A screened cable such as LiYCY only keeps interference out if its screen is connected properly at the ends. Most screen problems come from how the braid is cut and earthed in the panel, not from the cable itself.
What the screen does
| Interference | Typical source | What helps |
|---|---|---|
| Electric field | Power cables, contactor coils | An earthed screen. Even thin foil works. |
| 50 Hz magnetic field | Power cables, transformers, motors | Twisted pairs and distance. A copper screen does little. |
| High frequency | Variable speed drives, switching power supplies, radio | A screen bonded 360°, usually at both ends |
Don't use the screen of a signal or control cable as a protective conductor. Provide a separate protective conductor (CPC) where the equipment and installation design require one, sized as described in sizing protective conductors.
Braid or foil
| Screen | Good at | Limitations |
|---|---|---|
| Copper wire braid | Low resistance, flexing, easy to clamp | Small gaps let some high-frequency interference through |
| Foil with drain wire | Full coverage, thin, high frequencies | Tears if flexed; connected through the drain wire |
| Foil and braid | Demanding EMC conditions | Larger and stiffer |
Coverage is how much of the cable's surface the braid covers. LiYCY braids are typically about 85%, lighter control cables about 70%, and many drive motor cables 90% or more. Higher coverage generally screens better, but construction matters too, and a poor termination wastes even a dense braid. Screen quality is measured as transfer impedance: the lower, the better.
Tinned copper is normal for LiYCY braids. The tin helps the copper resist corrosion in humid climates such as Singapore, keeps good contact at the gland or clamp, and makes the braid easier to solder.
Earth at one end or both?
| Arrangement | Common for | Note |
|---|---|---|
| One end only (single-point), normally at the panel | Analogue instrument loops (4–20 mA, thermocouples, RTDs), where the system specifies it | Avoids ground loops, but gives less high-frequency protection |
| Both ends, 360° | Drive motor cables, encoders, fieldbus, data cables | Needs good bonding between the two ends |
The choice depends on the interference present and how well the earths at each end are bonded together, not on the signal type alone. With single-point earthing, trim the braid back and insulate it at the other end.
When a screen is earthed at both ends and the two earths are at different voltages, current flows along the screen. A bonding conductor run alongside the cable reduces that voltage difference, and with it the screen current.
Clamp it all round, don't twist it into a tail
A long twisted tail (a pigtail) to an earth terminal works at low frequency. At high frequency it acts like a small coil, and interference gets onto the cores right where the cable enters the panel.
A 360° termination contacts the braid all the way round:
- EMC cable gland into an earthed metal enclosure or gland plate. Prepare the braid as the gland maker instructs.
- Screen clamp on the exposed braid, onto an earthed screen bar close to the cable entry.
- Connector with a metal backshell for plug-in sensors and encoders.
If a terminal connection can't be avoided, keep the tail to a few centimetres. The same applies to the drain wire of a foil screen.
Preparing the braid
- Strip the sheath without cutting the braid strands underneath.
- Expose and dress the braid to suit the gland or clamp.
- Keep the screen intact as far as the equipment instructions require. Where it ends, trim it cleanly and remove every loose strand. One stray strand can short a terminal.
- Keep the cores outside the screen as short as possible, and keep pairs twisted up to the terminals.
- At an end that is deliberately not earthed, trim the braid back and cover it with heat-shrink.
Keep the screen continuous
A screen protects the whole run only if it is unbroken. In a single-point system, carry each screen through junction boxes on its own insulated terminal, and don't earth it again along the way. In a both-end system, screens are normally bonded to an earthed screen bar at each enclosure.
Common mistakes
- Long pigtails or drain-wire tails on drive and encoder cables
- Screens earthed at both ends without good bonding between the ends
- Single-point screens earthed again at junction boxes
- Screens left unconnected at both ends
- Using a signal cable screen as the protective conductor
- A cut or unravelled braid at the gland
- Signal cables run tight against power cables. Distance still helps; see ELV and power cables in the same containment.
Screened cables in the Goldberg range
- LiYCY: PVC data and control cable with a tinned copper wire braid, rated 250/250 V
- UL2464: multicore signal cable with an aluminium/polyester foil and a tinned copper drain wire
This article is general guidance. Follow the equipment and gland makers' instructions, and have a Licensed Electrical Worker (LEW) confirm the earthing arrangement for your installation.
Need screened cable? See our control and instrumentation range or ask our team for a quote.
