Analysis of process paths to meet the new safety standards for electric water heaters
The new safety standard GB4706.12-2006 “Special Requirements for Safe Storage Electric Water Heaters for Household and Similar Purposes” (hereinafter referred to as the standard) is equivalent to IEC60335-2-21:1997 and will be implemented on July 1, 2007. The main modification is the addition of Appendix AA. The specific content is “Additional requirements for Class I electric water heaters that can provide emergency protective measures when the grounding system is abnormal.” For solutions that comply with Appendix AA, there are power-off laws and regulations in the industry. There are two main options for the “charged water resistance method”. In this regard, the author believes that the “power-off method” is a better solution, and the “charged water resistance method” cannot fully achieve the safety purpose from the perspective of technical principles.
According to the title of Appendix AA “Additional requirements for Class I water heaters that provide emergency protective measures when the grounding system is abnormal”, the problem to be solved is grounding system abnormality, specifically grounding abnormalities other than appliances, which can be mainly divided into missing ground wires The following is a detailed analysis of the effects that the power-off method and the charged water resistance method can achieve in these two situations.
The power-off method is an effective method given by the international standard system to solve the problem of missing ground wires.
The lack of ground wire refers to the lack of ground wire in the fixed wiring, resulting in no grounding protection. It can also be classified as having a ground wire but not connecting it or the ground resistance is too large. Many households in rural China are often ungrounded due to low-voltage power supply systems. In fact, these households use a large number of Class I appliances.
At present, there are already mature recommended solutions internationally. Appendix P in the new version of IEC60335-1:2004 (National Standard GB4706.1-2005) is the solution to this problem. Appendix P solves the problem of using Class I appliances when there is no earth-protected power supply due to the lack of fixed wiring. The solution is to use a PRCD leakage protector (also known as the power-off method), which allows the configuration of a residual current Leakage protectors not exceeding 30mA, and more stringent standards are put forward for the experimental requirements of the equipment. For example, the leakage current does not exceed 0.5mA, the humidity resistance temperature t value is increased to 37°C, the ambient temperature is increased to 40°C, and the heating temperature rise is reduced to 15℃. If the experimental requirements mentioned above are met, the appliance is considered to have reached an acceptable level of safety when used in an environment where the external ground wire is missing.
In the 1980s, China began to vigorously promote leakage protectors, and the effect was very obvious. Related accidents dropped significantly, which fully proved that the additional protective effect of installing leakage protectors on electric shock protection was very effective and played a huge role in the security protection of the laser cutting bending aluminum power box. .
Is the charged water resistance method safe when the ground wire is missing? The charged water resistance method currently on the market refers to a method that uses the principle of water resistance to reduce the current at the water outlet. A common practice is to add a section of plastic pipe to the inlet and outlet pipe of the water heater. Once there is a risk of electric leakage, the water resistance formed by this section of plastic pipe reduces the outlet current to reduce the risk of electric shock.
According to the definition of the standard, the electric shock protection of Class I appliances not only relies on basic insulation, but also must include an additional safety protection measure, namely safety grounding protection. Only with these two protections can an acceptable safety level be achieved.
The absence of a ground wire means that the water heater has only basic insulation and no ground protection. So, can a section of plastic pipe with an electric water resistance method be equivalent to ground protection? The answer is obviously no. Without grounding protection, it is necessary to meet safety requirements, which is equivalent to meeting the requirements of Class II appliances. According to national standards, the electric shock protection of Class II appliances must not only rely on basic insulation, but must also provide additional safety protection measures such as double insulation or reinforced insulation. A section of plastic pipe with electric water resistance method obviously cannot be equivalent to supplementary insulation or combined with basic insulation to achieve reinforced insulation. Therefore, the charged water resistance method cannot be considered to achieve an acceptable level of safety in the absence of a ground wire.
The power-off method can better solve the problem of electrified ground wires
The live ground wire means that there is a ground wire in the fixed wiring, but if the ground wire does not play the role of ground protection, it is a source of danger. This situation is very dangerous for all Class I appliances.
At this time, there is an abnormal voltage or current on the ground wire, which is caused by many reasons, such as a phase-to-ground short circuit due to the aging of the phase wire insulation, and the contact resistance of plugs and sockets that are used for a long time increases and generates heat, causing a phase-to-ground short circuit. There is also the lack of necessary protection in fixed wiring.
IEC61540 provides relevant solutions to solve the problem of electrified ground wires. It is allowed to use RCDs with open and closed PE conductors when there is danger on the ground wire. There is already experience in using this type of RCD in other countries such as Australia and Germany. In response to China’s special use conditions, the new national standard GB20044-2005 “Mobile residual current devices (PRCD) without overcurrent protection for electrical accessories for household and similar purposes” also uses RCD to solve the actual situation in China. It has passed the test Abnormal conditions on L, N, and PE will quickly and automatically disconnect all connections with the power supply, laser cutting optic off the source of hidden dangers, eliminating the danger on the appliance body caused by the electrified ground wire, ensuring the safety of the user, and achieving a acceptable level of safety. National standard GB20044-2005 is a mandatory standard that has been implemented on August 1, 2006. Its publicity materials also fully explain the necessity of disconnecting the ground wire, the structural requirements when disconnecting, and the explicit method. .
With the charged water resistance method, when the ground wire is charged, Appendix AA of GB4706.12-2006 only assesses that the leakage current at the inlet and outlet of the electric water heater is below 5mA, and other parts of the appliance are not required to be assessed. According to the requirements of Article 27.1 of the standard, all accessible magnesium parts that may be live due to insulation failure must be connected to the ground terminal, such as shells and other accessible metal parts, which can be charged at this time. It is very dangerous to touch them. Considering the seriousness of the hazard, the publicity materials of GB4706.12-2006 also added assessment methods for other accessible metal parts. If the accessible metal parts adopt methods that cannot be grounded. According to the provisions of 5.14 in the main body of the standard, if a Class 0Ⅰ appliance or Class Ⅰ appliance has ungrounded accessible metal parts, and a grounded intermediate metal part is not used to separate it from live parts, the regulations for Class Ⅱ structures shall apply. According to the relevant requirements assessment, two solutions can be obtained, either separated by grounded intermediate metal parts, or according to the requirements of Class II structure. If such requirements are met, it can be considered an emergency protective measure.
Even if there is an alarm, the charged water resistance method cannot ensure safety.
Another focus of the GB4706.12-2006 standard is alarm measures. How effective are the alarm measures to ensure that the power supply is cut off? Standard AA22.7 stipulates that alarm measures should be taken on water heaters once an abnormality occurs in the external grounding system, and the alarm effect is guaranteed until the power is manually cut off. In this regard, the results achieved by the power-off method and the charged water resistance method are also different. In fact, from the perspective of the standard’s final acceptable safety level, alarming is not the purpose, but disconnecting the power supply is. The alarm is a means to achieve the purpose of disconnecting the power supply.
The power-off method has quickly cut off all connections to the power supply when the external grounding is abnormal, preventing personal electric shock. Moreover, the user can no longer power on the appliance and can detect problems in time. At this time, the appliance itself is no longer in any danger. , the purpose of safety is achieved, so in this sense, the necessity of alarming is not strong, even if it is needed at most to prompt the user that there is a problem with the grounding system of the home. This is a preferable solution. There is no need to use an alarm to ask the user to cut off the power supply. The user does not need to worry about the effectiveness of the alarm or the possible danger caused by not unplugging the plug in time.
The assessment of electric shock protection by the charged water resistance method, Appendix AA of GB4706.12-2006 only focuses on the accessible metal parts of the water inlet and outlet and the water outlet. Even if methods are used to make other accessible metal parts not electrified, the inside of the appliance, such as the water tank, is still electrified. , it is still very dangerous at this time, and the appliance can still work normally, so the danger has not been eliminated. Therefore, alarm measures are very necessary.
Moreover, the purpose of the alarm is to enable the user to unplug the power plug, which means that the user must hear the alarm and unplug the power cord to ensure safety. Due to the differences and non-professionalism of users, the effect of the alarm is very important. The alarm should be obvious and allow users to detect it in time. The installation location of the alarm and the location of the plug should also be considered so that users can detect it in time and make it easy for them to pull out the alarm. plug, so as to ensure that the danger is eliminated in time.
In summary, the power-off method fundamentally solves the dangers to personal life caused by abnormal external grounding. The solution to the danger of equipment is thorough and in line with the principle of safety. The charged water resistance method can only solve part of the problem. It must be cut off by the user to achieve ultimate safety. It is very risky. It relies on the user’s processing power and has great limitations. Th
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