How to locate the common-mode interference source of an electrical power measurement switch (PMS)?

Persistent common-mode interference in a PMS can cause equipment communication anomalies, measurement jumps, or immunity test failures. Accurately locating the source is a prerequisite for efficient rectification. Interference typically originates from high dV/dt nodes in switching power supplies, poorly shielded motor or inverter loads, and unbalanced grounding systems.

The source location process can be divided into three steps: First, use a near-field probe to scan the PCB, focusing on the heatsinks of switching transistors, transformers, relay contacts, and input/output terminal areas to find strong magnetic or electric field radiation points. Second, use a current probe to perform conducted emission diagnosis on power lines and communication lines, determining the frequency points exceeding the standard according to EN 55032. If the frequency exceeds the standard in the low-frequency range (150kHz-1MHz), it is often related to grounding impedance or insufficient filtering; if the frequency exceeds the standard in the high-frequency range (>30MHz), it may be related to layout or shielding. Finally, by unplugging and plugging in external cables (such as RS485, sampling lines) or temporarily adding a CMZ3225A-510T common-mode choke, observe whether the interference is significantly reduced, thus determining whether it is internal coupling or external input.

Systematic positioning can quickly narrow down the range of interference sources to specific modules or paths, providing a clear direction for subsequent targeted use of CMZ/CML series inductors or optimization of grounding design.