Knowledge of safety capacitors
Time：2021.09.29
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Safety capacitors are used in such occasions that after the capacitor fails, it will not cause electric shock and will not endanger personal safety. It includes X capacitors and Y capacitors. The x capacitor is the capacitor connected between the two lines of the power line (LN), and metal film capacitors are generally used; the Y capacitor is the capacitor connected between the two lines of the power line and the ground (LE, NE), and they usually appear in pairs. . Due to the limitation of leakage current, the Y capacitor value cannot be too large. Generally, the X capacitor is uF and the Y capacitor is nF. The X capacitor suppresses differential mode interference, and the Y capacitor suppresses common mode interference.
Safety grade of safety capacitor
Safety Capacitor Safety Level Allowable peak pulse voltage overvoltage level in application (IEC664)
X1 >2.5kV ≤4.0kV Ⅲ
X2 ≤2.5kV Ⅱ
X3 ≤1.2kV ——
Safety capacitor safety class Insulation type Rated voltage range
Y1 Double insulation or reinforced insulation ≥ 250V
Y2 basic insulation or additional insulation ≥150V ≤250V
Y3 basic insulation or additional insulation ≥150V ≤250V
Y4 basic insulation or supplementary insulation <150V
The capacitance of the Y capacitor must be limited, so as to achieve the purpose of controlling the magnitude of the leakage current flowing through it under the action of the rated frequency and rated voltage and the impact on the EMC performance of the system. GJB151 stipulates that the capacity of Y capacitor should not be greater than 0.1uF. In addition to complying with 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-circuit under extremely harsh environmental conditions. The withstand voltage performance of Y capacitors Protecting personal safety is of great significance.
Precautions
Capacitors for suppressing electromagnetic interference of power supply
When using capacitors to eliminate noise in power supply cross-line circuits, not only normal voltages, but also abnormal pulse voltages (such as lightning) must be considered, which may cause the capacitors to smoke or catch fire. Therefore, the safety standards for cross-line capacitors are strictly regulated in different countries, so safety-certified capacitors must be used.
It is not allowed to use DC capacitors as cross-line capacitors
The X2 type capacitor for suppressing electromagnetic interference of power supply should be suitable for occasions that will not cause the danger of electric shock when the capacitor fails, such as the power supply is connected across the line, and can withstand a pulse voltage of 2.5kV.
The Y2 capacitor for suppressing power supply electromagnetic interference should be suitable for occasions that will not cause electric shock when the capacitor fails. When the power supply is connected across the line, it can withstand 5kV pulse voltage impact without causing breakdown.
Safety capacitors adopt IEC standards, advanced production technology and strict quality assurance system.
According to IEC 60384-14, capacitors are divided into X capacitors and Y capacitors. 1. X capacitor refers to the capacitor across L-N, 2. Y capacitor refers to the capacitor across L-G/N-G. (L=Line, N=Neutral, G=Ground)
X capacitors are divided into X1, X2, X3. The main differences are: 1. The high voltage resistance of X1 is greater than 2.5 kV, and less than or equal to 4 kV, 2. The high voltage resistance of X2 is less than or equal to 2.5 kV, and 3. The high voltage resistance of X3 is less than or equal to 1.2 kV.
Y capacitors are divided into Y1, Y2, Y3, Y4. The main differences are: 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
X and Y capacitors are safety capacitors, the X capacitor is between the live line and the neutral line, and the Y capacitor is between the live line and the ground. They are used in the power supply filter to filter the power supply and interfere with common mode and differential mode respectively. It plays a filtering role. Safety capacitors are used in such occasions, that is, after the capacitor fails, it will not cause electric shock and does not endanger personal safety. The peak pulse voltage overvoltage level allowed in the safety level of safety capacitors (IEC664) X1> 2.5kV ≤4.0kV Ⅲ X2 ≤2.5kV Ⅱ X3 ≤1.2kV ——Safety capacitor safety grade Insulation type Rated voltage range Y1 Double insulation or reinforced insulation ≥ 250V Y2 Basic insulation or additional insulation ≥150V ≤250V Y3 Basic insulation or additional insulation Insulation ≥150V ≤250V Y4 Basic insulation or additional insulation <150V The capacitance of Y capacitors must be limited, so as to achieve the purpose of controlling the magnitude of the leakage current flowing through it under the action of the rated frequency and rated voltage and the impact on the EMC performance of the system . GJB151 stipulates that the capacity of Y capacitor should not be greater than 0.1uF. In addition to complying with 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-circuit under extremely harsh environmental conditions. The withstand voltage performance of Y capacitors Protecting personal safety is of great significance
There is an X capacitor on the filter circuit, that is, it is connected across the L-N line; the Y capacitor is the N-G line. In the safety standard, there are X1, X2, X3 capacitors according to the pulse voltage; Y1, Y2, Y3 according to the insulation level. (These are not classified according to what material, so I will learn more later.) As for the safety standards, there are some differences between countries, but the rated voltage is nothing more than 250 and 400. The safety capacitors made by major manufacturers are to meet the requirements of this safety standard. A safety capacitor can meet the requirements of Y capacitors, or it can be made to meet the requirements of X capacitors. Therefore, some safety capacitors are marked with X1Y1, X1Y2...
A capacitor connected between the live wire and the 0 wire is X, and a capacitor connected between the live wire and the ground wire is like a Y. Due to the direct capacitance between the live wire and the 0 wire, it is affected by the voltage peak to avoid short circuits. The more important parameter is the withstand voltage level, and there is no fixed limit on the capacitance value. The direct capacitance between the live wire and the ground wire is related to the problem of leakage safety, so the parameter it pays attention to is the insulation level. As James Bai said, too large capacitance will have an impact on the device after the power is cut off.
safety certificate
1. As a safety capacitor, the Y capacitor must be certified by a safety inspection 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 true DC withstand voltage is as high as 5000V or more. It must be emphasized that Y capacitors should not be replaced by ordinary capacitors such as nominal withstand voltage AC250V or DC400V.
2. The X capacitor, as one of the safety capacitors, also requires the certification of a safety inspection agency. X capacitors are generally marked with safety certification marks and withstand voltage AC250V or AC275V, but their real DC withstand voltage is as high as 2000V or more. Do not use ordinary capacitors such as nominal withstand voltage AC250V or DC400V at will.
Breakdown problem
If the Cx capacitor in the filter is broken down, it is equivalent to a short circuit of the AC grid, which at least causes the equipment to stop working; if the Cy capacitor is broken down, it is equivalent to adding the voltage of the AC grid to the housing of the device, which directly threatens personal safety and affects The safety of all circuits or equipment with a metal enclosure as the reference ground often leads to the burning of certain circuits or equipment.
actual needs
According to actual needs, the capacitance of the X capacitor is allowed to be larger than the capacitance 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 line from being plugged and unplugged due to the charging and discharging process of the capacitor. Cause the power cord plug to live for a long time. The safety standard stipulates that when the power cord of the working machine is unplugged, within two seconds, the charged voltage (or ground potential) at both ends of the power cord plug must be less than 30% of the original rated working voltage.
