Design of intelligent residual current electrical fire monitoring detector

1 Introduction

In recent years, the number of fire accidents in China has been on the rise. Among the fire accidents, the proportion of electrical fires remains high, accounting for 27% of the annual number of fires and 52% of the losses. The casualties and property losses caused by fires are huge every year. The task of preventing and effectively curbing electrical fires is imminent. The relevant national departments have successively formulated or revised relevant standards and specifications, requiring the installation of electrical fire monitoring and alarm systems in buildings. According to the new standard of electrical fire monitoring system (GB14827-2005), this paper designs an intelligent residual current electrical fire monitoring detector, which can be used alone or connected to the electrical fire monitoring equipment through RS485 bus to form a complete electrical fire monitoring system.

2 Design Basis of Electrical Fire Monitoring Detectors

2.1 Detector-related standards

The designed device should comply with standards such as "Electrical Fire Monitoring System" (GB 14287-2005), "Residual Current Electrical Fire Monitoring Alarm Device" (GB14287.2-2005), "Code for Fire Protection Design of Buildings" (GB 50096-2006), "Code for Fire Protection Design of High-rise Civil Buildings" (GB 50045-2005), "Requirements and Detection Methods for Electrical Fire Prevention in Buildings" and "Installation and Operation of Residual Current Operated Protective Devices" (GB13955-2005).

2.2 Introduction to electrical fire monitoring system

The electrical fire monitoring system is composed of electrical fire monitoring equipment and electrical fire monitoring detectors. The system should meet the requirements of "Electrical Fire Monitoring System" (GB 14287-2005). When the detected parameters in the protected line exceed the alarm setting value, the electrical fire monitoring system can send out alarm signals, control signals and indicate the alarm location.

Electrical fire monitoring equipment can receive alarm signals from electrical fire monitoring detectors, send out sound, light alarm signals and control signals, indicate the alarm location, and record and save alarm information.

Residual current electrical fire monitoring detectors can detect changes in electrical fire hazard parameters such as residual current and temperature in the protected circuit. They can be divided into two categories according to their working mode: one is an independent detector (a detector with monitoring and alarm function), and the other is a non-independent detector. This paper designs an intelligent independent detector.

2.3 Causes of residual current

Under normal circumstances, when there is no equipment leakage or grounding fault in the circuit, as shown in Figure 1, according to the circuit principle, the current phasor sum of the three-phase four-wire power supply is equal to zero, that is, Ia+Ib+Ic+In=0, and the vector sum of the magnetic flux generated in the current transformer is equal to zero. At this time, the induced current IL in the secondary coil is 0, so the line is powered normally. When equipment leakage or fault grounding occurs in the circuit, for example, the fault leakage current ZI appears, the current phasor sum of the three-phase four-wire power supply will not be equal to zero, that is, Ia+Ib+Ic+In≠0, and the vector sum of the magnetic flux generated in the current transformer is also not equal to zero. At this time, there is an induced current in the secondary coil, that is, the residual current IL≠0.

Figure 1 Working principle of residual current transformer.

2.4 Necessity of installing electrical fire detectors

Arc-type short circuit to ground in electrical grounding fault is an important cause of electrical fire. Arc-type short circuit to ground has a large impedance and voltage drop, which limits the fault current, making the overcurrent protector unable to operate or unable to operate in time to cut off the power supply, and the local high temperature generated by the leakage arc of several hundred milliamperes can reach more than 2000℃, which is enough to ignite the surrounding combustibles and cause fire. Moreover, electrical equipment is distributed in every corner of the building, and the scope of harm is wide. If the leakage of the system is not monitored and controlled, it will pose a threat to the safety of people and property, and there is a great fire hazard. The residual current electrical fire monitoring detector can accurately monitor the faults and abnormal conditions of the electrical circuit, and can effectively prevent the common electrical fire accidents in buildings caused by grounding arcs caused by leakage. In order to ensure the safety of people's lives and property, it is very necessary to install residual current electrical fire monitoring detectors at the power supply line and trunk line of the building.

3. Detector design

3.1 Basic functions of the system

The intelligent residual current electrical fire monitoring detector integrates the monitoring, analysis, alarm and control of electrical faults such as residual current, short circuit, overload, overvoltage and undervoltage (phase loss). It mainly has the following functions:

(1) It has leakage current, overcurrent long delay, overcurrent short delay and short circuit instantaneous protection functions to form the required protection characteristics. It can intelligently set the setting value and warning value of leakage current, overcurrent long delay, overcurrent short delay and overcurrent instantaneous. It also has over-voltage and under-voltage alarm functions and phase loss alarm functions.

(2) Display and store the line address, fault type, fault time, leakage current, and three-phase current value of the fault point. Up to 1,000 historical faults can be recorded and stored for a long time until deleted by command.

(3) By adopting RS485 bus communication technology, the bus and the host can form a master-slave monitoring system to achieve user networking. On one computer, 1 to 250 intelligent detectors can be remotely monitored online, the safe use of electricity by each user can be checked at any time, and the power supply lines of each user can be connected or disconnected at any time.

(4) It has a pre-alarm function, a humanized intelligent control strategy that gives an early alarm when approaching the action parameters and an alarm trip when exceeding the standard. It adopts an advanced active protection mode and an intelligent control structure to record and control the safety status of the power operation line, and can remotely trip the circuit breaker at the specified node.

(5) It can be connected with temperature sensors, smoke sensors, combustible gas detectors, etc., and linked with the fire automatic alarm system center to remotely cut off the load power supply, and a DC12V signal is fed back to the alarm center to trigger an alarm.

3.2 Overall Hardware Design

The residual current electrical fire monitoring detector is mainly composed of power supply, single-chip microcomputer PIC24FJ96, three-phase AC voltage and current detection circuit, residual current detection circuit, RS485 communication module, alarm, buttons and display, as shown in Figure 2.

Figure 2 Detector block diagram.

Its main working principle is: after conditioning the three-phase current, leakage and voltage signals obtained from the current transformer and linear optical isolator, they are input into the A/D conversion of the single-chip microcomputer, which samples and analyzes them, and outputs corresponding display and alarm signals, etc. The analysis results can also be transmitted to the host computer through the RS485 bus.

3.2.1 MCU circuit

The microcontroller used is PIC24FJ96, which is a high-performance CPU with improved Harvard architecture designed by Microchip. It is the core of the detector and completes various control functions of the detector, including sampling of three-phase voltage, three-phase current and leakage current, data processing, alarm output, communication with the host computer, LCD display and buttons, etc.

3.2.2 Residual current detection circuit

The residual current detection circuit is a zero-sequence current transformer. The protected phase line and neutral line pass through the annular core to form the primary coil of the transformer, and the winding wound on the annular core forms the secondary coil of the transformer. If there is no leakage, the vector sum of the current flowing through the phase line and the neutral line is equal to zero, so the corresponding induced electromotive force cannot be generated on the secondary coil. If leakage occurs, the vector sum of the current of the phase line and the neutral line is not equal to zero, which will generate an induced electromotive force on the secondary coil, and this signal will be sent to the intermediate link for further processing, as shown in Figure 3.

Figure 3 Residual current detection circuit.

The processed signal is sent to the single chip microcomputer, which samples 20 points in each cycle. The effective value of the residual current can be calculated according to formula (1).

Where X is the sample value.

3.2.3 Three-phase voltage and current detection

The voltage detection consists of a linear optical isolator, an operational amplifier and a rectifier filter circuit. Since the detector does not require high voltage accuracy, the use of an optical isolator can greatly reduce the size of the system.

The current detection is composed of a three-phase AC transformer, an operational amplifier and a rectifier filter circuit. The three-phase AC transformer converts the current into a voltage signal, which is conditioned by the circuit composed of the operational amplifier and then rectified and filtered and input to the A/D converter of the microcontroller for conversion.

3.2.4 RS485 bus hardware circuit

Figure 4 RS485 bus hardware circuit. The detector communicates with the host computer using the RS485 bus. One host can control up to 250 detectors. The RS485 communication system adopts a master-slave structure. The slave does not actively send commands or data. Everything is controlled by the host.

Therefore, in a communication system, only one host computer is used as the master, and other slave computers cannot communicate with each other. Even if there is information exchange, it must be forwarded through the host computer. The hardware circuit for communicating with the host computer is shown in Figure 4.

Figure 4 RS485 bus hardware circuit.

4. Detector Software Design

The software completes the functions of the entire detector and adopts a modular structured C language programming scheme. The C language has the advantages of high quality of generated code, high program execution efficiency, and good portability. The software part includes voltage, current and leakage sampling, data processing, alarm output, key input and LCD display. The system block diagram of the software is shown in Figure 5:

Figure 5: Software system block diagram.

Residual current detection alarm judgment is a more important part of software design. It analyzes, compares and judges the data after A/D conversion and transfers it to the corresponding subroutine. If the detected residual current value is less than the set value but greater than 0.8 times the set value, the detector will remind the duty personnel in the form of sound and light alarm. If the residual current is greater than the set value, in order to prevent interference, the detector continuously detects the residual current and trips after exceeding the set time.

The Modbus communication protocol is used between the host computer and the slave computer. The Modbus communication protocol is one of the mainstream communication protocols currently used by international intelligent instruments. When the host computer sends a communication command to the detector, the slave computer that meets the corresponding address code receives the communication command and reads the information according to the function code and related requirements. If the CRC check is correct, the corresponding task is executed, and then the execution result is returned to the host computer. The returned information includes the address code, function code, data after execution, and CRC check code. If the CRC check is wrong, no information is returned.

5 Conclusion

The single-chip microcomputer is used to carry out the intelligent design of the residual current electrical fire detector, realizing the combination and intelligence of the traditional circuit breaker functions, and the systematization and networking of circuit breaker control can be realized through the application of bus communication technology. After actual work testing, the residual current electrical fire detector in this article has achieved the expected purpose and passed the national fire electronic quality inspection and appraisal.