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To reach the new target of electric water heater safety requirements of the technical path analysis
The new safety standard GB4706.12-2006 "Household and similar safety storage water heaters special requirements" (hereinafter referred to as the standard) Equivalent to IEC60335-2-21: 1997, and will be July 1, 2007 implementation, The main modification is the addition of appendix AA, the specific content is "additional requirements for Class I electric water heaters that provide emergency protective measures in the event of an abnormal grounding system". For solutions conforming to Appendix AA, "Water resistance method" two main programs. In this regard, I believe that the "power-off method" is a better solution, "charged water resistance method" from the technical point of view can not fully meet the safety objectives. According to the title of appendix AA, "Additional Requirements for Class I Water Heaters Providing Emergency Protective Measures in the Event of Abnormal Grounding System", the problem to be solved is that the grounding system is abnormal, especially the ground anomalies other than appliances, And ground live two kinds of situations, the following on the power-off method and charged water resistance method in both cases can achieve the effect of a specific analysis. Power-off method is an effective way to solve the lack of ground wire given by the international standard system Grounding loss refers to the lack of ground wire in the fixed wiring caused by no ground protection, you can also have a ground wire but the ground or ground resistance is too large For this. Many households in rural China are more ungrounded due to the low-voltage power supply system, and in fact these families are also using a large number of Class I appliances. At present, there are mature proposals in the world, and the appendix P in the new version IEC60335-1: 2004 (GB4706.1-2005) is the solution to this problem. Appendix P solves the problem of how to use Class I appliances in the absence of a grounded power supply due to the loss of fixed wiring. The solution is to use a PRCD Leakage Protector (ie, a power-off method) that allows the configuration of a residual current Not more than 30mA leakage protector, at the same time the appliance requirements of the experiment put forward more stringent standards, such as leakage current does not exceed 0.5mA, increase the humidity resistance temperature t value of 37 ℃, the ambient temperature increased to 40 ℃, heating temperature reduced to 15 ° C. If the above mentioned experimental requirements are met, the appliance is considered to be used in an environment where the external ground is missing, reaching an acceptable level of safety. 80s of last century, China began to vigorously promote the leakage protector, the effect is very obvious, a substantial decline in accidents, fully proved that the installation of leakage protection for additional protection against electric shock protection is very effective, the power grid security has played a huge role . Charged water resistance method can also be safe when the ground is missing? Currently on the market charged water resistance method refers to the use of water resistance principle to reduce the outlet current method. The usual practice is to add a section of plastic pipe to the water heater inlet and outlet pipes, in the event of a leakage risk, the water resistance of the plastic pipe to reduce the outlet current to reduce the risk of electric shock. According to the standard definition, Class I appliances not only rely on the basic protection of the electric shock protection, and must include an additional safety precaution that safety ground protection, with these two protection, to achieve an acceptable level of safety. The lack of ground means that the water heater has only basic insulation and no ground protection. So, charged water resistance of a plastic pipe can be equivalent to the ground protection? The answer is obviously no. There is no grounding protection to achieve safety requirements, equivalent to meet the requirements of Class II appliances. According to national standards, Class II appliances have protection against electric shock not only by basic insulation, but also by additional safety precautions such as double insulation or reinforced insulation. A section of plastic pipe with water resistance method obviously can not be equivalent to additional insulation or combined with the basic insulation to achieve enhanced insulation. Therefore, the lack of live water resistance in the ground can not be considered an acceptable level of safety. Power-off method can be better to solve the problem of ground wire live Grounding refers to the ground wire in the fixed wiring, but if the ground does not play the role of ground protection, but is a dangerous source, this situation for all Class I appliances Are very dangerous. At this point, there is an abnormal voltage or current in the ground, it is due to many reasons, such as phase short circuit due to insulation aging, plug, socket contact resistance for a long time to increase the heat caused by phase short circuit, There is a lack of necessary wiring in the fixed protection. IEC61540 solves the problem of grounding the ground and provides solutions that allow the use of RCDs that open and close PE conductors when there is a hazard on the ground and experience of using such RCDs in other countries such as Australia and Germany. For China's special use of the national conditions, the new national standard GB20044-2005 "electrical accessories for household and similar purposes without overcurrent protection mobile residual current device (PRCD)" also uses the RCD to solve the actual situation in China, which passed the test L, N, PE on the abnormal situation, quickly and automatically disconnect all connections with the power supply, cut off the root causes of hidden dangers, eliminating the danger of the appliance body due to live ground charged to ensure the safety of users, to achieve a Acceptable level of safety. National standard GB20044-2005 is a compulsory standard, August 1, 2006 has already begun to implement, in its propaganda material also fully illustrates the need to disconnect the ground, disconnect the structure of the requirements and express methods . Live water resistance method in the case of live ground, GB4706.12-2006 Appendix AA only examines the inlet and outlet of electric water heater leakage current 5mA below, the other parts of the appliance did not require assessment. As required by Standard 27.1, all accessible metal parts that may be subject to insulation failure are to be connected to ground terminals, such as housings and other accessible metal parts, in which case power may be applied as long as access is very dangerous. Taking into account the seriousness of the danger, publicity materials GB4706.12-2006 also increased the assessment of other accessible metal parts, if the accessible metal parts can not be grounded approach. According to the standard text in 5.14, if Category 0I appliances or Class I appliances with ungrounded accessible metal parts, and did not use a grounded intermediate metal parts separated from the live parts, according to the provisions of the structure of type II Of the relevant requirements of the examination, which can be two solutions, or by grounding the middle of the metal parts separated, or according to the requirements of Ⅱ class structure requirements. If such a requirement is met, it can be considered an emergency precaution. Charged water resistance method even if there is an alarm, it can not ensure safety Another focus of GB4706.12-2006 standard is the alarm measures, alarm effect how to ensure that the power cut off. In the standard AA22.7 provides that the water heater should be taken once the external grounding system, anomalies occur when the alarm measures, the alarm effect to ensure that manpower cut off until the power. In this regard, the power-off method and charged water resistance method can achieve the results are different. In fact, the alarm is not an end in terms of the standard, ultimately acceptable level of safety, and the purpose of disconnecting the power supply is to provide a means of disconnecting the power supply. Power-off method due to the external ground fault has been cut off quickly with all the power connection to prevent the occurrence of personal shock, and the user can no longer use electricity appliances, has been able to promptly find a problem, this time the body has no danger of appliances , To achieve the purpose of security, so in this sense, the necessity of the alarm is not strong, even if the need to remind the user's home grounding system is at fault. This is a more desirable solution, so users do not have to worry about the effectiveness of the alarm and do not have to worry about the danger that may be caused by the unplugged plug in time. Charged water resistance method for the assessment of electric shock protection, GB4706.12-2006 Annex AA is only accessible to the inlet and outlet of accessible metal parts and water, even if the method used to make other accessible metal parts are not charged, but the appliance is also charged inside the tank , This time is still very dangerous, and the appliance can also work properly, the danger has not been eliminated. Therefore, the alarm measures are very necessary. Moreover, the purpose of the alarm is to allow the user to unplug the power plug, which means that the user wants to hear the alarm and unplug the plug to ensure safety. Due to user differences, non-professional, so the effect of the alarm is very important, the alarm should be obvious, allowing users to find out, and where the alarm installation location and the plug must also take into account allows users to find timely and user-friendly Plug, so as to ensure the timely removal of the danger. To sum up, the power-off method fundamentally solves the danger to the human body caused by the abnormality of the external grounding, and the solution to the danger of the utensil is thorough and in line with the principle of safety. The charged water resistance method can only solve part of the problem, must be cut off the power with the user in order to achieve the ultimate safety, there is a big risk, it depends on the user's processing power, there are great limitations, the appliance itself does not necessarily guarantee safety .