“Across-the-Line” and “Line-to-Ground” Capacitors

“Across-the-Line Capacitors”, or X Capacitors, serve a crucial role in many EMI (Electromagnetic Interference) and RFI (Radio Frequency Interference) applications. “Across-the-Line” Capacitors suppress electrical noise, provide protection against shock and provide operational insulation. Generally the dielectric of these “safety” capacitors are encased in a flame retardant case.

X Capacitors can be classified into three subclasses: Class X1, Class X2 and Class X3. The most common of these classes are X1 and X2. X1 Capacitors have enhanced impulse ratings and are used where very high transients (or high amplitude sounds) are likely to occur. X2 Capacitors are more often used in more general applications.

Another type of “safety” capacitor that suppresses RFI is a Y Capacitor or “Line-to-Ground” Capacitor. Unlike the X Capacitor, the Y Capacitor is designed to be connected between a current carrying conductor and a grounded conductor. The X Capacitor, on the other hand, is connected between two current carrying conductors or AC lines. Y Capacitors are manufactured in four classes; Y1, Y2, Y3 and Y4. The most common are Y1 and Y2.

Both X and Y Capacitors are rated according their voltage and peak pulse voltage. The peak pulse voltage indicates the extreme conditions, such as power surges and lightning strikes, these capacitors are able to withstand. Most household appliances contain X2 or Y2 capacitors. X2 Capacitors are impulse tested to 2.5kV and Y2 Capacitors are impulse tested to 5kV. However, when over-voltage occurs, these “safety” capacitors can fail. “Across-the-Line” Capacitors failure may result in an applications poor performance or a blown fuse but, there is no imminent danger to the user. “Line-to-Ground” Capacitors, however, can result in electrical shock or worse when the ground connection is lost.

Overall, these “safety” capacitors are designed to handle high voltage impulses while protecting the circuit and the user from harm.