How to configure safety capacitors for I/O modules?

Properly configuring safety capacitors in I/O modules is fundamental to ensuring safety and EMC performance. The capacitor connected between the L and N lines is the X capacitor, used to suppress differential-mode interference. Its capacitance value is selected based on the expected noise frequency attenuation, typically ranging from 0.1μF to 2.2μF, and a discharge resistor must be connected in parallel to meet voltage discharge requirements after power failure. The capacitor connected between the L-PE or N-PE lines is the Y capacitor, used to suppress common-mode interference and conduct noise to ground. Its capacitance value is strictly limited to control leakage current, typically ranging from 1nF to 4.7nF. Y capacitors should be placed as close as possible to interference sources, such as between the primary and secondary windings of the transformer in a switching power supply. Y capacitors are also required at DC power ports, between the positive and negative lines and the chassis ground. All safety capacitors must be certified models, such as X1/Y1 or X2/Y2 grades, which have high reliability and withstand voltage. In PCB layout, the lead traces of safety capacitors should be short and thick to reduce parasitic inductance and ensure low impedance of the high-frequency noise discharge path. After configuration, a leakage current test must be performed to ensure compliance with safety standards.