Grounding, shielding and EMC basics
Protective earth vs functional grounding, equipotential bonding, when to ground shields on one or both ends, cable separation and VFD-related interference — practical rules for control cabinets and field wiring.
Protective earth and equipotential bonding
- PE (protective earth) protects people: exposed conductive parts are connected so that a fault trips protection. Never remove or misuse it for signals.
- Equipotential bonding connects metal parts of machines, cabinets and cable trays with low impedance so that they stay at the same potential. It is the basis for good shielding: if two cabinets differ in potential, current flows through cable shields.
Shields
| Practice | Use |
|---|---|
| Shield grounded at both ends, 360° clamp | Digital/fieldbus cables (PROFIBUS, PROFINET, CAN), VFD motor cables — effective against high-frequency interference; requires good equipotential bonding |
| Shield grounded at one end | Sometimes for low-level analog signals in installations with poor bonding to avoid low-frequency ground-loop currents |
Follow the device manufacturer: fieldbus guidelines generally require both-end grounding plus bonding. Connect the shield at the cabinet entry with a clamp over the full circumference — a long pigtail ("drain wire" twisted into a terminal) degrades shielding at high frequency.
Cable separation
Group cables by category and keep distance or use separate, metallic, bonded trays:
- power and motor cables (especially VFD outputs);
- 24 V digital signals;
- analog and communication cables.
Cross at right angles where they must meet.
VFD-specific measures
- Shielded symmetrical motor cable, shield clamped at drive and motor.
- EMC filter and grounding per the drive manual; short, wide grounding connections.
- Output reactors or du/dt filters for long motor cables.
- Keep analog references to the drive shielded and away from the motor cable.
Symptoms of EMC problems
Analog values jumping when a motor starts, communication errors under load, PLC inputs switching without reason, sporadic resets. Check bonding and shield clamping before replacing devices.
Connections
Open in Knowledge Graph →Loading the map…
Sources
Updated 2026-10-02