The electricity supplied to consumers contains surges, voltage spikes, and electromagnetic noise. Voltage spikes and surges are mainly caused by heavy current draws while EMI/RFI noise is primarily due to electromagnetic and radio frequency interferences. These unwanted components can significantly affect the performance of electronic devices and appliances.

To reduce the effect of these harmful surges, voltage spikes, and EMI/RFI noises on electrical equipment and devices, an AC line filtering capacitor or EMI/RFI suppression capacitor is added to the circuit. Apart from suppressing electrical noise, these safety capacitors provide protection against fire and electrical shock.

Capacitors for AC line filtering applications

Most electrical equipment and appliances require AC line filtering. Capacitors are added to electronic circuits of electrical appliances to suppress harmful surges, voltage spikes, and EMI noises. These safety capacitors are commonly known as EMI/RFI suppression capacitors, and they provide necessary protective measures to electronic devices and appliances. Capacitors for AC line filtering are classified into Class-X and Class-Y capacitors.

Class X and Class Y capacitors are usually connected directly to the AC power input. Unlike ordinary capacitors, these components are exposed to voltage transients, power surges, lightning strikes, and over-voltages. The most common types of transients include transients from atmospheric interference, transients from on/off operation of electrical equipment, transients from network failures, and pulses from electrical equipment. Under these operation conditions, AC line filtering capacitors have a high chance of failing.

When selecting an AC input filter for a specific application, it is important to understand the source and characteristics of the interference. The positioning of the safety capacitor greatly depends on the location of the source of interference. It is advisable to place the safety capacitor as close to the source of interference as possible. This is commonly done to suppress interference generated by electrical machines such as motors, welders, and air conditioning units. In electronic ballasts, inverters, and switched mode power supply systems, the interference is distributed. In such cases, the filter is usually placed near the entry point of the AC cord.

Class X (line-to-line) safety rated capacitors

A Class-X safety capacitor is used to suppress symmetrical interference, and it is usually connected between a line and another line or between a line and a neutral. This capacitor is commonly referred to as the across-the-line capacitor. In some line-to-line connections, this safety capacitor is placed in series with the line. The dielectric materials that are mostly used for manufacturing X class capacitors include ceramic, polypropylene, polyester, and paper.

Failures in Class-X capacitors are primarily due to exposure to transients and voltage surges. When this capacitor fails, the failure is more likely to result in a short circuit. This short can trigger overcurrent protective devices such as fuses and circuit breakers to open. In some cases, this failure can result in fire. As long as a Class-X capacitor fails short, the failure is unlikely to result in electrical shock hazards. If a Class-X capacitor fails open, the performance of the electrical appliance is affected because there is no component to suppress interference.

Class-X capacitors are further subdivided according to their peak voltage rating and peak impulse voltage rating. These capacitors are sub-divided into three subcategories: X1, X2, and X3. Subclass X2 capacitors are mostly used in household applications while subclass X1 capacitors are commonly used for industrial applications. The characteristics of each subclass are defined by the International Electrotechnical Commission’s IEC 60384-14 standard.

Class Y (line-to-ground) safety rated capacitors

A Class-Y capacitor is used to bypass asymmetrical interference, and it is usually connected between a line and a ground or a neutral and a ground. These safety capacitors are commonly referred to as “line-to-ground” capacitors. Y class capacitors are commonly used in motors, electronic ballasts, inverters, and so on. These capacitors are mostly manufactured using paper or ceramic dielectric.

When a Y class capacitor fails, the line to ground connection is opened. This mode of failure exposes electronic devices and appliances to electrical noise and EMI/RFI interference. When a Class-Y capacitor fails open, it does not expose users to fatal electrical shock hazards. However, in case this safety capacitor fails short, the ground connection is lost, and this can result in fatal electric shocks.

Just like Class-X capacitors, Y class capacitors are sub-categorised according to voltage rating and peak impulse voltage rating. The subclasses of the Class-Y capacitors and their characteristics are defined by the International Electrotechnical Commission’s IEC 60384-14 standard. There are four sub-classes of Class-Y capacitors (Y1, Y2, Y3, and Y4), and each has its own voltage rating and peak impulse voltage rating. Subclass Y2 capacitors are commonly used in household appliances while Y1 capacitors are mostly used in industrial machines.

Although some Class-Y capacitors can be used in place of some Class-X capacitors, the latter cannot be used to replace the former. This is primarily because Class-X capacitors do not meet the line-to-ground safety requirements. However, some safety capacitors are designed to be used as either Class-X or Class-Y capacitors. Such capacitors usually have a combination designation. For instance, the designation X1/Y1 means that the capacitor can be used as either a subclass X1 capacitor or subclass Y1 capacitor.

Safety certified capacitors are usually subjected to various tests to ensure that they meet specific performance and reliability requirements. In most cases, safety agencies subject safety certified capacitors to endurance tests, impulse voltage tests, and active flammability tests. Moreover, safety certified capacitors come in a variety of sizes, dielectrics, and lead spacing options.


The electricity that is supplied to consumers contains unwanted and potentially harmful components. These components can significantly affect the performance of electrical appliances. Safety certified capacitors are used to suppress harmful components and reduce their effect on the performance of electronic devices. These capacitors are categorized as Class-X and Class-Y capacitors, and they are connected directly to lines. Class-X capacitors are usually used in across-the-line (line-to-line) applications while Class-Y capacitors are intended for line by-pass (line-to-ground) applications. Capacitors for AC line filtering or EMI/RFI suppression are tested, rated, and certified in accordance with the requirements of national, regional, or international safety agencies. Class-X and Class-Y safety certified capacitors are further grouped into subclasses. When selecting a capacitor for AC line filtering, it is important to ensure that the component meets the safety standard requirements.

Voltage Ratings

SubclassPeak Voltage
X1>2.5kV, ≤4.0Kv

SubclassRated Voltage
Y2150VAC≤ V <300VAC
Y3150VAC≤ V <250VAC