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.
This is used to describe safety approved capacitors whose failure in a circuit will present danger of electrical shock. For example, a capacitor connected between the AC power line and the ground conductor in a circuit to reduce common mode noise. (UL and CSA refer to this a "Line-by-pass noise"). The capacitance is limited up to 4700pF so leakage current can be controlled.