# Selection parameters of a X & Y rated capacitors

In a capacitor drop supply, X rated capacitors are responsible for dropping the AC voltage across them. I've been looking at why only X rated capacitors are used. After spending some time searching the internet, I found that X rated capacitors are connected in series (line to line) with the circuit, while Y rated capacitors are connected in parallel (line to ground).

The X rated capacitor is designed for 250, 400, 600 VAC. The effective impedance (Z), rectance (X) and the mains frequency (50 – 60 Hz) are the important parameters to be considered while selecting the capacitor. The reactance (X) of the capacitor (C) in the mains frequency (f) can be calculated using the formula:

$$X = \frac{1}{2 \pi fC}$$

So at 0.22uF the reactance will be $14.4k\Omega$. Now the effective impedance can be calculated by using

$$Z = \sqrt{R + X}$$

Now this Z is responsible for the dropping of the high voltage. So just because of high reactance (is it X rated capacitor property? Correct me if I am wrong), is it used to drop the supply voltage? What happens if we add another capacitor in series except X rated? Does the other one also work as a X rated capacitor?

• This site is family friendly; no X rated capacitors here... Aug 2, 2013 at 4:42
• @JYelton Means What??
– Atom
Aug 2, 2013 at 4:43
• Sorry SHASWAT, it's a joke referring to motion picture ratings. Aug 2, 2013 at 4:44
• oh great.Now I got your point.......
– Atom
Aug 2, 2013 at 4:46
• As far as I know the X and Y designation is the letter code for the low temperature coefficient of Class 2 ceramic capacitors. I'm not sure that alone could be classified as a capacitor "rating," hence the humor. Aug 2, 2013 at 4:48

X and Y type capacitors (as it turns out) are types of safety capacitors, usually for higher voltage. (The X and Y letter codes can also mean a low temperature coefficient for class 2 ceramic capacitors.)

Here are the relevant points from a Safety Capacitor Basics document from Tecate Group:

• X Capacitors are also known as "across the line" capacitors. They are used between the "live" wires carrying incoming AC current. They're used in applications where failure of the capacitor will not lead to risk of electrical shock to the user.

• Y Capacitors are also known as "line to ground" capacitors. They are used in applications where failure of the capacitor could lead to the danger of electrical shock to the user, if the ground connection is lost.

• So Y rated e.g can be used in EMI suppression?
– Atom
Aug 2, 2013 at 5:02
• Yes, page 2 of the linked document states "The function of these capacitors is to protect against surges and transients, as well as providing EMI filtering." Aug 2, 2013 at 5:03
• So if I connect another capacitor instead of X rated cap,then it can also works as same as X rated or something else happens??
– Atom
Aug 2, 2013 at 5:12
• Sorry SHASWAT I don't know that much about them. You might want to post that as a separate, specific question. Aug 2, 2013 at 5:16
• @SHASWAT, From an eletrical function point of view these are just capacitors. From a safety point of view these are constructed to be robust and to fail safely. Capacitors not requiring safety considerations are smaller and cheaper. X and Y capacitors will have been tested to withstand certain surges / spikes and not fail in a dangerous way. Aug 2, 2013 at 7:10