Both belong to the same family. IEC 60384-14 groups them as electromagnetic interference suppression capacitors and splits that family into Class X and Class Y. A part from one class cannot be substituted for the other, and neither can be replaced by an ordinary film or ceramic capacitor of the same value.

Where Each Capacitor Sits, and What It Filters

The placement is the whole point. Inside an AC input filter, an X capacitor bridges line and neutral. Noise that travels out on one wire and returns on the other, which engineers call differential-mode noise, sees a low-impedance path straight through the capacitor instead of heading out along the mains cable.

A Y capacitor does the opposite job. Two of them are normally fitted: one from live to earth and one from neutral to earth. Common-mode noise, which pushes current in the same direction on both lines, is diverted to the chassis through these parts rather than leaving the enclosure on the power lead.

In an isolated supply, a Y capacitor often bridges the primary and secondary grounds as well. That gives high-frequency noise currents a controlled path across the isolation barrier instead of letting them radiate. The value used there has to sit well above the parasitic winding capacitance, which is why parts from 1 nF to 1 µF turn up in that position.

The Failure Modes That Explain the Ratings

The two classes are rated differently because they fail differently, and the standards are written backwards from the consequence of that failure.

An X capacitor that breaks down usually goes short circuit. It sits across the mains, so a short draws fault current through the input fuse and trips it. Nobody gets a shock from a part that is already sitting between two live conductors. What the design has to prevent is the capacitor overheating and igniting before the fuse clears, which is why X parts are built from self-healing metallized polypropylene film in a flame-retardant case.

A Y capacitor cannot be given that freedom. If it shorted, a live conductor would be connected to the metal chassis a user can touch. That is a shock hazard rather than a fuse problem. Y capacitors are therefore made to fail open, are given the highest impulse withstand in the family, and carry a much tighter limit on leakage current. The insulation class marked on a Y part, basic or reinforced, describes exactly this question.

X Capacitor Sub-Classes: X1, X2 and X3

Class X parts are graded by the peak impulse voltage they have to survive in service.

Above 1 µF the test voltage is derated so the product of voltage and capacitance stays constant, roughly 4/√C for X1 and 2.5/√C for X2. That detail matters when you compare a small part and a large one on the same datasheet, because the larger capacitor is tested to a lower voltage by design.

X2 is the workhorse. Almost every switch-mode supply, charger and appliance filter running on 230 V mains uses X2 parts, usually somewhere between 0.1 µF and 2.2 µF. X1 appears where the mains is expected to see heavier transients, and X3 has largely fallen out of use as X2 prices came down. Our Class X2 capacitor range covers the common 275 VAC and 310 VAC ratings in both boxed film and dipped constructions.

Y Capacitor Sub-Classes: Y1 Through Y4

Y parts are graded by insulation class and by the impulse they must withstand, which is a different question from the one Class X asks.

The split between Y1 and Y2 is about the insulation they are allowed to bridge rather than about capacitance. Y1 can sit across reinforced insulation, which is what a part needs if it bridges primary and secondary. Y2 is enough for line-to-earth duty in a normal Class I product. Y3 and Y4 cover lower-voltage situations, and in some designs they are placed in series to reach a reinforced rating.

Ceramic is the common dielectric here, in disc form, using Y5U, Y5V or Y5P material depending on the temperature range and stability required. Film versions of Class Y capacitors exist as well and get chosen when stability over temperature matters more than size.

What Really Limits the Capacitance Value

Two limits decide the capacitance you can actually use, and neither comes from filtering needs.

A Y capacitor’s value is capped by earth leakage current. The capacitor is a permanent path from the mains to the chassis, so it passes a small AC current the whole time the product is plugged in. For a single capacitor that current is:

I = 2πf × V × C

At 230 V and 50 Hz, a 4.7 nF capacitor passes roughly 0.34 mA. Once the second Y capacitor on the neutral side is added, the total rises again. Where that total has to land depends on the product and the standard it is built to. UL 1950 allowed up to 3.5 mA of earth leakage for information technology equipment, while medical and some household standards are far tighter, in the region of 0.35 to 0.7 mA. Tight limits are why Y values cluster between 1 nF and 4.7 nF and rarely pass 10 nF, no matter how much filtering you would like to have.

An X capacitor has the opposite constraint. Because it stores charge straight across the mains, it has to be discharged quickly once the plug is pulled, or the pins stay live. IEC 62368 and IEC 60335 set the limit: the voltage across the capacitor must fall to a safe level within about a second of disconnection, commonly quoted as below 34 V. UL 1950 expressed the same idea as a time constant, requiring a capacitor larger than 0.1 µF to fall to 37% of the peak input voltage within one second for type A equipment, or ten seconds for type B.

A capacitor on its own cannot meet that, so an X capacitor is almost always paired with a bleeder resistor across it. The resistor value comes straight out of the time constant, and it has to survive mains voltage continuously. That is why X2 parts with an integrated bleeder resistor have become common in compact designs. A typical 0.1 µF X2 part rated 275 VAC with UL and SEMKO approval shows the shape of what this looks like in practice: 0.1 µF X2 safety capacitor, 275 VAC, flame retardant.

Choosing Between Them for a Real Design

In practice the choice is set by position, then narrowed by ratings.

The order of decisions that keeps a design out of trouble is: confirm the position, work out the impulse and overvoltage category the mains can present, pick the sub-class that covers it, then check the capacitance against the leakage or discharge limit before checking anything else. Price and package size come last.

Certification and Temperature Details

A safety capacitor is only a safety capacitor if it carries the approvals for the market it ships into. Look for the marks that matter in your region: UL and CSA for North America, VDE and ENEC for Europe, CQC for China, plus SEMKO and others depending on the end product. A part without those marks offers no guarantee on the failure behaviour described above, even when the value and voltage look identical on paper.

Temperature is another trap. Older certificates covered −40 °C to +85 °C, while current safety approvals commonly require −40 °C to +125 °C. A design that copies a legacy part number can end up with a component that no longer matches the certificate the rest of the board was qualified around.

FAQ

Can I use a Y capacitor where an X capacitor belongs?

No. The two are rated for different positions. A Y part is optimised to fail open and to hold leakage down, and it is not tested for the pulse duty that an X position across the mains presents.

Why are Y capacitors so much smaller than X capacitors?

Their value is limited by earth leakage current rather than by filtering need. A few nanofarads is usually the most you can pass to earth before the product breaks its leakage limit.

What does the X2 marking actually mean?

It means the part is rated to survive peak impulse voltages up to 2.5 kV in service and has passed the corresponding test in IEC 60384-14. On its own it says nothing about capacitance or case size.

Do X capacitors need a resistor?

In practice, yes. A bleeder resistor is what brings the stored voltage down after the mains is removed, and the discharge time constant the safety standards ask for cannot be met by the capacitor alone.

Xuansn manufactures Class X2 and Class Y safety capacitors with UL, VDE, CQC and ENEC approvals, in both film and ceramic constructions, and we can supply the certificate documentation your product file needs. If you are working through a filter design or a cross-reference, send the position, working voltage and any leakage limit to coco@xuanxcapacitors.com, or contact us here with the specification, and we will come back with a part that fits the certificate your product needs.

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