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.
#UF_capacitors
is a professional ISO9001 factory in safety capacitors disc_capacitors #Y1 #Y2
#X2 for more than 27+ years, meeting international
safety standards such #ENEC, #UL, #CQC, #KC/KTL an #VDE. #UL recognized lab
test in factory & could issue UL test report directly. Compared to #Kemet,
#Murata, #AVX #Vishay etc L/T more than 20~40+ weeks, we could manage #fast_
lead_time around 4 weeks at competitive.
