalance, neutral then superimposed on the current imbalance would be more serious after heating. Some developed countries to fully zoomed in neutral wire cross section, and even get to the line section. But in some places, neutral wire cross section as customary practices, only the line section 1/2 or even 1/3. If the three-phase load imbalances fairly serious and there is a greater harmonic currents, then unbalanced currents and harmonic currents, may make the neutral damage even blow, causing damage to the insulation of electric equipment, easy-to-phase devices burned, or even fire. Therefore, the electric circuit design installation must adapt to the new electrical technology development requirements, otherwise the old suffering from an addition adds new bad, cause my electric fire trends will not decrease, but increases.

The old building of aging 2.3 electrical line — — very important electrical fire hazard

Old buildings in large numbers to use aluminum conductor wires, cables, electrical circuit design too frugal, line capacity is low, the aging serious line of fire, electrical fire is a very important reason. Since 1999 in China national mandatory standards the residential design code clearly requires the use of copper wires within the House.

2.410kV network small resistance earthing system — and a new electrical fire hazard

In power grids, 10KV power grid lines in a large number of used high-voltage cable power supply, and lines are long, the line of the capacitor current increases, therefore, urban 10KV power supply network increasingly past neutral system (or compensated) into the small resistors (high current) grounding system, although this has many advantages, but also there is a risk. The original is not grounded system ground fault current to normal phase capacitive current, ground fault voltage only about 100 Volt. To add a small resistance earthing system, select the General earthing small resistance, the current flows through the next location to hundreds of Ontario, ground fault transient overvoltage up to hundreds of volts and even 1-2KV. If 10/0.4KV substation 10KV ground fault occur within, and the transformer voltage side of neutral and high voltage transformer shell, 10KV power distribution cabinet used a grounding, the fault voltage along the PEN line or PE line to adopt protection zero users, low-voltage electrical equipment shell may produce contact voltage endanger personal security, low-voltage equipment, insulation, especially old equipment insulation will bear such a high voltage, short circuit could easily be breakdown resulting fire hazard, it is also the technical development of electrical and adverse effects.

2.5 design considerations not week and construction quality difference — is also a significant fire hazard

In recent years, due to the extensive use of air conditioning in summer, some of the buildings of the electrical circuit cross-section design capacity is small, low overwhelmed, frequent trip, even more serious is the electrical circuit overload, resulting in a long time, become an insulating difficult fire hazard. In addition to the design line cross-section is small, our country has no wires, cables, current national standards such as IEC standard 2.5mm2 copper conductor plastic line ampacity to 26A, but in China, some of the information by 30 ~ 32A, IEC standards higher than 20%. While the design is not taken into account a number ofWire wear dark laying pipe heating caused by a reduction of current carrying capacity, these factors make the selected line cross-section more is small, but also to leave for future use. In the construction process, electrical wiring not specifications, construction technology, wire connections are not real, poor contact, insulated shaving etc is also an electric fire occurred. Especially the neutral wire connection quality is poor, such as creating neutral line breaks, damaged equipment insulation, single-phase devices burned, or even fire. Additionally, for high-power lamps should be well insulated fireproof treatment.

3 electric fire main countermeasures

3.1 actively promoted with leakage protection function of the circuit breaker

To prevent short circuit arc resistance of earthing should vigorously promote the use of belt leakage protection of circuit breakers, general building, apart from the end of the installation of 30mA circuit leakage current protective device (RCD), into the line should be installed with leakage protection function of three-phase four-wire circuit breakers, earth leakage current optional 300mA or 500mA, belt 0.15-0.3 seconds delay. With leakage protection function of the circuit breaker for the delay feature and 30mA RCD at the second level, implementation of selective protection and 500mA following ARC energy not yet sufficient to ignite the fire, so if the building in China's power line installation with leakage protection of circuit breaker, we can effectively eliminate ground arcing hidden, thus substantially reducing our single-phase short circuit caused by the fire.

3.2 prevent higher harmonic fire hazards caused by

Current harmonic to residential customers in more than a small number of public buildings, now only take increasing line section, especially increasing the neutral section to reduce circuit impedance, which reduce the harmonic current in the circuit impedance on the harmonic voltage, reduced lines of harmonic content. For public buildings, preventing harm, in addition to harmonic reducing line impedance, can also be fitted with source (or passive) harmonic filters, harmonic cancellation to filter out or offset the harmonic components.

Renovation of the old line, 3.3 eliminate electrical fire hazard

For old buildings their aging line should use copper wires (cable) to update, eliminate fire hazards. Aluminum wiring and copper terminal connection terminals should be lined aluminium Tin, copper and aluminum wire connection should be used copper and aluminium transition joints. The new building project should be strictly in accordance with the domestic design code GB50096-1999 requirements for design, "electrical lines should be in compliance with security and fire safety requirements of laying wires, conductors should be copper, each residential gas line section should not be less than 10mm2, branch circuit section should not be less than 2.5mm2".

3.4 via low resistance grounding of 10KV power grid, substation should equipotential connection or set apart for the grounding network

To prevent the small resistance grounding of 10KV power supply network grounding failure is too high for transient overvoltage conduction to low user devices, you can set the substation power system protection earthing and grounding of the low voltage system separation settings, and they should have a certain distance, making the transient overvoltage cannot be performed by the original common grounding grid transmission to low user. Alternatively, you can significantly reduce the grounding resistance value of substation work, and adjust the 10KV via low resistance grounding system of small resistor to ground of power supply control 10KV short-circuit current in a certain range, so that the transient overvoltage not reached dangerous values. If substation located in high-rise building, the transformer protective grounding and the low voltage side of the Earth through the equipotential connection and share a grounding, the buildings of steel, metal pipes, cables, metal jacketing and so do the equipotential connection occurs when substation 10KV ground fault, the buildings all metal parts together in the same potentials, not likely to cause an electric shock accident. Therefore equipotential connection of high-rise building with low-voltage system grounding the transformers, high-voltage Cabinet protection earthing common grounding device.

3.5 appropriate increase design standards to ensure the quality of construction

With the economic development of the future will have more electrical equipment into the home and office buildings, electrical design should be based on the development of vision select wire cross section, it is necessary to meet the future needs. It also should take full account of the relatively large flow of conductors, strictly control line length does not exceed the allowable range, eliminate risk. In construction should be strictly according to norms, do line connection, high-power lamps of fire, protect line insulation, preventing neutral line breaking, eliminating unnecessary risk. In addition, China's national standards for building fire safety design specification, electrical construction and acceptance, and IEC standards have a gap, some electrical fire hazard prevention in these specifications without specific provisions should therefore be to improve national standards and norms of electrical safety level, and to develop national standards for conductors flow to prevent electrical fire measures implemented.