Since capacitors in EMI filters are connected to ac power lines, these capacitors can fail due to over-voltages and transients. They are classified according to their use in the circuit. Capacitors connected line-to-line are called “X capacitors,” also called “line to neutral” capacitors. Those connected from line-to-ground are called “Y capacitors,” also referred to as “line bypass capacitors.”
X capacitors are used for differential-mode EMI filtering. Y capacitors are used for common mode EMI filtering bypassing the interference from the wires to ground. Since safety capacitors are directly connected to the mains voltage, they can be subjected to voltage transients, power surges, overvoltage conditions, and other stresses resulting in device failure. They are designed for specific failure modes. Failure of X capacitors could result in a fire. X capacitors are designed to fail shorted, which causes a fuse or circuit breaker connected to the device to open, preventing the possibility of a fire.
The shorting of a Y capacitor could present a fatal shock hazard for personnel using the equipment. As a result, Y capacitors are designed to fail open and prevent any possibility of a shock hazard. While the equipment is shut down by failure of an X capacitor and the subsequent tripping of an overcurrent protection device when a Y capacitor fails, the equipment could continue operating, but EMI filtering would be significantly reduced.
Safety capacitor standards
Because a shorted Y capacitor could lead to the danger of an electric shock, Y capacitors are held to a higher operating standard compared with X capacitors. During the certification process, two key tests performed are the impulse and endurance tests. These are done to verify that the X/Y capacitor can withstand ten impulses of alternating polarity, followed by a 1000-hour endurance AC life test. After completing these two tests, the capacitors must still perform reliably in the circuit under AC voltage conditions. These tests are part of the IEC 384-14 certification requirements.
A design consideration when using X capacitors is the need to automatically discharge the capacitor upon loss of power to the power converter to comply with safety standards. Several suppliers offer integrated circuits that can be placed in series with bleed resistors (also called discharge resistors) that automatically discharge the energy in the X capacitor when the mains voltage is disconnected, diverting the energy away from the exposed AC plug and protecting equipment users.
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