Capacitor Frequently Asked Questions

What is an “X” capacitor?

An “X” capacitor is used in Across-the-Line applications. Typically between the Hot and the Neutral power lines in North American applications and between L-1 and L-2 in European and other power line applications. The definition for “X” capacitor application is: “where damage to the capacitor will not lead to the danger of electrical shock”. The X classification, is divided into three sub-classifications, X1, X2, X3 as defined by IEC664 categories I, II III.

What is a “Y” capacitor?

A “Y” capacitor is used in Line-Bypass applications. Typically between the Hot line and Ground or Neutral and Ground in North American applications and between L-1 or L-2 and Ground in European and other power line applications. The definition for “Y” capacitor application is: “where damage to the capacitor may involve the danger of electrical shock” The “Y” classification is divided into four sub-classifications, Y1, Y2, Y3, Y4 as defined by EN132400.

Why are safety agency approvals required for X and Y capacitors?

Across-the-Line(X) and Line-Bypass(Y) capacitors are subjected to the rigors of the AC Power Line. This exposure can be quite severe considering the opportunity for the power lines to conduct electrical noise cause by natural and manmade sources. To assure the ability of X & Y capacitors to survive these surges, test levels have been devised, by international safety agencies, to assure the proper function of capacitors, in power line applications.

What are the common uses for these capacitors?

The most common use for X & Y capacitors is in EMI/RFI filtering applications where they are directly connected to the AC Power line. These types of capacitors may also be used to suppress electrical noise generated by motors, inverters and electronic ballasts. They are also used in solid state relay snubbers and spark quenchers for suppression of inductive transients.

How do I select a capacitor for my application?

The first consideration is the voltage requirements for the capacitor. Typically, most X and Y capacitors have a voltage rating of at least 250VAC. This means that the are acceptable for use in any application with a voltage requirement up to and including 250VAC. Note that 250VAC equates to 350VDC. Add the +10% tolerance onto both of these values and the actual maximum ratings are 275VAC and 385VDC.

The next consideration should be the temperature rating. The typical temperature rating for X &Y caps is 85C. However, many series are now being designed with 100C ratings. This is particularly helpful in applications where parts are placed extremely close together and heat may be a factor.

Finally, the capacitance value required will establish the insertion loss characteristics required for the application and determine the physical size of the capacitor(s).

Over recent years there has been a major thrust to reduce the size of all components in the power supply section of electronics products. This has caused the physical size of X&Y caps to shrink significantly. Continuing design work is developing smaller capacitor sizes as new materials and manufacturing techniques are employed in capacitor design.

Until recently the capacitance value of the X cap has been limited to 2.2mFd. Recently, due to customer demand, these values have been increased and are now available up to 10mFd and 300VAC. This is particularly important in the larger filter designs such as those used in 3-phase applications.

What if my voltage requirement is greater than the capacitor voltage rating?

Remember that if you put two capacitors in series the capacitance value decreases, but the voltage value increases. If you required a capacitor in an application of 5mFd at 480VAC what would you do? By using two 10 mFd, 300VAC capacitors in series you would have a circuit capacitance of 5mFD and a voltage rating of 600VAC. The correct capacitance value, and a voltage rating greater than the 480VAC requirement.