X and Y capacitors, also known as "AC line filter safety capacitors" and "EMI/RFI suppression capacitors," serve as crucial components of electrical equipment and electronic machinery, to which they supply operational insulation and various protective measures. Safety capacitors often consist of a ceramic disc or a metallised self-healing film made of paper, polyester, or polypropylene encased in a flame retardant case.
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X and Y capacitors suppress electrical noise and provide protection against electrical shocks and fires. They prevent equipment and machinery from sending and receiving electromagnetic and radio frequency interference. When connected between line phases (across the line), X capacitors effectively check symmetrical interference. Y capacitors, on the other hand, when connected in a line bypass -- that is, between a line phase and a point of zero potential -- keep out asymmetrical interference.
Nearly all EMI and RFI filtering applications make use of safety capacitors directly connected to an AC line. Generally, X capacitors work for across-the-line applications, and Y capacitors work for line bypasses or line-to-ground applications. A Y capacitor can take the place of an X capacitor in an across-the-line application only when absolutely necessary, though this is highly uneconomical. An X capacitor cannot perform well in a line bypass, and would fail most, if not all, line-to-ground safety standards. Common uses of safety capacitors also include motors, inverters, and electronic ballasts, as well as solid-state relay snubbers and spark quenchers.
Results of Capacitor Failure
Because of safety capacitors' direct connections to AC lines, capacitor failure can sometimes result from exposure to over-voltages and transients. Failure of an X capacitor in an across-the-line connection would not cause electrical shock, but it could open safety fuses or circuit breakers and cause fire. Y capacitor failure, on the other hand, can give the user a potentially fatal electric shock because of the loss of the ground connection. Various safety agencies closely monitor the performance of safety capacitors by subjecting them to impulse voltage tests, endurance tests and active flammability tests.
Safety standards classify X and Y capacitors according to their rated voltage and the peak pulse voltage they can safely withstand. Peak pulse voltage refers to extreme conditions the capacitor may have to experience, such as sudden power surges and lightning strikes in overhead cables. Manufacturers produce X capacitors in classes X1 to X3, and Y capacitors in classes Y1 to Y4. The most common classes available commercially are X1, X2, Y1, and Y2, which are impulse tested to 4 kV, 2.5 kV, 8 kV, and 5 kV, respectively. Of these, X2 and Y2 capacitors find the most widespread application in common household appliances.
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