Measures to deal with interference in PLC working environment
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How do you deal with interference in a PLC working environment?
Four strategies cover most interference problems: shielded cabling (braided shield on analog and communication lines, grounded at the source end only), proper grounding (single-point earth with low resistance, no daisy-chaining between devices), power conditioning (dedicated supply circuit, isolation transformer or UPS where the process warrants it), and ferrite beads near I/O connections for high-frequency noise. The first diagnostic step is always the same: check what changed in the facility recently, because the interference source is often a new installation rather than an exotic phenomenon.
An earlier version of this page carried field-experience percentages and an attributed engineer statement that could not be verified. They have been removed; the strategies below stand on the electrical engineering behind them.
Where interference comes from
| Source | Mechanism | Effect on the PLC |
|---|---|---|
| Variable-frequency drives and large motor starts | Electromagnetic radiation and conducted noise on shared power | Communication errors, spurious input changes, resets |
| Grid switching and capacitor banks | Voltage sags, spikes and harmonic distortion on the supply | Resets, data corruption, power-supply stress |
| Welding equipment and radio transmitters | Radiated electromagnetic energy | Induced voltages in signal cables, packet loss |
| Static discharge (ESD) | Human contact or dry-environment discharge | Component damage, memory corruption |
| New LED lighting or charging stations on shared circuits | Conducted high-frequency noise | Intermittent input flicker, analogue drift |
Scroll the table horizontally to view all columns.
The protection strategies
- Shielded cabling. Braided shield on analog and communication cables absorbs electromagnetic energy before it reaches the conductors. Ground the shield at the source end only; connecting both ends creates ground loops that introduce new noise.
- Proper grounding. A single-point earth with low resistance provides the fault-current and noise path. Daisy-chaining grounds between devices creates loops; use dedicated earth, not structural steel.
- Power conditioning. A dedicated supply circuit for the PLC system, with an isolation transformer or UPS where noise or dips are known. Sharing circuits with heavy loads invites conducted interference.
- Ferrite beads. Placed near I/O connections, magnetic cores absorb high-frequency noise on signal cables. Ineffective on power cables; fit for purpose on signal lines.
Example: a real interference diagnostic
A controller's analog inputs drift for no obvious cause. The wiring and shielding are sound. The question that solves it: what changed? A new variable-frequency drive was installed two panels away last month, sharing the same cable tray. Separating the trays and adding a ferrite ring on the analog cable resolves the drift. The lesson: interference diagnosis is as much a facility survey as an electrical measurement.
Common questions
Why does the shield only get grounded at one end?
Grounding both ends creates a loop with the earth path, and any potential difference between the two points drives current through the shield, which re-introduces the noise the shield was meant to block. One end gives the noise a path to earth without the loop.
What ground resistance should the PLC earth have?
Low enough to dissipate fault current and noise effectively; a common engineering target is a few ohms at the controller's earth point. The measurement that matters is between the controller's earth and true earth, not between two cabinet panels.
What if the interference source cannot be removed?
Increase the separation (physical distance is the cheapest filter), upgrade the shield, add ferrite suppression on the affected lines, and verify the controller's supply is conditioned. If the source is inside the plant, coordinate the fix with whoever owns the equipment causing it.
Sourcing help
KOEED supplies shielded cables, ferrite suppression, isolation transformers and the controllers they protect, across the major brands. Send the interference description and part list for a quote.
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