Technical research on test method of the socket and switch of temperature monitoring alarm function

The emergence of sockets and switches with fire protection monitoring and alarm functions had realized the forward detection of the fire risk monitoring gateway, which had played an important role in preventing the occurrence of electrical fires. In 2014, the national standard GB31252 “Fire Monitoring and Alarm Sockets and Switches” was promulgated and implemented, which detailed the product functions, performance and test methods of fire protection monitoring and alarm sockets and switches. This article is based on the difficulty of accurately controlling the heating process when measuring the temperature alarm value of the socket and switch. A new type of simulated temperature alarm test method was designed, and a detection device for the temperature test of the fire monitoring alarm socket and switch was developed. And the testing device was tested and verified. The test results show that the testing device designed in this paper can meet the technical requirements of the temperature part test in GB31252-2014.


Introduction
Electrical fires generally refer to fires caused by self-combustion or ignition of other combustibles due to energy release caused by electrical wiring, power supply equipment, electrical appliances, and transformer and distribution equipment due to faults or other reasons, such as high temperature [1], electric arc [2], electric spark [3] and energy released by non-fault reasons.According to related statistics [4], from 2008 to 2017, the proportion of electrical fires in the total number of fires in the country had increased from 29.7% to 35.7%, and the number of electrical fires had increased by 147.9%, accounting for the proportion of the total number of fires in the country an increase of 20.2%.It can be seen from historical data that not only the number of electrical fires is large, but they are also increasing rapidly.In addition, electrical fires also account for a large proportion of particularly serious fire accidents.
In order to effectively prevent and contain the occurrence of electrical fires, according to the process mechanism of electrical fires from heating, storing heat to generating open flames, research institutions and related enterprises in the field of electrical fires had applied electrical fire monitoring technology to develop various types of electrical fire monitoring products.By collecting the residual current [5], fault arc, temperature and other data generated in the process of fire, mathematical models were established to analyze the process data, and then to determine whether there is a hidden danger of electrical fire, and the goal of early warning of electrical fire was achieved [6][7][8].The electrical fire monitoring products currently produced on the market are mainly divided into 4 categories, residual current electrical fire monitoring detectors, temperature measuring electrical fire monitoring detectors, fault arc electrical fire monitoring detectors and measuring thermal particle electrical fire monitoring detectors device.As a supplement to the scope of use of the above-mentioned electrical fire detection products, fire monitoring and alarm sockets and switches with electrical fire detection and alarm functions have appeared on the market in recent years.Different from the above-mentioned electrical fire detection products, they are usually installed on the branch of electrical circuits.Fire monitoring alarm sockets and switches are generally installed at the end of the electrical circuit, which effectively realizes the forward movement of the electrical fire alarm monitoring gateway.The importance of protecting the personal and property safety of the broad masses of people's families is also selfevident [9,10].
How to ensure the quality of fire protection monitoring alarm sockets and switch products and improve the detection and test level of electrical fire detection products is also an important research hotspot.Based on the particularity of the fire monitoring alarm socket and switch temperature detection method, this article according to the national standard GB31252-2014 "Fire monitoring alarm socket and switch" in the fire monitoring alarm socket and switch temperature test method requirements [11].developed the temperature monitoring alarm function test device to ensure the implementability and compliance of the product standards for fire monitoring alarm sockets and switches.

Test standard
In order to standardize the product quality of fire protection monitoring and alarm sockets and switches, the national standard GB31252-2014 "Fire protection monitoring and alarm sockets and switches" was promulgated and implemented in 2014.The standard specifies the classification, requirements, test, inspection rules and identification of fire protection monitoring alarm sockets and switches.According to the requirements of GB31252, fire protection monitoring and alarm sockets and switches are divided into fire protection monitoring and alarm functions: 1) sockets and switches with the function of detecting residual current; 2) sockets and switches with the function of detecting temperature; 3) sockets with the function of detecting fault arcs and switch .
This article focuses on the detection test method and realization of the socket and switch alarm function with temperature alarm function.According to the provisions of paragraph 4.3.2 of Article 4.3 of GB31252-2014, the trigger condition of the socket and switch monitoring alarm function with temperature detection function is 70°C ±5°C.GB31252-2014 5.2 stipulates the requirements for the monitoring and alarm function test, and the test method in 5.2.2 stipulates that "on the premise of ensuring that the test sample is in normal working condition, increase the temperature of the sample protection circuit at a rate of not more than 1°C/s.To 60°C±3°C, keep for 10min; then continue to increase to 70°C±5°C, keep for 1min; record sample status and alarm time".

Test method
GB31252-2014 had put forward higher requirements in the monitoring and alarm function test for temperature rise, temperature control, speed, error, etc.However, when switches and sockets are working, the most heat-generating parts are often located at the terminal, and the size of the terminal is only the size of a person's pinky finger.In such a small space, it is difficult to control the temperature increase and ensure the temperature increase rate and error.
During the test of the socket and switch monitoring and alarm function, measuring the temperature alarm value of the socket and the switch requires precise control of the heating process.The specific technical solutions include air type, energized heating type, liquid type and contact type.The air measurement method directly heats the switch and the socket as a whole through air heat conduction.We have verified the scheme, put the sample in a closed-loop thermostat, and placed it near the fire monitoring alarm socket and the temperature sensor of the switch.The thermocouple detects the temperature, and the wind speed inside the thermostat is 0.8m/s.Taking into account the possible influence of different heating rates on the alarm value, the heating rate of 1°C/min, 5°C/min, and 30°C/min is selected for heating.The heating test starts at 25°C, and after 20 minutes of stabilization, the temperature starts to rise.The temperature in the closed-loop incubator is selected to reach 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, Read the temperature value inside the fire protection monitoring alarm socket and switch at 120°C, 130°C, and 140°C.
The specific temperature value is shown in Figure 1 below.From the air-type measurement data in Figure 1, it can be seen that the air-type measurement method has a slow heat transfer rate, and the internal temperature distribution of the test sample is uneven when the temperature rises at a high speed, especially during rapid heating, the heating rate required in GB 31252-2014 is not more than 1°C/s.Such a heating rate is in line with the actual heating of the socket or switch terminal, but it is difficult to simulate by air heating, and cannot meet the requirements of the socket and switch product alarm time.
The energized heating type is a scheme that heats the switch and socket by controlling the current of the connecting conductor to make the conductor overload operation and heating.However, because the heating condition of each point in the circuit is different, the actual heating of the switch and the socket is reproduced by controlling the current.But it is difficult to ensure the accuracy of the measurement results depending on the selected monitoring point and the different environment [11].
The liquid measurement method requires the product to be directly penetrated into the liquid environment such as the oil tank, and it is also not applicable to the test on switches and sockets.The contact measurement method, as an ideal and feasible measurement method at present, is not able to meet the heating requirements of small-sized parts because the existing contact methods are heated by temperature plates and the like.For temperature alarms of switches and sockets, it is necessary to directly heat their terminals, and to be able to control the temperature accuracy of the heating rate.None of the above four methods can satisfy the reality of direct and rapid heating of switches and sockets, and does not affect the normal operation of the product demand [8,12].
Based on the above analysis, this paper has developed a new type of detection device for fire protection monitoring alarm socket and switch temperature test.The device integrates a thermocouple in the probe of the heating part to simulate the protection circuit.During the test, the heating probe of the device is installed in the probe with detection, the temperature function alarm switch or the terminal joint of the socket should be tightly connected.With controlling the temperature accuracy, it can not only simulate the actual situation of the temperature change of the switch and socket terminals caused by the high current heating, but also effectively control the temperature accuracy, which meets the requirements of the temperature part test in GB 31252-2014.

Working principle
The fire monitoring alarm socket and switch temperature test device use high-precision temperature collectors for closed-loop feedback and collection of temperature.The high-performance MCU main Temperature of the thermostat / ℃ Temperature of the sensor / ℃ control chip realizes communication and control functions.The dual isolation design of power and control signals ensures the safety and reliability of the equipment.The heating source uses advanced (MCH) metal ceramic heater, which has the characteristics of small size, fast heating, corrosion resistance, high temperature resistance, environmental friendliness, and long service life.At the same time, the heater and temperature sensor are encapsulated into a silver shell to form an integrated temperature measurement and control heating head, which improves its pressure resistance and thermal conductivity, and further improves its service life.
The working principle block diagram is as shown in Figure 2(1) below: In the above Figure 2(1), the main control chip MCU has a communication serial port connected to RS232 and temperature collector.A serial port connected to a monitor to realize display and setting data interaction.A serial port connected to a USB debugging port.An I/O interface with PWM output function, and the chip clock configuration can meet the duty cycle of 10×10-5.
K-type thermocouple has the characteristics of high linearity, small temperature drift, small probe volume, etc.The test device integrates the KX-2*0.3mmtype K thermocouple into the silver probe of the heating part.While heating the terminal, the AT4508 temperature sampler is used to collect temperature data every 1s, and the calibration is used to ensure accuracy range, temperature probe is compensated by external calibration and internal set error value.
Test heating and temperature acquisition probes are shown in Figure 2(2) and 2(3).The heating circuit adopts DC24V voltage power supply, the power is 38W when heating at full speed, and the current is 1.6A.When heating at full speed, the duty cycle of the heating core is high, the heating rate is faster, and the fastest heating rate can reach 1°C/s.The duty cycle is different, so the power will be different.According to the collected data, the temperature curve is displayed on the screen.The temperature curve is fitted by the temperature data of the temperature probe collected by the host computer software with time (s) as the horizontal axis and temperature (°C) as the vertical axis, and the horizontal and vertical axes are divided.The degree value can be customized.
The control principle of the heating rate is as follows: As described in Formula (1), PID is used to automatically adjust the duty cycle of the heating core to achieve a specific heating rate.And the temperature of the object to be heated is detected in three consecutive acquisitions.
Where ∆U is the duty cycle variable that needs to be changed; K is the proportional constant; K is the proportional integral constant; K is the proportional differential constant; T , T , T are temperature rise rates measured for three consecutive times; And the initial duty cycle U is 20%.

Function realization
The fire monitoring alarm socket and switch temperature test device have a constant temperature function.The brake program is entered at 3°C before the constant temperature, and the duty cycle is reduced to the heating rate.At this time, the temperature curve shows that the heating rate slows down, and it is between the detected temperature and the set temperature.Continue to enter the PID adjustment method to adjust the duty cycle of the heating core randomly to ensure a constant temperature.The fire monitoring alarm socket and switch temperature test device are shown in Figure 2(4).

Fire monitoring alarm socket and switch temperature alarm test
In the fire protection monitoring alarm socket and switch temperature alarm experiment stage, we set the temperature alarm value of the fire monitoring alarm socket and switch to 70°C to simulate the actual installation of cables and switches.Install the test heating and temperature collection probe on the fire monitoring alarm socket and switch terminal, and make it in the normal monitoring state according to the GB31252-2014 standard.Set the heating rate to 5°C/min, 3°C/min, and 1°C/min through by the temperature control interface of the test device.Start heating from 25°C increase to 60°C according to the set heating rate, then keep for 10min, then continue heating to 70°C for 1min, and finally to 75°C for 1min, observe the collected temperature curve and the operation of the fire monitoring alarm socket and switch state.The heating curve are shown in Figure 3.It can be seen from Figure 3 that when different heating rates (5°C/min, 3°C/min, 1°C/min) are used to raise the temperature to 70°C, the fire monitoring alarm socket and switch both send out temperature alarm signals.After the test, it can be seen from the heating curve that the temperature linear control of the test device is accurate and the temperature remains stable.

Conclusions
Different from traditional air type, energized heating type, liquid type and contact type fire protection monitoring alarm socket and switch heating control method, this paper designs and implements a new type of fire protection monitoring alarm socket and switch temperature test detection device, the thermocouple is integrated in the silver probe of the heating part, and the probe is tightly connected with the terminal joint of the alarm switch or socket.The temperature accuracy is controlled while heating, which not only simulates the actual situation of switch and socket terminal temperature changes caused by high current heating, but also effectively controls the temperature accuracy, which meets the technical requirements of the temperature part test in GB 31252-2014.

Figure 2 .
Figure 2. Fire monitoring alarm socket and switch temperature test device.