conducted interference

Conducted interference is unwanted electrical noise that reaches a device by traveling directly through conductive paths, typically power supply lines, signal cables, or ground connections. Unlike radiated interference, which couples through the electromagnetic field in space, conducted interference physically rides on the wiring that connects components. It enters through the same conductors meant to carry useful signals or power, making it insidious because the noise and the intended signal share the same physical path.

The sources are concrete and widespread in industrial settings. Switch-mode power supplies generate high-frequency switching noise at tens of kilohertz to megahertz frequencies. Variable frequency drives produce harmonics and transients during motor control. Relay contactors create sharp voltage spikes when switching inductive loads. Switching action in any large inductive circuit, from solenoid coils to transformer secondaries, produces di/dt (rate of change of current) that couples into adjacent conductors. These noise sources inject directly into the common network of power distribution, contaminating every device connected to it.

Conducted interference manifests differently depending on frequency and circuit sensitivity. Low-frequency components (50 Hz to a few kilohertz) may cause digital logic errors, flicker in analog displays, or phase noise in precision oscillators. Higher-frequency components can demodulate onto audio or instrumentation signals, producing audible hum, measurement errors, or false triggers in control circuits. Sensitive measurement instruments, audio equipment, and precision analog circuits are most vulnerable. A 12-bit analog-to-digital converter may see its lowest bits corrupted by conducted noise well before the interference is audible as hum.

Separation and filtering

Practical mitigation begins with source separation: run power feeds and signal cables in separate conduits when possible, and group conductors by function rather than laying everything in a single bundle. Ferrite cores on cables, chokes in power lines, and multiple-stage LC filtering on supply rails all attenuate conducted interference at the source and entry points. Star-point grounding, where all signal grounds reference a single node rather than spreading through a mesh, prevents ground loop currents from carrying noise throughout the system. Input filtering on device power pins, often a 100 nF ceramic capacitor plus a 10 microfarad or larger electrolytic, catches high-frequency noise before it reaches internal circuits.

Industrial standards like EN 61000-4-4 specify test levels for conducted immunity, injecting calibrated disturbances directly onto power and signal lines to verify equipment robustness. Equipment rated for industrial environments must tolerate 4 kV fast transients on power supplies and several hundred volts of RF injection on low-voltage signal lines. Equipment designed for office use is typically rated only to 2 kV or less, which is why industrial-grade devices cost more but remain reliable near motor starters and variable frequency drives where office-grade equipment fails.