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Understanding Ceramic Safety Capacitors (Class X & Y)

Class-X and Class-Y capacitors are safety-certified and generally designed and used in AC line filtering in many electronic device applications. These safety capacitors are also known by other names, including EMI/RFI suppression capacitors and AC line filter safety capacitors. (EMI stands for electromagnetic interference and RFI stands for radio-frequency interference; RFI is simply higher-frequency EMI.)

Class-X and Class-Y capacitors help to minimize the generation of EMI/RFI and the negative effects associated with received EMI/RFI. 

In order for these capacitors to perform their EMI/RFI filtering tasks, they are directly connected to the AC power input, that is, the AC “line” and the AC “neutral” (see Figure 2 below). And because of this direct connection to the AC voltage, the capacitors may be subjected to overvoltages and/or voltage transients—lightning strikes, power surges. Thus, capacitor failure is a very real possibility.

When a Class-X capacitor, also referred to as an "across the line capacitor"—the capacitor placed between line and neutral—fails because of an overvoltage event, it is likely to fail short. This failure, in turn, would cause an overcurrent protective device, like a fuse or circuit breaker, to open. Therefore, a capacitor failing in this fashion would not cause any electrical shock hazards.

If a Class-Y capacitor, also known as the "line to ground capacitor" or "the line bypass capacitor"—the capacitor placed between line and ground—fails short, this could lead to a fatal electric shock due to the loss of the ground connection. Class-Y safety capacitors are designed to fail open. A failure will cause your electronic device to be subjected to the noise and interference that the capacitor would normally filter out, but at least there will be no fatal electric shock hazard.

Applications for Class-X and Class-Y Capacitors

Subclass X2 and Y2 are the most commonly used safety-certified capacitors. Depending upon your own application and requirements, they are probably the ones you'll want to use. This is assumed because X2 and Y2 safety capacitors are used in common appliances that operate from ordinary household wall outlets. To be clear, you should select your Class-X and Class-Y capacitors according to your design's purpose and requirements.

Whereas X2 and Y2 caps are appropriate for household applications, X1 and Y1 safety capacitors are used in industrial settings. As an example, a subclass X1 safety capacitor would be used for an industrial lighting ballast that is connected to a 3-phase line.

Of course, you could always use subclass X1 and Y1 in non-industrial applications, but you'll be spending more money and the larger sizes may prove inconvenient.

You might be asking, are X2 and Y2 safety capacitors interchangeable?

A Y2 capacitor can safely be used in place of an X2 capacitor, but an X2 capacitor should not be used in place of a Y2 capacitor. This is because, although an X2-type capacitor would work and filter noise sufficiently, it would not meet the line-to-ground safety standards. Y2 safety capacitors are more robust, are able to withstand higher peak impulse voltages, and are designed to fail open as opposed to failing short.

There are also safety caps that combine aspects of X and Y types, such that they have met both X and Y safety requirements and standards. So for an X1/Y1 combination, this simply means that the capacitor can be used either as an X1 capacitor in a line-to-line application or as a Y1 capacitor in a line-to-ground application.

Because Class-X and Class-Y capacitors must be connected directly to AC lines (line-to-neutral or line-to-ground) in order for them to perform their EMI and RFI filtering functions, they must be rated and certified as "safety capacitors."

Both Class-X and Class-Y capacitors have subclasses: subclass X1, X2, and X3, and subclass Y1, Y2, Y3, and Y4. Subclass X2 and Y2 are the most common type of subclass for applications that use 120VAC (USA) or 220/240VAC (Europe). X/Y combination capacitors are also available, so you might consider using one of these, as well.