Electronics

Classification of safety capacitors

Safety regulation capacitors are divided into X-type and Y-type. AC power input is divided into three terminals: live line L / zero line n / ground line g, (L = line, n = neutral, g = ground). Across “L-N”, that is, between “live line and zero line”, is x capacitance; Across “L-G / N-G”, that is, between “live wire ground wire or zero wire ground wire”, is the Y capacitance. A capacitor between the live wire and the zero wire is like an “X”, while a capacitor between the live wire and the ground wire is like a “Y”. These are not divided according to what material.

X capacitor: since the connection position of this capacitor is also critical, it also needs to comply with relevant safety standards. X capacitor is also one of the safety capacitors. According to the actual needs, the capacitance value of X capacitor is allowed to be larger than that of Y capacitor, but at this time, a safety resistor must be connected in parallel at both ends of X capacitor to prevent the power line plug from being charged for a long time due to the charging and discharging process of the capacitor when the power line is unplugged. The safety standard stipulates that when the power line of the working machine is unplugged, the charged voltage (or potential to ground) at both ends of the power line plug must be less than 30% of the original rated working voltage within two seconds.

As one of the safety capacitors, the X capacitor must also obtain the certification of the safety testing organization. X capacitors are generally marked with safety certification mark and withstand voltage AC250V or ac275v, but their real DC withstand voltage is more than 2000V. When using, do not arbitrarily use ordinary capacitors such as nominal withstand voltage AC250V or dc400v.

Generally, polyester film capacitors with large ripple current are mostly used as X capacitors. The instantaneous charge and discharge capacitance is large, but its internal resistance is also large.

Y capacitor: the connection position of these two Y capacitors is critical and must comply with relevant safety standards to prevent electric leakage of electronic equipment or electrification of casing, which is easy to endanger personal safety and life. They all belong to safety capacitors, so the capacitance value must not be too large, and the withstand voltage must be high. Generally, the earth leakage current of machines working in subtropical zone shall not exceed 0.7ma; For machines operating in temperate zone, the leakage current to ground shall not exceed 0.35ma. Therefore, the total capacity of Y capacitor shall not exceed 4700pf (472).

The capacitance of Y capacitor must be limited, so as to control the leakage current flowing through it and its impact on the EMC performance of the system under the action of rated frequency and rated voltage. Gjb151 stipulates that the capacity of Y capacitor shall not be greater than 0.1uF. In addition to meeting the corresponding grid voltage withstand voltage, the Y capacitor is also required to have sufficient safety margin in terms of electrical and mechanical properties to avoid breakdown and short circuit under extremely harsh environmental conditions. The withstand voltage performance of Y capacitor is of great significance to protect personal safety. In particular, it is pointed out that the Y capacitor as a safety capacitor must obtain the certification of the safety testing organization. The appearance of Y capacitor is mostly orange or blue. Generally, it is marked with safety certification mark (such as UL, CSA, etc.) and withstand voltage AC250V or ac275v. However, its real DC withstand voltage is more than 5000v. It must be emphasized that ordinary capacitors such as nominal withstand voltage AC250V or dc400v shall not be used as Y capacitors at will

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