Causes and prevention of safety hazards of household appliances

Household appliances are closely related to daily family life. People come into direct contact with them in the process of controlling and using them. While dazzling household appliances bring great convenience and enjoyment to people, they also bring huge safety hazards to people. Household appliances generally use mains electricity as a power source. This 220V, 50Hz alternating current is an unsafe voltage and is very dangerous to the human body. Of course, household appliance safety cases are only low-probability events, and the possibility of accidents, especially malignant accidents, is extremely small. But behind the low probability is a huge absolute value. According to statistics, the number of deaths caused by electric shock caused by household appliances in my country exceeds 1,000 each year, and the economic losses caused by fires caused by household appliances are huge. Since the users of household appliances are very complex, including the elderly, children, women and many people who have not received electrical knowledge training, the safe use of household appliances is very important.

2 Reasons why household appliances pose safety risks

The safety hazards caused by household appliances include electric shock, fire, mechanical injury, radiation, etc., among which electric shock and fire are the most prominent.

The main reasons are as follows.

2.1 Product quality issues

Accidents such as electric shock and fire caused by substandard quality of household appliances occur frequently, a considerable part of which is caused by substandard quality of power cords.

(1) Electric frying pans, electric rice cookers, household electric stoves, electric soldering irons and liquid heating appliances (including electric cups, electric thermos bottles, electric pots, milk cookers, electric teapots, electric coffee pots, electric pressure cookers, water boilers, etc.) should use fiber braided or rubber sheathed flexible cables, but they do not use such products as required by the standards. Among them, household electric stoves, electric soldering irons and various liquid heating appliances do not use power cords as required, which is a serious problem.

(2) Vacuum cleaners, electric food processing appliances, etc. should use rubber-sheathed flexible cables or polyvinyl chloride sheathed cables, but such products are not used as required by the standard.

(3) Products that do not use PVC insulated wires are not allowed to be used as specified in the standard. These products are mostly products with external metal parts with a temperature rise of more than 75K, such as electric irons and portable heaters. For massage appliances, when the temperature rise of the external metal parts exceeds 60K, PVC insulated wires cannot be used. However, there is still a phenomenon of using PVC insulated wires in these appliances on the market.

The structural design of some household appliances does not meet the requirements of national standards, and lacks effective and necessary protection for live or overheated parts and abnormal operation, resulting in safety accidents during the use or maintenance of the products; some products are not strictly controlled during the production and processing, and the components and internal insulation materials used are not heat-resistant and flame-retardant, causing the products to have electric shock hazards or fire hazards. Some manufacturers have deviations in their understanding of product standards and unreasonable product designs. Some do not pay enough attention to product labeling, and the product labeling and warning signs are incorrect and non-standard. The instructions and precautions are not detailed, and there are no warning words, which affects the safety and correct use of consumers. Some companies entrust OEM production after obtaining compulsory certification certificates, lack product quality control.

2.2 Grounding protection issues

Since most of the general residences and high-rise buildings do not have a dedicated ground wire, the ground wires (or ground wires required) of household appliances in offices or households do not know where to connect, so many electric shock casualties have occurred when using household appliances. It is understood that the grounding of household appliances is very chaotic. Some leave the ground wire empty, some change the single-phase three-pin plugs and sockets with grounding heads into single-phase two-pin plugs or sockets, and some hang the ground wire on the water supply and gas supply pipes (it should be noted that since the pipe joints are wrapped with non-conductive materials such as jute, they are electrically "discontinuous" and have a large grounding resistance. If it is a gas pipeline, it will explode once a spark is generated). Some also connect the housing of the appliance to the neutral wire through the zero-connection pile (binding post) in the socket that directly connects to the power supply. This is very dangerous. If the live wire and the neutral wire are connected in reverse, the live wire will be connected to the housing of the appliance. Even if they are not mixed, once the neutral wire is disconnected, the housing of the appliance may carry a voltage of 200V.

According to a survey conducted by relevant departments in 2006, only 57% of households in my country's large cities can provide zero-grounding protection for Class I household appliances, and almost 100% of households in rural areas do not provide grounding protection for Class I household appliances.

At present, the sockets in rural houses are two-hole sockets without grounding wires. The grounding wires of three-hole sockets are not reliably grounded, and there is no protective grounding measure at all. Therefore, the metal shell leakage protection of Class I electrical appliances should be given high attention by the power department and the majority of electricity users, especially rural users, and effective measures should be taken to solve the grounding or zeroing problem of household Class I electrical appliances to ensure safety.

2.3 Expired use of household appliances

In recent years, there have been more and more safety accidents caused by the use of "overage" home appliances. Home appliances are large durable consumer goods, and most consumers have the mentality of "replacing them only when they are no longer usable" when purchasing and using them. However, experts remind consumers that all home appliances have a safe service life, just like food has a safe shelf life. If they exceed the shelf life, they should be scrapped, otherwise there will be increased safety hazards. According to professionals who have been engaged in home appliance repair for many years, when a TV exceeds its safe service life, as the electrical components age, the TV will have problems such as unclear images and tremors, and the radiation of the TV will also increase. The aging TV is subjected to vibration, impact, collision, sudden cooling, sudden heating, excessive dust and dirt in the TV, or local overheating caused by short circuit of wires, which can cause the cathode ray tube to explode; if the refrigerator is too old, the preservation and sterilization functions will degenerate, resulting in food odor and inability to preserve freshness. At the same time, the refrigerant will leak, pollute the environment, and endanger health; overage washing machines often have minor problems such as water seepage, and in severe cases, they will leak electricity.

As for household electrical appliances themselves, safety is generally fully considered in design and manufacturing to ensure personal safety. For example, the leakage current of the product to the ground is limited, the shell is grounded, sufficient leakage distance and electrical clearance are ensured, double insulation or reinforced insulation structure is adopted, safe voltage is used, the bending resistance of the power connection line is ensured to prevent it from being twisted or disconnected, and the risk of electric shock caused by capacitor discharge, etc. Therefore, as long as the newly purchased electrical appliances are used correctly, there are generally no safety hazards. The problem is with old household appliances.

3 How to prevent electrical safety issues when using household appliances

3.1 Improving insulation performance and design safety

3.1.1 Improving insulation performance

The better the insulation performance, the smaller the leakage current and the lower the voltage between the shell and the ground. If the shell of the electrical appliance is made of insulating materials (including the commonly used knobs, etc.), it can avoid contact between the human body and the metal, which is more conducive to personal safety. For household appliances such as transformers, if the "live" phenomenon occurs, the simplest way is to change the insertion direction of the two poles of the power plug. However, this method is not applicable to household appliances containing heating wires and motors. Use insulating materials to enhance the insulation ability. For example, people can stand on dry wooden brackets or wooden boards to ensure good insulation between people and the ground. When using electrical appliances such as electric irons, even if the shell leaks electricity, because the insulation resistance between people and the ground is very large, the current flowing through the human body is extremely small (no more than 1mA), and it will not cause electric shock accidents.

3.1.2 Improving design safety

(1) Install a thermal fuse

To prevent overheating of electric heaters controlled by thermostats, temperature fuses can be installed. When the temperature rise exceeds the limit, the fuse will melt and cut off the power supply. It is suitable for the range of 80℃ to 230℃. In addition, bimetallic thermostats can also be used. By adjusting the temperature control appropriately, the temperature can be prevented from being too high and the electric heater from overheating.

(2) Use leakage protection device

The use of leakage protection devices ensures the safe use of household appliances. This has been confirmed by the practice of low-voltage distribution networks at home and abroad.

According to actual needs, branch leakage protectors for electrical equipment and total leakage protectors for residential lines can be installed to achieve multiple protections or graded protection.

3.2 Safe Installation

(1) Meet the requirements of the electrical installation environment

When buying household appliances, you should first carefully check the technical specifications in the product manual, such as whether the power supply type is AC or DC, whether the power supply frequency is the general industrial frequency of 50Hz, whether the power supply voltage is 220V for civilian use, how much power the appliance consumes, whether the existing power supply capacity of the family can meet the requirements, especially the plug socket, fuse, electric meter and wire. If the load exceeds the allowable limit, the insulation will be damaged by heat, which will cause an electrical accident. Only after the above content is checked correctly can you consider installing and powering on. When installing household appliances, you should check the requirements for the installation environment in the product manual, and pay special attention to not installing household appliances in a humid, hot, dusty environment, or inflammable and corrosive gas environments under possible conditions.

(2) Must be grounded or connected to a common neutral line

Household appliances such as refrigerators, washing machines, electric fans, microcomputers, and air conditioners are all Class I appliances. Their characteristics are that the power leads use three-pin plugs, in which the top pin of the three-pin plug is connected to the metal shell of the appliance. According to the safety requirements for Class I appliances, the metal shell must be grounded or connected to a common neutral line when in use, which is the so-called protective grounding and protective zeroing.

(3) Correct wiring

When laying power lines, the phase line and the neutral line should be clearly marked and consistent with the wiring of household appliances. They must not be connected incorrectly. When connecting household appliances to the power supply, there must be a switch or plug connector that can be disconnected. It is forbidden to directly insert the wire into the socket hole. For those that require protective grounding or protective neutral connection, three-pin plugs and three-hole sockets should be used, and the grounding and neutral pins and sockets should be strictly distinguished from the corresponding pins and sockets.

It is forbidden to use symmetrical two-pin plugs and two-pin sockets instead of three-pin plugs and three-pin sockets to prevent incorrect connection and plugging, which may cause the metal casing of household appliances to be electrified and cause electric shock accidents. Although the grounding and neutral wires are not electrified under normal circumstances, for safety reasons, their wire specifications should be larger than the phase wires, and switches or fuses shall not be installed on them. It is also forbidden to connect them to tap water, hot water, gas or other pipelines at will.

3.3 Safe Use

(1) Ensure that the leakage protector works properly

The leakage protector should be tested regularly during operation, usually at least once a month to check its reliability. The method is: when the power is on, press its test button, and it will trip immediately, indicating that it is working.

Do not blindly supply power after the protection device is activated. Find out the cause before restoring power. Because the protection device is usually activated due to a circuit failure or leakage of the metal casing of the household appliance. If you force power on without finding out the cause, it will often cause damage to the power supply line or electric shock accidents.

It is particularly important to note that after a fuse burns out, it is absolutely not allowed to use copper wire, iron wire, aluminum wire, etc. to replace the fuse to force power supply, nor can it be replaced with a fuse with a larger rated current.

(2) Timely elimination of “overage” old appliances

The General Rules for the Safe Use Period and Recycling of Household and Similar Electrical Appliances (hereinafter referred to as the General Rules) approved and issued by the National Standardization Administration have come into effect, and the related Detailed Rules for the Safe Use Period of Household Electrical Appliances (hereinafter referred to as the Detailed Rules) have also been launched at the same time. The General Rules stipulate the "retirement" age of household appliances. Manufacturers must indicate the safe use period of the household appliances they produce, and stipulate that the safe use period starts from the date of purchase by consumers. Within the safe period marked by the manufacturer, if a safety accident occurs when consumers use household appliances normally, all responsibilities will be borne by the manufacturer.

(3) Regularly measure the insulation resistance of electric household appliances

Generally, professionals are invited to use a 500V megger to measure the insulation resistance between the winding and the housing of electric household appliances every three years. Generally, appliances with basic insulation can be used with confidence if the resistance is above 2MΩ; appliances with reinforced insulation can be used with confidence if the resistance is above 7MΩ. If the insulation resistance does not meet the above requirements, the cause should be found out and the fault should be eliminated. The appliance can only be used when the insulation resistance meets the requirements. The measurement time should be selected in the summer when the air is relatively humid.

In addition, the insulation resistance of electric household appliances that have not been used for a long time (generally more than 3 months) or have been exposed to moisture should also be measured before use to see if they meet the requirements.

(4) If any abnormality occurs in the appliance, immediately cut off the power supply

If you find any abnormality when using household appliances, such as abnormal voltage increase or decrease, abnormal noise, odor, temperature, smoke, flame, etc., disconnect the power supply immediately and ask a professional to repair it.