Calm down Aunty Ethel! I'm not talking about "X Rated!" rather we will discuss "X Rated!" as well as Y and Z rated. Still not clear, then read on........
In the olden days, the X rating certificate, was issued between 1951 and 1982 by the British Board of Film Censors as a result of the Wheare Report on film censorship. From 1951 to 1970, it meant "Extremely graphic, where only those aged 16 and over can be admitted," and from 1970 to 1982 it was redefined as meaning "Suitable for those aged 18 and over". The X certificate was replaced in November 1982 by the 18 certificate.
I remember those straight laced days and me, I never even saw a naked lightbulb until I was 21 and to this day, I only ever see Mrs Mullard Magic divesting her kevlar foundation wear in pitch blackness whilst aided by a WW2 vintage Tabby Zamboni pile powered IR monoscope, however, I digress......
These days, we use X rated and Y rated to identify and characterise Class-X and Class-Y capacitors which minimise the generation of EMI/RFI and the negative effects associated with received EMI/RFI.
In order for these capacitors to perform their EMI/RFI filtering tasks, they are directly connected to the AC mains power input, that is, the AC “live” and the AC “neutral” and because of this direct connection to the AC voltage, the capacitors may be subjected to over voltages and/or voltage transients—lightning strikes and power surges, hence, capacitor failure is a very real possibility.
When a Class-X capacitor, also referred to as an "across the live capacitor"—is placed between live and neutral and fails because of an overvoltage event, it is likely to fail short. This failure, in turn, would cause an overcurrent protective device, like a fuse or circuit breaker, to open. Therefore, a capacitor failing in this fashion would not cause any electrical shock hazards.
If a Class-Y capacitor, also known as the "live to ground capacitor" or "the live bypass capacitor" — is placed between live and ground and fails short, this could lead to a fatal electric shock due to the loss of the ground connection. Class-Y safety capacitors are designed to fail open. A failure will cause your electronic device to be subjected to the noise and interference that the capacitor would normally filter out, but at least there will be no fatal electric shock hazard.
Going back to the old times again when My Mother-In-Law, the redoubtable Dowager Duchess June roamed the earth along with the dinosours some Erie capacitors were Y & Z but unfortunately not X rated as well as you can see from the following colourful advert from 1965 by Erie: -
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