Why is the Y-capacitor named so?

Question

I guess X-caps are named so since they are connected across the Line and return. Similarly is there any reason for naming Y-caps, which are connected between L/N and GND ?

Answer 1

Be careful that there are two uses to these terms:

• Officially: they are terms in the originating standards; 'X' and 'Y' may be implied as said, or are simply sequential letters chosen to label the locations. Likely the standards do not explain this, and you'd have to dig into notes by the working group that produced the standard, or ask actual members/participants, if they know why the letters were chosen.

• Informally: as capacitors of these ratings are so commonly used for these positions, the reverse meaning tends to be adopted. One might therefore refer to a "Y capacitor" in a generic DC-DC converter that doesn't even have mains connection, understood to mean the between-isolation EMI capacitor -- but even where such ratings might not be needed. Understand that this is only ever a shorthand "this is wrong but you know what I mean" kind of meaning; or to put it another way, the concepts are nearby in brain-space and one may perhaps use that thought-vector in reverse to convey nearby thoughts, with the downside that this behavior dilutes the specificity of the original definition.

  Or to put it more generally: language gon' language, and we observe evolution in meaning / word use for various reasons, often for expediency, and... rarely in the directional of technical accuracy.

As for typical standards, Safety Capacitors First: Class-X and Class-Y Capacitors | All About Circuits offers some ideas. In the EU, IEC 60384-14 describes capacitors for this purpose, and analogous (not necessarily harmonized!) standards exist in other countries (UL, CSA, etc.).