How to distinguish between X capacitors and Y capacitors?

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How to distinguish between X capacitors and Y capacitors? [Copy link]

At the input end of the AC power supply, three safety capacitors are generally needed to suppress EMI conduction interference. The AC power input is divided into three terminals: live wire (L)/neutral wire (N)/ground wire (G). The capacitor connected in parallel between the live wire and the neutral wire is generally called an X capacitor. Since the position of this capacitor connection is also critical, it also needs to comply with relevant safety standards. The X capacitor is also one of the safety capacitors. According to actual needs, the capacitance of the X capacitor is allowed to be larger than that of the Y capacitor, but at this time a safety resistor must be connected in parallel at both ends of the X capacitor to prevent the power cord plug from being charged for a long time due to the charging and discharging process of the capacitor when the power cord is unplugged.

Safety standards stipulate that when the power cord of a working machine is unplugged, within two seconds, the voltage at both ends of the power cord plug (or the ground potential) must be less than 30% of the original rated working voltage.

The capacitors connected in parallel between the live wire and the ground wire and between the neutral wire and the ground wire are generally referred to as Y capacitors. The connection positions of these two Y capacitors are critical and must comply with relevant safety standards to prevent leakage of electronic equipment or charging of the casing, which may easily endanger personal safety and life. They are all safety capacitors, which requires that the capacitance value cannot be too large and the withstand voltage must be high. Generally speaking, for machines working in the subtropical zone, the ground leakage current is required not to exceed 0.7mA; for machines working in the temperate zone, the ground leakage current is required not to exceed 0.35mA. Therefore, the total capacity of Y capacitors generally cannot exceed 4700PF (472).

Specially pointed out: As one of the safety capacitors, X capacitors must also be certified by a safety testing agency. X capacitors are generally marked with a safety certification mark and a withstand voltage of AC250V or AC275V, but their actual DC withstand voltage is as high as 2000V or more. When using them, do not use ordinary capacitors with a nominal withstand voltage of AC250V or DC400V as substitutes.

As a safety capacitor, Y capacitors must be certified by a safety testing agency. Y capacitors are mostly orange or blue in appearance, and are generally marked with safety certification marks (such as UL, CSA, etc.) and withstand voltage AC250V or AC275V. However, its actual DC withstand voltage is as high as 5000V or more. It must be emphasized that Y capacitors must not be replaced with ordinary capacitors with a nominal withstand voltage of AC250V or DC400V. Usually, X capacitors are mostly polyester film capacitors with relatively large ripple current resistance. This type of capacitor is large in size, but the current it allows for instantaneous charging and discharging is also large, and its internal resistance is correspondingly small. The ripple current indicators of ordinary capacitors are very low, and the dynamic internal resistance is relatively high. Using ordinary capacitors to replace X capacitors, in addition to the capacitor withstand voltage not meeting the standard, the ripple current indicator is also difficult to meet the requirements. According to IEC 60384-14, capacitors are divided into X capacitors and Y capacitors. 1. X capacitors are capacitors across LN, 2. Y capacitors are capacitors across LG/NG. (L=Line, N=Neutral, G=Ground) X capacitors are further divided into X1, X2, and X3. The main differences are: 1. X1 withstands high voltage greater than 2.5 kV and less than or equal to 4 kV, 2. X2 withstands high voltage less than or equal to 2.5 kV, 3. X3 withstands high voltage less than or equal to 1.2 kV Y capacitors are further divided into Y1, Y2, Y3, and Y4. The main differences are: (DC voltage rating) 1. Y1 withstands high voltage greater than 8 kV, 2. Y2 withstands high voltage greater than 5 kV, 3. Y3 withstands high voltage n/a 4. Y4 withstands high voltage greater than 2.5 kV. They are used in power filters to filter the power supply, and filter the common mode and differential mode industrial interference. Safety capacitors are used in such occasions that after the failure of the capacitor, it will not cause electric shock and will not endanger personal safety. It includes X capacitors and Y capacitors. X capacitors are capacitors connected across the two lines of the power line (LN), and metal film capacitors are generally used; Y capacitors are capacitors connected across the two lines of the power line and the ground (LE, NE), and generally appear in pairs. Based on the limitation of leakage current, the value of Y capacitors cannot be too large. Generally, X capacitors are uF level and Y capacitors are nF level. X capacitors suppress differential mode interference, and Y capacitors suppress common mode interference. Safety level of safety capacitors Safety level of safety capacitors Peak pulse voltage overvoltage level allowed in applications (IEC664) X1 ＞2.5kV ≤4.0kV Ⅲ X2 ≤2.5kV Ⅱ X3 ≤1.2kV —— Safety level of safety capacitors Insulation type Rated voltage range Y1 Double insulation or reinforced insulation ≥ 250V Y2 Basic insulation or supplementary insulation ≥150V ≤250V Y3 Basic insulation or supplementary insulation ≥150V ≤250V Y4 Basic insulation or supplementary insulation <150V The capacitance of Y capacitor must be limited to achieve the purpose of controlling the leakage current flowing through it and its impact on the EMC performance of the system under the rated frequency and rated voltage.

GJB151 stipulates that the capacity of Y capacitors should not exceed 0.1uF. In addition to meeting the corresponding grid voltage withstand voltage, Y capacitors are also required to have sufficient safety margins in terms of electrical and mechanical properties to avoid breakdown and short circuits under extremely harsh environmental conditions. The withstand voltage performance of Y capacitors is of great significance to protecting personal safety. One more thing to note when choosing: 1. Capacitors for suppressing electromagnetic interference in power supply When capacitors are used in power cross-line circuits to eliminate noise, not only normal voltage, but also abnormal pulse voltage (such as lightning) must be considered, which may cause the capacitor to smoke or catch fire. Therefore, the safety standards for cross-line capacitors are strictly regulated in different countries, so capacitors that have passed safety certification must be used. 2. DC capacitors are not allowed to be used as cross-line capacitors: For X2-type capacitors used to suppress power supply electromagnetic interference, they should be used in situations where there is no risk of electric shock when the capacitor fails, such as when the power supply is connected across the line, and can withstand a 2.5kV pulse voltage. For Y2-type capacitors used to suppress power supply electromagnetic interference, they should be used in situations where there is no risk of electric shock when the capacitor fails. When used for power supply cross-line connection, they can withstand a 5kV pulse voltage impact without breakdown. 1. X capacitors, polystyrene (film ethylene) capacitors, polystyrene has a higher withstand voltage and is suitable for high-voltage pulse absorption in EMI circuits. 2. Capacitance calculation: Generally, for two-stage X capacitors, the first stage uses 0.47uF, and the second stage uses 0.1uF; for single-stage, 0.47uF.There is no convenient calculation method at present. (The size of the capacitor has no direct relationship with the power of the power supply) X capacitor refers to the peak voltage, not the rated voltage. X capacitors are generally MPX (metallized polypropylene mold capacitors). Their rated voltage is also several hundred volts. For example, an X2 is marked as MPX-X2-250VAC/275VAC.