X capacitor and Y capacitor

Capacitors in modern circuits and electronic applications are indispensable passive components for smoothing and filtering power lines and bypassing signals. In AC/DC EMC filtering applications, the two special X and Y capacitors used to filter AC power noise are generally referred to as "safety capacitors".

This article will explain how to use X or Y capacitors and why these safety capacitors are critical in modern circuit design.

What is X capacitor?

X-capacitors are commonly referred to as "line-to-line" or "across-line" capacitors and are used to reduce EMI/RFI caused by differential noise on the AC power supply. X-capacitors are connected across the live and neutral wires to filter out the negative effects caused by conducted interference, voltage surges, and voltage transients. X-capacitors provide a clean AC signal to the circuit and are therefore affected by all AC line variations, which can be dangerous if the voltage or power threshold of the capacitor is exceeded. Due to the special design of X-capacitors, their failure mode is short circuit, which triggers the circuit breaker or fuse to open under overstress. If the overcurrent protection device is not installed or does not respond, X-capacitors will pose a great risk of causing fire.

What is a Y capacitor?

Y capacitors are commonly called "line-to-ground" or "bypass" capacitors and are connected across the live and ground wires to handle EMI/RF caused by common mode noise. Y capacitors can also be affected by AC line variations due to conducted interference, overvoltage surges, and voltage transients, which can cause the capacitor to fail and become dangerous if the capacitance threshold is exceeded. The special design of Y capacitors is different from X capacitors, and its failure mode is open circuit, which causes the circuit to be affected by unfiltered AC power. Y capacitors have a lower risk of causing fire.

Safety capacitor classification

X capacitance (line to line)

Y capacitor (line to ground)

Image source: All About Circuits

Safety capacitor classification

Like many safety-critical components, the capacitance and threshold of safety capacitors are marked in various standards and corresponding classifications. There are many standards that define safety capacitors, such as IEC 60384-14, UL 1414, UL 1283, CAN/CSA C22.2 No.1, and CAN/CSA 384-14. The most common standard, IEC 60384-14, defines the peak pulse voltage for the safety level of X and Y capacitors.

According to IEC 60384-14, X capacitors are classified as follows:

-Category X3 – Impulse voltage less than or equal to 1.2 kV

-Category X2 – Peak impulse voltage less than or equal to 2.5 kV

-Category X1 – Peak impulse voltage between 2.5 kV and less than or equal to 4.0 kV

According to IEC 60384-14, Y capacitors are classified as follows:

-Category Y4 – Rated voltage less than 150VAC

-Category Y3 – Rated voltage less than or equal to 150VAC to 250VAC

-Classification Y2 – Rated voltage less than or equal to 150VAC to 300VAC

-Category Y1 – Rated voltage less than or equal to 500VAC

Types of X and Y capacitors

X and Y safety capacitors filter AC signals and reduce EMI, so they are directly connected to dangerous AC power supply voltages, so they must be certified as "safety capacitors" to ensure safe operation under these conditions. There are various types of safety capacitors used in safety filter circuits. Although the applications are not limited to this, ceramic surface mount and ceramic disc safety capacitors are commonly used in interference suppression and AC line filtering applications, while film capacitors are usually used to attenuate conducted emissions.

Ceramic or film capacitors can be used as X or Y capacitors, but factors such as the shape and characteristics of the capacitor make one type of capacitor more suitable for certain applications. Ceramic capacitors are often used in antenna coupling, primary and secondary coupling such as switching power supplies, and suppression of line interference such as high-speed switching control of motor control, relays, switching power supplies and inverters. Film capacitors have self-healing properties, making them ideal for capacitive power supplies, meters, automotive applications, and harsh environmental conditions.

in conclusion

X and Y capacitors filter AC power lines to reduce EMI and RFI in sensitive electronic applications. X and Y capacitors are classified and have different levels of continuous AC, peak pulse voltage and voltage transient capabilities. Regardless of the application, safety capacitors are essential to power signal-sensitive electronic equipment while reducing the risk of fire or electric shock.