It can be seen from the setting requirements of residual current type electrical fire monitoring detector in Section 9. The new fire code 2 considers that when the residual current of the power supply line is less than 500 mA, it can be considered that setting detectors in the next level distribution cabinet (box) does not meet this regulation; However, it is difficult to ensure the effectiveness of the detector when it is set at the head end of the low-voltage distribution system above 500 mA.

Because of article four. 2. 2 in GB14287. 2-2005 《 Electrical Fire Monitoring System Part 2: Residual Current Type Electrical Fire Monitoring Detector 》 requires that the alarm value of residual current type electrical fire monitoring detector should be not less than 20 mA and not more than 1 000 mA, and the alarm value of the detector should be 80% ~ 65438.

The alarm threshold of the detector is generally 300 ~ 500 mA (300 mA is the condition that residual current generates arc to ignite absorbent cotton under laboratory conditions, and the ignition point of combustible materials or combustible materials in the engineering site is higher than that of absorbent cotton, so it is reasonable to take 300 ~ 500 mA). The alarm value refers to the alarm value set on the basis of filtering the inherent residual current of the line. If the residual current of the line is greater than 500 mA, it is obviously difficult to ensure that the alarm value of the detector does not exceed 600 Ma. ?

2. The position of the detector:

Based on 500 mA residual current, when the loop is all computer load, the detector is set at the head end of the low-voltage distribution system, and the corresponding maximum calculated current Ic = 500/2. 63 =190 a. The residual current of distribution line trunk line, branch line, branch line and distribution box is not considered in the above calculation. The residual current in this part can be 100 mA, which is roughly estimated as follows: trunk line 0. 15 km(YJV- 185 mm2), with branch trunk line 0. 5 km(YJV-25 mm2) and branch line 65445.

Therefore, when the circuit is full of computer load, the corresponding maximum calculated current IC = (500- 100)/2. 63 = 152a, and the minimum rated current of the selected molded case circuit breaker is 160 A. When the circuit is full of 30w/ lamp (including ballast power) T5 fluorescent lamp load, the corresponding maximum calculated current IC = (500-100)/2.2 =1.

Because the lighting and sockets in civil buildings usually use trunk circuits, it is reasonable to take the rated current of the plastic case circuit breaker in the front section of the lighting socket circuit corresponding to the residual current of 500 mA as 160 A. Therefore, when the rated current of the plastic case circuit breaker selected according to the lighting socket circuit is less than or equal to 160 A, a detector should be set at the head end of the low-voltage distribution system. Only when it is greater than 160 A, it should be considered to be set in the next level distribution cabinet (box).

There are a large number of single-phase low-power electrical equipment (such as computers, televisions, liquid crystal displays, energy-saving lamps, fluorescent lamps, etc.). ) In the low-voltage distribution system of civil buildings. These devices have low power and large residual current, and the load access system is random and scattered, so it is difficult to accurately estimate the residual current of the lighting socket circuit.

For lighting and socket circuits, the rated current of molded case circuit breaker is 160 A, which is the minimum limit. In addition to the office building lighting socket circuit, other lighting socket circuits or other types of buildings can be increased according to the load situation (because the above analysis is conservative, including estimating the residual current and power factor of the line, etc.). ), can improve 1 ~ 2 level or so. However, when the rated current of the plastic case circuit breaker selected according to the lighting socket circuit is greater than or equal to 300 A, it is difficult to ensure that the residual current of the line is not greater than 500 mA, so it is recommended to set it in the next level distribution cabinet (box).

In addition to lighting sockets, buildings also include air-conditioning electricity (multi-connected system, central air-conditioning system) and power electricity (elevator, water pump, non-air conditioning ventilation electricity). The residual current of this high-power load is very small, generally not exceeding 0. 5 mA/A, and the rated current of the circuit breaker at the outlet of the low-voltage cabinet generally does not exceed 630 A, which fully meets the condition that the residual current of the power supply line is less than 500 mA, only when a high-current bus is used.

For special electricity consumption (information and intelligence center, big kitchen, swimming pool, gym, laundry room, etc.). ), we can refer to the power consumption of lighting and sockets. Because of the particularity of this kind of equipment and installation environment, it is best to follow the actual operation.

3. Power supply and distribution mode of detector position setting:

If the estimated residual current of the line is close to or greater than 500 mA, the detector should be set in the next level distribution cabinet (box). When radial power supply is adopted, the detector should be set at the exit of the next level distribution cabinet (box), not at the entrance. When mains power supply is used, the detector can be set on the incoming line or outgoing line of the next level distribution cabinet (box) according to the load situation.

The specific analysis is as follows.

The power supply radius of low-voltage distribution system is generally less than 250 m, and the maximum power supply radius of trunk circuit is less than 200 m, so:

(1) When the cable is used for radial power supply, the maximum value of its inherent leakage current is only 2 × 38 × 0. 2 = 15.2 mA (according to the rated current of the plastic case circuit breaker at the head end of the distribution line, two cables are considered as YJV- 185 mm2), which accounts for the maximum value of the head-end detector.

Therefore, setting a detector at the head end of the low-voltage distribution system, which is almost the same as the incoming line of the next-level distribution cabinet (box), can also monitor the main line insulation, which is more conducive to play its role; The detector should be located at the head end of the low-voltage distribution system or at the outlet of the next-level distribution cabinet (box).

(2) When the bus duct is used for radial power supply, it generally supplies power for super-high power equipment, such as large air-conditioning host. The residual current of this kind of equipment is not large, so it is enough to set the detector at the head end of the low-voltage distribution system.

(3) When the cable trunk line is used for power supply, the maximum rated current of the molded case circuit breaker at the head end of the low-voltage distribution system is not more than 400 A, and the cable section is not more than 240 mm2. The rated current of the incoming switch of the secondary distribution box separated from the trunk line is generally not very large. When the power supply circuit is lighting and socket circuit, and it is greater than 160 A, detectors can be set at the incoming or outgoing line according to the load situation.

(4) When the bus duct is used for mains power supply, the rated current of the incoming switch of the secondary distribution box separated from the mains is generally large, so it is necessary to determine whether the detector is set in the incoming or outgoing line according to the load of the secondary distribution box switch; The lowest limit value of lighting socket circuit can be 160 A, and the level of 1 ~ 2 can be appropriately increased according to the load situation; The power and air conditioning circuit can be directly installed at the incoming line of the power distribution cabinet (box).

References:

Baidu Encyclopedia-Residual Current Electrical Fire Monitoring Detector