Safety capacitor is divided into x type and y type. The AC power input consists of three terminals: live Line L/ Neutral line N/ Ground line G, (L=Line,N=Neutral, G=Ground). Straddling the "L-N", that is, the "live wire to zero wire", is the X capacitance; Straddling the "L-G/N-G" between the "live wire - ground wire or neutral wire - ground wire" is the Y capacitance. The indirect capacitance of live wire and neutral wire is like "X", while the indirect capacitance of live wire and ground wire is like "Y". These are not divided by what material.
X capacitor: As the location of this capacitor connection is also critical, it also needs to meet relevant safety standards. X capacitor is also one of the safety capacitors. The capacitance of the X capacitor is allowed to be larger than that of the Y capacitor as required. However, a safety resistor must be connected to both ends of the X capacitor in parallel to prevent the plug of the power cable from being charged and discharged for a long time when the power cable is removed and inserted. The safety standard states that when the power cord of a machine in operation is removed, the voltage (or potential to ground) at both ends of the plug of the power cord must be less than 30% of the original rated operating voltage within two seconds.
As one of the safety capacitors, X capacitors must also be certified by safety testing institutions. X capacitors are generally marked with safety certification mark and voltage resistant AC250V or AC275V words, but their real DC voltage resistance is up to more than 2000V, do not use ordinary capacitors such as nominal voltage resistant AC250V or DC400V to substitute.
Generally,X capacitors choose polyester film capacitors with relatively large ripple current. This type of capacitor has a larger volume, but it also allows a large amount of current to be charged and discharged instantaneously, and its internal resistance is correspondingly small.
Y capacitor: The connection position of the two Y capacitors is critical and must meet relevant safety standards to prevent electrical leakage of electronic equipment or electrization of the casing, which is easy to endanger personal safety and life. They are all safety capacitors, which requires that the capacitance value cannot be too large, and the voltage must be high. Generally speaking, the ground leakage current of the machine working in the subtropical zone should not exceed 0.7mA. Working in the temperate zone, the earth leakage current should not exceed 0.35mA. Therefore, the total capacity of the Y capacitor cannot exceed 4700PF (472).
The capacitance of Y capacitor must be limited so as to control the leakage current flowing through it at rated frequency and rated voltage and the impact on the EMC performance of the system. GJB151 specifies that the capacity of the Y capacitor should not be greater than 0.1uF. In addition to conforming to the corresponding power grid voltage and voltage withstand, Y capacitor is also required to have sufficient safety margin in terms of electrical and mechanical properties to avoid breakdown and short circuit in extremely harsh environmental conditions. The voltage withstand performance of Y capacitor is of great significance for personal safety. It is especially pointed out that Y capacitor, as a safety capacitor, must be certified by the safety testing institution. Y capacitors are usually orange or blue in appearance, and are marked with safety certification labels (such as UL and CSA) and voltage resistant AC250V or AC275V. However, its true DC withstand voltage is up to over 5000V. It must be emphasized that the Y capacitor should not be arbitrarily substituted by a nominal AC250V or DC400V capacitor.