In daily life, the production methods of staple food are nothing more than steaming, frying, boiling, baking and branding. The "baking" is widely used not only in the catering industry, but also in the food processing industry. Baking food requires the use of ovens, and charcoal ovens have almost disappeared. At present, in addition to gas ovens and oil ovens, the largest number of electric ovens are used in hotel kitchens and food factories. Compared with other ovens, electric ovens do not pollute the environment, are easy to install and use, and improve the working conditions of operators, so they are widely popular. However, as an electrical product, if the electric oven is purchased, installed and used improperly, it will become an electric tiger, which may endanger the safety of personal and property. In recent years, electric shock injury, fire and other malignant accidents caused by electric oven have occurred from time to time.
China has implemented the production license system for food electric ovens since the 1980s. In 1997, it promulgated and implemented new product safety standards and conducted national supervision and random inspection on food electric ovens for many times, so as to improve product quality and protect the safety of people's lives and property. In the second quarter of 2003, the General Administration of quality supervision, inspection and Quarantine of the people's Republic of China again organized the national supervision and random inspection of food electric ovens. The total qualified rate of products was only more than 60%, and the safety situation of food electric ovens was still not optimistic.
What kind of electric oven is safe? According to some important provisions of national standards, combined with the quality problems found in national supervision and random inspection and product safety accident analysis over the years, this paper briefly introduces several main safety requirements of electric oven.
electric-shock safeguard 
Anti electric shock protection is also known as "protection against touching live parts". When the electric parts, the insulating cover or the protective cover are not required to be removed by the user when the electric parts are operated, covered or maintained, and all the electric parts shall not be touched by the user when the electric parts, the insulating cover or the product are not used.
During the inspection, it was found that in order to save labor and materials, some factories covered the exposed live parts such as the end of the electric heating pipe of the oven with a baffle, but it was subject to the condition that the eyes could not see it. Fingers could still touch the live parts from the lower part of the baffle or through gaps and holes; In order to facilitate the maintenance and replacement of parts by the manufacturer in the future, the door of the electrical box is designed to be opened easily, and the temperature controller and other electrical components on the panel can be disassembled by hand, so that the live parts are exposed and unprotected; The bare wire is not used as a heating element, but no protective measures are taken. The owners of these factories subjectively believe that users know which parts of the electric oven have electricity and will not touch it, which is a wishful thinking fluke. If the anti electric shock protection items of such products fail to meet the qualified requirements, it will bring serious hidden dangers to users and users.
Insulation resistance, leakage current and electrical insulation strength
For the requirements of preventing personal electric shock, it is obviously not enough for the product to only have the protective measures that the guarantor cannot touch the live parts. The product should also ensure that the electrical insulation between all surfaces of the electric oven that can be touched by people and its internal live parts is safe and reliable, otherwise people may still be in danger when touching the non live shell of the oven. Check whether the electrical insulation meets the requirements. Generally, three indicators are used: insulation resistance, leakage current and electrical insulation strength. Because the instrument used to measure insulation resistance (commonly known as "megger") is cheap and easy to use, the method of measuring insulation resistance is widely used in electric oven production enterprises to judge the electrical insulation quality of products, which is also a simple and practical method for users to check whether the electrical insulation of oven is damaged. However, GB4706 Series of national mandatory safety standards stipulate that leakage current and electrical insulation strength are used as the judgment basis for the electrical insulation quality of electric oven.
Leakage current: when any electrical product is in use, if the person standing on the ground touches the ungrounded shell of the product, there will be current flowing from the human body. This current is called leakage current. Under normal circumstances, the leakage current of electrical products is very small and people will not feel it. However, when the performance of insulating medium is reduced due to moisture and high temperature, the leakage current may exceed the safe value and personal electric shock accident may occur. In order to prevent this danger, special instruments must be used to simulate human body and environmental conditions, and test the leakage current of the product to confirm that it does not exceed the specified limit.
Electrical insulation strength: also known as electrical strength or insulation strength, commonly known as high voltage resistance. The power supply voltage of China's municipal power supply is single-phase 220V and three-phase 380V. However, due to the existence of inductive load and other reasons, the power grid may instantaneously produce overvoltage much higher than the power supply voltage under some conditions, which requires the electrical insulation of products to have a certain ability to withstand overvoltage. The ability of electrical insulation to withstand breakdown voltage is called electrical insulation strength, which is a very important index to assess the safety and quality of electrical insulation. The test method is to use a special high-voltage tester to apply a high test voltage between the live parts and the shell of the electric oven and maintain it for a certain time to observe whether the insulation breaks down or flashover occurs. According to different insulation properties, the test voltage value is generally 1000V to 3750V.
The long-term storage of electric oven does not cause insulation moisture, or the long-term continuous use of high temperature causes insulation overheating, which are the two extreme states that are most likely to degrade the electrical insulation performance and cause leakage and breakdown accidents. Therefore, the leakage current and electrical insulation strength tests must be carried out respectively under the condition of simulating these two worst states. Generally, the test simulating wet state is called cold state or wet state; The test simulating overheating is called hot state or working temperature. All electrical insulation of the product shall be able to withstand all specified cold and hot tests before it is considered to meet the requirements.
The insulation of the electric heating tube of the food electric oven is the weakest link of the electrical insulation of the whole product. Leakage and breakdown often occur in these parts. In addition, the use of inferior temperature controllers, switches and wires, the use of insulation materials with poor heat resistance, and the use of hygroscopic asbestos and silicate cotton as electrical insulation are all important reasons for the leakage current and electrical insulation strength of products in hot or cold state not meeting the qualified requirements, and even causing personal safety accidents. Strictly controlling the purchase of electrical components related to safety and conducting incoming inspection of key components such as electric heating tubes are important links to greatly improve the safety and quality of electric ovens.
Grounding measures
For the electric food oven, the so-called grounding measure is to install a grounding electrode (grounding bolt) on the main body of the metal shell of the product, and then reliably connect all other exposed metal parts that are not connected with the main body with wires to the grounding electrode as long as possible; There must be a yellow and green protective grounding conductor in the power line. The inner end of the line is reliably connected with the grounding electrode on the main body of the metal shell, and the outer end is connected with the grounding electrode of the power plug; For the oven without plug, connect the protective ground wire of the power supply during installation. The electric oven with reliable grounding of metal shell will not cause any danger to people standing on the ground and touching the metal shell of the oven under the condition of insulation failure and dangerous voltage on the shell, because the potential difference between the oven shell and the earth is equal (potential difference is equal to zero, no voltage).
It can be seen that the electrical insulation and grounding of the electric oven constitute a double protective measure to prevent personal electric shock. Double protection is an important safety principle in GB4706 national standard. It runs through the provisions of the standard and greatly improves the safety of food electric oven, which has important practical significance.
In addition, we can also say that grounding measures are the last protective barrier to prevent electric shock. Almost 100% of those who have electric shock accidents do not reliably ground the metal shell of the product. There are three reasons: first, the product is not installed with qualified grounding electrode and protective ground wire; Second, the product grounding measures meet the requirements, but the protective ground wire of the power supply is not connected during on-site installation; Third, the power supply system at the product installation site has no protective ground wire and cannot be connected. The key to solving the first and second points is that the producers and users of products must truly understand that grounding is a matter of human life and must strictly implement the standards without carelessness. The third point involves the supporting problems of the design and installation standards of civil building electrical system and the standards of electrical products such as electric ovens, which is more complex. But it is certain that an ungrounded electric oven must not be used.
Abnormal operation
Abnormal operation is a very important group of safety items of food electric oven. It is used to assess whether the product can still ensure the personal and property safety of users and users, and avoid disastrous accidents such as electric shock, fire and so on under the abnormal conditions of human misoperation or some fault during the use of the product.
Abnormal operation is relative to normal operation. For example, the electric oven is generally equipped with a set of temperature control system, which automatically turns on and off the heating elements when the oven is in normal working state to control the food baking temperature. Once the temperature control system fails and cannot automatically cut off the heating power supply, the oven will be in an abnormal working state. The heating is out of control, and the temperature inside and outside the oven will rise without limit, which will eventually lead to the oven burning or even fire. Some ovens are equipped with manual temperature control function. According to the operation regulations, the operator shall not leave when using manual temperature control. However, the operator may misoperate due to negligence, which will also cause the abnormal working state of heating out of control. This requires that the oven should be installed with a set of overheat protection system independent of the temperature control system to quickly cut off the heating power supply in case of temperature control failure, so as to prevent catastrophic accidents.
The electric oven should ensure the safety of people and the environment under normal working conditions, and also ensure the safety of people and the environment under abnormal working conditions. This is another important safety principle of GB4706 Series Standards, which is of great practical significance to improve the safety and quality of electric oven.
To realize the safety protection of abnormal work, we must increase the product cost. It is impossible for those inferior products that compete in the market by pressing the price to install overheating protection system. Some factories use inferior parts, which can not achieve the purpose of overheating protection at all. All these food electric ovens without overheating protection function are unqualified products that do not meet the national mandatory standards, and there are serious potential safety hazards. Consumers should not buy them cheaply. In case of accidents and losses caused by the use of such nonconforming products, laws shall be applied to protect their legitimate rights and interests.