Design of a composite electrical fire monitoring detector

my country is a country with frequent electrical fires. Since the end of the 20th century, the number of electrical fires has always ranked first among all types of fires, close to 30% of the total number of fires, and the losses account for as much as 50% of the total fire losses, and have remained high. In foreign countries such as the United Kingdom, the number of electrical fires each year is only 17% of the total number of fires, the United States is only 10%, and Japan is only 13%, indicating that the incidence of electrical fires in my country is relatively high, and there is a big gap compared with developed countries. It can be said that the prevention and effective control of electrical fires has reached a point of urgency. In order to significantly reduce the number of electrical fires that affect the social fire safety in my country, a composite electrical fire monitoring detector based on the detection of residual current and temperature came into being.

1 Causes of electrical fires and the significance of monitoring

There are many reasons for electrical fires, which are mainly manifested in the following aspects:

(1) Electric sparks or arcs. The main cause of electric sparks or arcs is single-phase grounding faults, which are mainly manifested as insulation damage of the conductor, which causes the two conductors to be broken down and generate arc voltage. The arc will generate very high temperatures. For example, an arc current of 2 to 20 A can generate a local high temperature of 2,000 to 4,000 °C. In fact, an arc current of 0.5 A is enough to cause a fire.

(2) Poor contact. When the working current passes through, a large amount of heat is generated on the contact resistance, which increases the temperature of the connection. The high temperature further increases the contact resistance, forming a vicious cycle, damaging the insulation of the nearby electrical system, causing a short circuit and causing a fire. It may also directly ignite nearby combustibles and cause a fire.

(3) Combustible materials. Combustible materials can be divided into solid combustible materials and liquid combustible materials. Usually, the ignition point temperatures of liquid and solid combustible materials are 300°C and 400°C respectively.

Electrical fire monitoring, also known as residual current alarm, is a method of detecting and warning before a fire occurs, which truly means "preventing problems before they occur."

Unlike ordinary fire alarm systems, which are mainly used to reduce fire losses, installing an electrical fire monitoring system is to curb fires from the root, which has different meanings and is more practical. The incidence of electrical fires remains high, and the country is paying more and more attention to electrical fire prevention, which further highlights the role and significance of electrical fire monitoring. Both enterprises and individuals need to understand electrical fire protection products and the current market situation. More people paying attention to the electrical fire protection market will definitely play a role in regulating and stabilizing market development.

2 Residual current detection principle

In short, residual current is the vector sum of the currents in the main circuit of a residual current operated protective device. It is commonly known as leakage current, leakage current or excess current, but is uniformly referred to as "residual current" in the national standard "Installation and Operation of Residual Current Operated Protective Devices" (GB13955-2005).

Residual current electrical fire monitoring detectors use the principle of current transformer detection of current to detect the size of residual current to prevent the occurrence of electrical fires.

The detection principle is shown in Figure 1, where IA, IB, and IC are phase currents, IN is the neutral current, Id is the residual current to the ground of the phase line at point a, and S is any closed surface. According to Kirchhoff's law, the sum of the effective value phasors of the current flowing into any closed surface S is equal to zero, so IA+IB +IC -IN-Id =0, which can be rearranged to IA+IB +IC -IN=Id. Under normal circumstances, the vector sum of the three-phase current is equal to the current flowing in the N line in magnitude and opposite in direction, and they cancel each other out. If the line insulation deteriorates or other reasons cause the A phase line to generate a current to the ground at point a, a current proportional to the residual current Id will be induced in the coil of the current transformer at S in the figure, and its value reflects the leakage of current in the distribution line and electrical equipment.

Figure 1 Residual current detection principle[page]

3 System composition and detector setting principles

At present, the mainstream electrical fire protection product is the overload protection circuit breaker. Its basic function is to cut off the power supply in time, accurately and automatically when the protected electrical equipment generates current overload for some reason, so as to prevent the overload current from generating current thermal effect and causing combustion. Its defect is that it cannot adjust itself after the rated current is set. The working circuits with different rated currents must match the corresponding overload protection circuit breaker, and it can only monitor the current flowing through itself, but cannot monitor the leakage phenomenon of the working circuit.

3.1 System composition

This composite electrical fire monitoring detector consists of the following parts: electrical fire monitoring equipment, residual current electrical fire monitoring detector, and temperature measurement electrical fire monitoring detector. The system uses the current magnetic field effect and temperature effect to transmit the current change and temperature change of the working line to the monitoring equipment. When a sudden change in current or temperature occurs, the detector analyzes the change amplitude and compares it with the alarm setting value, sends out an audible and visual alarm signal, and sends the alarm signal and alarm address to the monitoring equipment.

After receiving the alarm signal, the monitoring equipment can display the alarm location, alarm value, alarm quantity and other information, reminding the on-duty personnel to quickly handle the working line and send the alarm information to the centralized control console. This is the main feature of the electrical fire monitoring system, which uses technical means to monitor the abnormal state of the electrical line based on the principle of electrical fire. The system composition schematic diagram is shown in Figure 2.

Figure 2 System composition diagram

3.2 Setting principles

The residual current detector is set up based on the principle of end detection, and should be set up at the main power supply end and the head end of the branch line. When the load and leakage current at the end of the power supply are very small, and the load conditions and natural leakage current of the upper line still meet the requirements for setting up the residual current detector, it can be set up at the power supply of the upper level. The alarm value of the residual current detector must be suitable for the detected electrical circuit, and the leakage current of the detector alarm shall not be less than 2 times the maximum leakage current of the protected electrical circuit and equipment during normal operation.

The rated current, rated voltage and other indicators of the residual current detector should meet the requirements of the protected line. The residual current detector should be used for alarm, and can only automatically cut off the power supply when protecting places and equipment with low power supply requirements. The number of levels of residual current detector detection should be determined according to the nature of the building and the specific conditions of the low-voltage distribution line, and the number of detection levels should not exceed three. When selecting a residual current detector, the inherent residual current of the power supply system should be calculated (when the power supply system has been completed, it should be actually measured), and a detector with appropriate parameters should be selected to try to make each detector fully play its role and reduce the number of detectors that constitute the monitoring system.

The setting of temperature measuring detectors should be based on the basic principle of detecting abnormal heating of electrical systems, and should be set at heating parts such as cable joints, cable bodies, and switch contacts. When the detection object is a low-voltage power supply system, a contact-arranged temperature measuring detector should be used. When the object to be detected is an insulator, the temperature sensor of the detector should be directly set on the surface of the object to be detected, using a contact arrangement. When the detection object is the temperature change inside the distribution cabinet, a non-contact arrangement can be used, but it should be set close to the heating components.

4 Conclusion

To sum up, the main function of this composite electrical fire monitoring detector is to detect the current (mainly line residual current) and temperature changes of distribution lines, transformers and distribution equipment, and important electrical equipment before a fire occurs, and to alarm the addresses of hidden dangers and fault locations.

The design of electrical fire monitoring detectors must first clarify the fire hazard level of the protected object, and determine the protection level in combination with the design of the power distribution system and the load characteristics of the project; at the same time, the design of electrical fire monitoring detectors should focus on line residual current detection and alarm, and increase the temperature of appropriate parts of the distribution equipment and lines according to the load characteristics and operating conditions, such as transformers, cable trays and cable interlayers in electrical shafts, etc., to improve the cost performance of the system. In addition, when designing the power distribution, the influence of line load growth factors and laying conditions on the current carrying capacity should be fully considered, sufficient neutral line cross-sections should be selected, the design of the total equipotential and local equipotential connections of the project should be carefully done, and the reasonable selection of residual current action protection devices is also crucial to the protection of electrical fires.