Learn Automation with MINATA #48: Earthing and Screening — There Is No Rule Called \"Connect Everything Somewhere\"
Earthing and Screening: There Is No Rule Called "Connect Everything Somewhere"
Earthing and screening do not reduce to "connect everything somewhere and you are done". Earthing serves two different goals — electrical safety and signal reference or screening — and mixing those two goals up is the source of a great many irritating faults.
The two roles of earth
- PE, the protective earth: it carries fault current back to the source so the protection can operate, and holds equipment enclosures at a safe potential. This is mandatory and must never be cut.
- Signal reference and screening: it provides a stable 0V reference and drains noise away from sensitive signals. How a screen is connected follows the equipment documentation, not personal preference.
Disconnecting PE to "get rid of the noise" puts an electric shock risk into the system. When there is noise, check bonding, cable routing and layout first — leave PE alone.
Bonding and connection points
Enclosures, mounting plates, doors and the earth bar should be well bonded so they share one reference potential. A single main earth bar in the panel, PE conductors of adequate cross-section and clean contact surfaces — not insulating paint — are the foundation. A potential difference between earth points is what creates the ground loop that ends up in your signal.
Screening, cable by cable
| Cable type | Usual screening rule |
|---|
| Drive motor cable | Screen bonded to PE at both ends, 360° clamp at the panel |
| Analogue and encoder | Usually bonded at one end, the panel end, to avoid a loop current |
| Industrial Ethernet | According to the cable and connector standard and the device documentation |
A 360° termination — clamping the screen around its whole circumference — works far better than a pigtail twisted into a single wire. Whether a screen goes down at one end or both depends on the goal: both ends work well against high-frequency noise but can create a loop current where the earth potentials differ. Follow the manual of the equipment involved.
A worked engineering situation
After a drive is added to a panel, an analogue value starts to jitter. The team re-examines the analogue cable route against the motor cable, how the screens are connected, whether the trunking is separated, and the filter configuration — rather than simply raising the digital filter in the PLC until the jitter disappears. The cause usually lies in the cable route and the screening, not in the software.
The earthing diagram, the screening rules and the bonding all have to follow the installation standard and the equipment manuals.
Common mistakes
- Cutting or neglecting PE to reduce noise, creating a shock risk.
- Bonding an analogue screen at both ends and creating a ground loop.
- Using a long pigtail instead of a 360° clamp on a motor cable.
- Bonding surfaces still covered in paint or rust, giving poor contact.
- Raising the PLC filter to hide a physical wiring problem.
Earthing and screening checklist
- [ ] PE is of adequate cross-section, uncut, with clean bonding surfaces.
- [ ] One main earth bar, with enclosures and mounting plates bonded to it.
- [ ] Each cable type is screened according to the rule for that equipment.
- [ ] Motor cables use a 360° clamp; analogue loop currents are dealt with.
- [ ] Noise is traced through layout and cabling before any digital filter is raised.
Correct earthing is the foundation of both clean signals and electrical safety. #49 continues into signal noise: the sources of it, and the cable routes that carry it.
Read more automation knowledge at MINATA: https://minatavn.com/en/blog/industrial-automation
Previous — #47: Control panel layout: https://minatavn.com/en/blog/automation-47-control-panel-layout
Next — #49: Mitigating noise on signals: https://minatavn.com/en/blog/automation-49-signal-noise-mitigation
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